There are several benefits to anodising, but we should make an early distinction between hard anodising, which applies a relatively thick oxide surface on aluminium components (hard anodising is not used on titanium), and the decorative version of the surface treatment, which applies a far thinner and more transparent layer. The hard anodising treatment gives a significant increase in hardness of the component’s surface, and a big improvement in wear resistance, to an aluminium component. It can also reduce the coefficient of friction, especially if the anodised surface is sealed with a low-friction polymer such as PTFE. Hard anodising is often used to prevent wear problems in piston ring grooves, for example, and we commonly find it used on aluminium pulleys of both the conventional and polyvee types.

The more decorative version of anodising does not give the same level of wear resistance as the  much thicker hard anodising process: with the oxide layer being far thinner, it has only the substrate to support it, rather than a thick layer of the stiff oxide. However, decorative anodised processes allow dyes to be used, and aluminium components can thus be colour-coded to aid identification or to add a little colour to an engine that perhaps looks otherwise a little dull, although on the grounds of taste some people object to the use of multi-coloured components ‘adorning’ the exterior of an engine. Decorative coatings do provide a degree of corrosion resistance though, which can be an important factor. There is a huge number of applications in engines and transmissions for these thinner processes.

The downside of anodised surface treatments is the loss of fatigue strength that results from their use. This loss can be significant, but depends very much on the exact process used and the alloy being processed. If the area requiring the coating does not have a significant stress concentration, it may be possible to locally mask the ‘danger area’ to avoid compromising its strength. For example, on polyvee pulleys the base of the grooves (which are the area of greatest stress concentration) can be masked by O-rings, while the wear surfaces – namely the tips and flanks of the teeth – are anodised.

To mitigate the deleterious effects of hard anodising on the fatigue strength of aluminium, some success has been found by using shot peening, although when peening is combined with hard anodising, the peening must be done before anodising. The improvement that comes with pre-anodising peening can be dramatic – a 30% decrease in fatigue strength without peening has been turned into a 10% increase in strength compared to an identical component that was neither peened nor anodised.

Reference

Champaigne, J., “Shot Peening Overview”, available at www.shotpeener.com, 2001

Written by Wayne Ward

Although not widely used for this purpose, there are applications where precision pickling produces tight-tolerance final dimensions on components, owing partly to the simultaneous effects of improvement of surface finish (roughness reduction) and removal of burrs from production that would require ‘artisan’ – that is, manual and non-repeatable – intervention. Depending on the acid and the material being treated, pickling can produce a surface with high quality and low roughness. As the acid preferentially attacks and removes material with a high ratio of surface area to volume, it quickly removes undesired features such as burrs as well as peaks in the surface finish of the material.

However, while pickling or etching to a precise final dimension is not widely used, the general principle of using strong acids to remove material as a way of reducing the thickness of sheet materials can be used both on raw materials and finished components, especially fabricated parts. The selective pickling of thin fabricated assemblies can be accomplished by masking critical areas (typically welds) followed by immersion in a bath of acid so that the remaining unmasked areas are ‘eroded’ to the desired thickness.

The amount of time required depends on the alloy, the particular acid used and its strength and temperature. It is therefore necessary to carry out regular tests of the rate of material removal before use. As the cumulative effect of pickling/etching on the acid is to weaken it and reduce the rate of material removal, if these tests are neglected then you may find produce components that are oversize and/or overweight. The advantage of using acid pickling/etching is that materials that are too thin to weld reliably can be specified, so it is possible to fabricate components such as oil tanks and water tanks, for example, that are much lighter by etching down to the desired thickness after welding.

Pickling is also used to remove surface contamination from some components. For example, stainless steel or titanium fabrications can be pickled to remove traces of steel from their surface, which can cause problems in later processing or might provide an initiation site for a corrosion problem. In the same way, some castings are acid-etched to remove the surface layer, which can include some foreign material (casting sand or investment casting mould materials) that might be a fatigue crack initiation site.

This technique is likely to be used on titanium gearbox main-case castings, as found in Formula One for example. Ferrari pioneered the use of titanium for gearbox main cases, although this was as a fabricated assembly rather than a casting. Minardi used cast-titanium main cases from 2000 onwards, and the technique was quickly adopted by other teams.

The rate of material removal varies widely with material type and alloy, so care needs to be taken and sufficient data gathered before using the process. It should also be noted that, because material is preferentially removed where the ratio of surface area to volume is high, sharp external corners are likely to be removed. This may or may not be the desired effect.

Written by Wayne Ward

There is a multitude of applications for vibratory finishing in engines and transmissions, and the main thing they have in common is that most of the treated components are in some form of sliding or rolling contact with other components. The merits of improved surface finish in such contacts are widely understood: by removing the high spots on any surface through a surface treatment such as vibratory finishing, we reduce friction and increase reliability by giving the oil film a higher margin of safety. Until the oil film is sufficiently developed – as in it becomes thick enough to separate the components – parts of the components are in contact, and friction is a function not only of the partially developed oil film but also of the solid contact, hence the high friction.

With poor lubrication conditions such as this, wear can rapidly cause a component to fail. The smaller the surface finish height, the thinner the oil film can be. So, particularly at start-up or low speeds, the improved surface finish is a real bonus. Components such as gears, cams and cam followers benefit from such treatments. Vibratory finishing is also used sometimes on crankshafts, again improving surface finish on bearing surfaces.

In terms of energy, there are some components where we definitely want a high-energy process capable of removing burrs and taking off sharp edges. For example, the edges of cam lobes and gear teeth are better when the edges are thoroughly de-burred and given a small radius.

However, there are applications in engines where we definitely want to retain sharp edges. On some machined edges on engine poppet valves, the sharp edges on the back of the valve are important, and choosing to put a nice radius on the wrong one of these can bring a measurable performance penalty.

The reason is that some sharp edges are carefully developed features to enhance the energy in the flow over the valve, acting in a similar way to a flow trip. By increasing turbulence in intake valve flows, for example, there are circumstances where the flow through the valve seat is greater because there is a reduced degree of separation. There are specific designs of valves to create these conditions, and if the critical sharp edges are turned into small radii then the valves can represent a step backwards compared to a conventional valve design, rather than giving the intended improvement.

There are some extremely high-energy vibratory finishing processes that are characterised as peening techniques, whose aim is to create significant residual compressive stresses in component surfaces, but the energies involved are far beyond those that can be imparted by conventional vibratory finishing treatments.

Written by Wayne Ward

Low-pressure carburising minimises the distorting effect of carburising, which has been one of the main drawbacks with the process. In order to control the properties of the carburised material, it is necessary to quench the component, and it is at this stage where traditional processes can introduce distortion, especially where liquids such as oil are used for the quenching. It is very hard to achieve an even rate of heat removal when an extremely hot component is plunged into cool oil.

One solution is to quench using a gas. Even though the carburising process is low pressure, the pressure of the gas quench can easily be 10 bar or more. In order to improve surface finish, inert gases are used and, in contrast to traditional oil-quenched carburising processes, the parts emerging from a vacuum carburising process followed by an inert gas quench remain bright and shiny in appearance. Typically nitrogen is used as a quench gas, but helium is also finding some use as it offers higher rates of heat removal owing to its lower introduction temperature and high specific heat capacity. Argon is also occasionally used.

If we consider where our carburised part fits in the engine, we might find that we need to fasten it to another component, perhaps even using threaded holes; we may also need to have thin areas of material. The danger with threaded holes is not limited to distortion but also the fact that the whole thread might be completely carburised, rendering it very brittle and prone to failure. In the same way, very thin sections of material might be hardened completely through.

Fortunately though we can indicate to the carburising supplier which areas of the component are to be treated, those that must not and those that are optional, by supplying them with a drawing. The part of the drawing that deals with heat treatments and surface treatments is often the key to success in component design, so it is important to ensure any general notes, instructions on drawing views or attempts to specify which areas are not to be treated are specific and clear.

The areas that are not to be carburised can be mechanically masked, but are most often plated or painted to prevent a carbon-rich case being formed. Copper plating is an effective method of preventing carburising; it’s a process known as ‘stopping off’. Also, copper-bearing paints can be applied either by spraying, brushing or dipping, and a number of the more modern stop-off paints are water-based and so water soluble, so can simply be washed off after use.

Written by Wayne Ward

What is less well known though is that nitriding can be used as a surface treatment for other materials, especially the more modern plasma nitriding processes which can be controlled more easily.

After steels, titanium is the most commonly nitrided material. Titanium is often coated with titanium nitride, but it is also possible to form the nitride directly at the surface, and for the nitride layer to be diffused into the surface as per conventional nitriding of steels. The advantage of forming titanium nitride directly from the substrate rather than applying a discrete layer is that the nitride formed in the process is an integral part of the component and does not suffer from the adhesion problems sometimes associated with coatings. One example from motorsport that compares the methods of nitride coating of titanium versus directly forming the nitride layer is of a steering rack, as cited by Huchel and Strämke*. The PVD-coated rack lasted one race, whereas the nitrided component lasted between up to ten races.

The nitriding of titanium was for a time restricted to quite simple geometry, because the surface could easily be damaged by high heat input, but now pulsed plasma nitriding methods are available that reduce local heat input and therefore allow more complex titanium components to be processed.

The benefits of nitriding titanium cannot be translated directly from experience of using the process on steel. It does not, for example, confer on titanium components any degree of improvement in fatigue life in the same way it does for steel. It does impart some wear-resistance to titanium, although only against adhesive wear. The layer thickness achieved at 1-3 microns is sufficient to prevent micro-welding, but is not sufficient to prevent abrasive wear which is caused by a ‘ploughing’ effect to a greater depth than nitriding is able to protect against.

Nitriding titanium poppet valves is a common application, as the stem and valve seat faces can commonly wear if not coated.

In the past, attempts have been made to coat titanium components with steel, which was then ground to size and subsequently nitrided in order to improve the wear characteristics of titanium. However, with improvements in plasma-nitriding methods, such elaborate processes are not required.

It is also possible to plasma-nitride aluminium, and there has been research in this area in order to provide wear-resistant surfaces on aluminium components.

* U. Huchel, S., and Strämke, “Pulsed Plasma Nitriding of Titanium and Titanium Alloys”, paper presented at the 10th World Conference on Titanium, 2003

Written by Wayne Ward

The magical powers of peening are due to the residual compressive stresses it imparts on the workpiece. At the risk of banging on endlessly on the same point, it is always worth setting the new reader/novice engineer on the path of discovering what help residual compressive can give us.

Shot peening has a number of parameters that can be controlled in order to change the magnitude and distribution of the compressive stresses in the surface of a given component. Good examples here are the size and material of the peening media, and peening intensity.

By introducing further variables, however, we can ‘play tunes’ with the shot-peening process in order to increase the compressive stresses beyond that achievable with what is a highly controlled blasting process. Readers should refer to issue 74 of Race Engine Technology and the article on Surface Treatments for more detail on the processes described briefly below.

Warm peening or hot peening is, as the name suggests, a process undertaken at temperatures higher than ambient. The control of temperature is important, as the magnitude and depth of compressive stress are very sensitive to peening temperature. The mechanism by which the process differs from ambient peening lies in the fact that the yield strength of the workpiece decreases with temperature. There is nothing to be gained by peening steel at 100 C for example; the peening temperature depends on the alloy being peened.

Stress peening places the workpiece under load during the process. Defining and achieving the optimum load in practice for most components would be prohibitively time-consuming and expensive, but it is used to very good effect on racing valve springs, which can be loaded simply and accurately to produce a predictable result. Research, reported in the above Race Engine Technology article, found it is possible to increase the compressive stress due to peening by around 50% in spring steels. Beyond the case of helical and torsion springs, there are few components we can place into realistic states of stress with sufficient ease to make it commercially viable.

Combining elevated temperature and stress while peening can improve on both of the above variations on shot peening, but the complexity and expense of achieving the correct parameters means this is almost certainly going to be only of academic interest as far as motorsport is concerned; such complex processes are likely to be used only in industries such as defence and aerospace. Again, if the process is to be used in race engines, the component which is likely to derive the most benefit and which is going to be the easiest and most cost-effective to treat will be valve springs.

Written by Wayne Ward

In this article, we will look at how such surface treatments might be used to advantage for transmission applications, specifically gears. Phosphated surfaces offer a number of potential advantages for gears, despite the surface being much softer than the carburised surface which is most often specified.

There are several reasons for this, some of which are a result of the softness of the treated surface. Phosphate surface treatments convert a very small depth of the surface, rather than simply depositing a material onto it, as with a coating. There is some confusion here though: there are sprayed coatings that are phosphate based, and it is a common mistake to call phosphate conversion treatments coatings. Although not a hardening treatment, phosphate conversion coatings are similar in principle – the resulting surface is different, but with an integral part of the component having been formed by a chemical reaction.

However we have finished the surface of a component, whether this is by machining or grinding, there are high-points on each surface, known as asperties. It is these asperities that first cause any hydrodynamic oil film to fail, as these are the points on the surface that will first come into contact with the mating part. As a chemical conversion process, phosphating reacts with an iron-bearing surface, and the rate of conversion is proportional to surface area. In the same way that finely atomised fuels burn more quickly, so asperities – with their high ratio of surface area to volume – are converted more fully to a soft phosphate layer. While the process may convert only microns of a surface, that is enough in many cases to change a hard asperity into a soft and weak high point, which will simply and harmlessly be deformed or detached in service.

Phosphate surface treatments are also well known for their ability to hold oil within the structure of the converted surface. This, and the softer nature of the surface itself, helps the running-in behaviour of gears and is effective in minimising scuffing of gears during early life. Bergseth (1) cites research on gears that shows that although the soft phosphate surface is quickly worn away during running, it is very effective in preventing scuffing. Sjöberg (2) carried out tests showing that under extreme contact pressures of 7 GPa (1015 ksi) phosphate treatments on standard specimens had a lower coefficient of friction and resisted scuffing, even though the same specimens without phosphate coating had scuffed at less than 1.9 GPa contact pressure.

Phosphate surface treatments are also useful, especially when they have been oiled, for inhibiting corrosion. Where transmission gears stand for periods of time, corrosion pitting may start, and phosphating can help prevent early-life corrosion. This can be especially important where transmissions are built and packed in a relatively warm environment and are then exposed to cold, as in an aircraft’s cargo hold, causing condensation.

It is not all good news though. Devlin et al (3) present research that shows that phosphate coatings may detrimentally affect the fatigue life of gears, although their study was conducted on low-hardness gears. 

References

1. Bergseth, E., “Influence of surface topography and lubricant design in gear contacts“, Kungliga Tekniska Högskolan academic thesis, ISBN 978-91-7415-427-6 

2. Sjöberg, S., “On the running-in of gears”, Kungliga Tekniska Högskolan academic thesis, ISBN 978-91-7415-656-0 

3. Devlin, M.T., Turner, T.L., Thompson, K., Kolakowski, K., Garelick, K., Guevremont, J.M., and Jao, T., “Effect of Phosphate Coatings on Fatigue and Wear”, paper presented at the Annual Meeting of the NLGI, 2007 

Written by Wayne Ward

Plating works by using zinc, say, as a sacrificial coating, the oxygen in the air preferentially reacting with the zinc rather than the aluminium or steel substrate. The chromate layer is highly tenacious, and can significantly increase the time it takes for the substrate to become corroded.

Chromate treatments are also used as an ‘undercoat’ for other processes on transmission casings. Magnesium, for example, is very often chromated as a minimum to prevent rapid corrosion. Although chromate coatings in isolation aren’t particularly good at preventing corrosion of magnesium, they are better than nothing and also better than phosphate treatments, both in preventing corrosion and acting as an undercoat.

Chromate treatments do a good job of ‘tidying’ magnesium castings – a new coating of a ‘black’ chromate can make an old casting look much ‘fresher’. This process was reasonably popular when bespoke race engines still widely used magnesium, and some historic F3000 and Formula One engines still sport chromate-treated magnesium cam covers.

The ‘black’ chromating treatment can be anything from a dark brown to black depending on the substrate. The two main chemical solutions used are based on sodium chromate with additions of manganese or ammonium sulphates. Neither process removes any material, so they are suitable for components with tight tolerances on machined bores, and are therefore excellent for new components as well as restoration work.

A more aggressive chromate treatment is based on a more acidic solution, again using sodium chromate but with additions of nitric acid. This is a very much faster process and results in a golden-yellow colour on magnesium. However, it can remove small amounts of material (up to 0.025 mm/0.001 in) so it’s not suitable for bores and other features with tight tolerances on finished size.

Magnesium still remains a popular material for transmission casings in racing, and chromate coatings therefore remain widespread, although they are often combined with other treatments such as painting or resin coatings, since chromate-treated magnesium offers increased adhesion in these applications. This is a real synergy as, if properly applied, the combined coating offers a much longer period of time before significant substrate corrosion is seen than if either the chromate or the paint/resin coating was used in isolation. However, some magnesium materials – including some high-strength wrought magnesium alloys – do not respond to chromate coatings.

Despite the FIA having seen fit to ban magnesium for engine components in Formula One, there are still tens of thousands of new race engines with all manner of magnesium components. Since many race engines are based on production units, there seems little point in replacing the highly engineered, lightweight magnesium components that have routinely been used for well over 25 years – production motorcycles in particular use magnesium cam covers, clutch covers, generator covers and so on. The car manufacturers have not been oblivious to this, and also use the material where it offers an advantage, and very often these components are chromate-treated and painted. 

Written by Wayne Ward

The anodised layer thickness may be specified and there is a ‘half and half’ rule that can be used to predict component growth. Half of the oxide layer is internal – that is, the existing metal is ‘replaced’ by its oxide – and half of the layer thickness manifests itself as component growth. For example, a component 10 mm thick and given a 0.04 mm thick anodising treatment will see a growth of 0.02 mm per side, 0.04 mm in total, giving a post-anodising thickness of 10.04 mm

The resulting oxide layer is very porous, and it is always ‘sealed’ to fill in the pores. There are several options for sealing, each of which has its own advantages. Hot-water sealing is commonly used, either on its own or in conjunction with one or more alternatives. Sealing of hard anodic surfaces gives treated components a greater degree of corrosion resistance and gives the coating increased toughness. It also improves dielectric (insulating) strength.

Using hot water fills the pores in the anodised surface and creates hydrated aluminium oxide, which is less dense than the as-anodised film. The anodised film is very hygroscopic, and will tend to seal itself slowly over time by drawing moisture from the air. Water sealing is done at close to the boiling point of water: the speed of the process depends very much on temperature, and the higher the temperature then the quicker an effective seal can be formed.

Another method, nickel fluoride sealing, uses a cold solution of the nickel salt but takes a long time, although the process can be accelerated by a warm water rinse after initial sealing. The nickel fluoride reacts with the aluminium oxide to produce aluminium fluoride and nickel hydroxide in the pores. Cold sealing is said to offer maximum hardness and wear resistance, and surfaces thus sealed are less prone to finger-marking.

Sealing using a hot sodium or potassium dichromate solution gives the anodising a military-style drab-olive colour, and is particularly effective in terms of corrosion prevention.

Anodic films can be sealed with PTFE to give a hard-wearing surface with relatively low friction and good water-shedding properties. It can have a slightly ‘milky’ appearance, especially if the anodised surface has been dyed black beforehand.

Sealing the anodised surface is very important if you intend to rely on its dielectric properties. If left unsealed, the dielectric strength over a fissure or pore drops to the same level as if an air gap equivalent to the anodised film were present. By sealing, we don’t necessarily improve the insulating properties of a pore-free section of the film, but we improve the film where it is at its weakest.

A note of caution: hard anodising is known to affect the fatigue strength of aluminium alloys, with the effect varying with the substrate alloy, the anodising process and the thickness of the coating. It is possible though to restore some of the loss in fatigue strength by the use of processes such as shot-peening.

Written by Wayne Ward

Compared to EDM, there are several practical advantages. The surface finish possible with ECM is typically better than with EDM: ECM can produce mirror-finished parts. EDM tools suffer wear through use, thus losing precision over time; this affects both die-sinking and wire EDM. Die-sinking is what is commonly called ‘spark erosion’, where a formed electrode is plunged into a workpiece. The wear on the EDM tool is one reason why it is necessary for the wire (which is the ‘tool’ in wire EDM) to be continuously fed during machining. ECM suffers from no such tool wear and so, provided that the machine control is repeatable, there should be no loss of accuracy.

However, the main objection to EDM machining has traditionally been the fact that a layer known as recast is formed on the surface of the part being machined. As the name suggests, recast is a layer that has been melted and then solidified, and is essentially a very thin layer of cast material on the surface of the part. This can lead to a surface having a lower fatigue limit than the underlying material, and has led to EDM being shunned to a large extent in the past. However, recent advances in the technique have reduced recast layer thickness significantly, and there are finishing techniques that can reduce this.

By contrast, ECM offers the chance to machine without the recast layer being formed. Compared with EDM, which removes material via arcing through a non-conducting fluid (the reason why the process is often called spark erosion), ECM does not machine by arcing. The fluid itself is one of the electrodes, as it is an electrolyte (conductive fluid). ECM is sometimes described as the reverse of plating: where electroplating deposits metal from the fluid onto the surface of the component, ECM takes material from the surface of the component into solution.

However, despite the fact that no recast layer is formed, there are problems associated with ECM that can reduce a component’s fatigue strength. The first is one that you might have considered when I described the process as being similar in concept to electroplating, since one of the dangers of electroplating is hydrogen embrittlement. Hydrogen is absorbed into the surface of a part and can form hydrides that render the surface brittle. In the ECM process, the hydrogen is evolved at the surface of the tool rather than the workpiece. However, owing to the proximity of the tool to the workpiece, the hydrogen formed can easily be absorbed by the workpiece.

Titanium alloys are one type of material that can be seriously affected, owing to the strong tendency of titanium to form a brittle hydride. Another problem is intergranular attack, which happens because the different elements within the workpiece material are taken into solution at different rates. This can lead to material at the grain boundaries being taken into solution much faster than the grains themselves, essentially forming micro-cracks at the surface of the workpiece in between grains. Nickel alloys are noted as being particularly affected among heat-resistant materials*.

So, the moral here is to consider the advantages and disadvantages of electrochemical machining carefully before using the process.

* Kozak, J., “The Effect of Electrochemical Machining on the Fatigue Strength of Heat Resistance Alloys”, Fatigue of Aircraft Structures, vol. 1, 2011

Written by Wayne Ward

In terms of friction, components with lower roughness surfaces require thinner oil films to separate them completely; once separated, the friction coefficient is a function of the lubricant and the film geometry only – there is no component of solid-to-solid (coulomb) friction. The durability aspect of surface finish is well documented in textbooks on engineering design. For example, Shigley and Mischke* give detailed coefficients for different surface finishes to allow calculation of more accurate fatigue strengths of components.

One way to achieve a very high level of polish on a surface is through vibratory finishing, which can be applied to a wide range of engine and transmission components, from tiny ‘widgets’ to crankshafts and gears. Components are placed into a vibrating ‘bath’ of media, which may be hard or relatively soft and which may be dry or contain fluids. Relatively soft media can be something like finely divided walnut shell, maize or ground corn cob, and this can work well, even for some hard materials. I have seen very highly polished press tools, engine components and transmission gears which have been finished using walnut shells.

The same technique can be used with success to de-burr components while polishing them.

The action of the media continuously rubbing against the part removes asperities and improves the surface finish further by removing the top layer of material, increasing the ‘flat’ area on the metal surface and decreasing the depth of the valleys in the surface.

There are some advantages over other surface finishing techniques using ‘loose media’ such as rumbling, where the polishing media is much larger. Owing to the size of the polishing media, the improvement in surface finish works well on internal surfaces such as holes. The limit on the size of an internal feature that is able to benefit from the process is a function of how large the media is. For example, if the media size is 0.5 mm, it is unlikely to polish a 0.4 mm ‘internal’ radius effectively, such as might be found on many turned parts. Another advantage is that the process is can be run with the top of the machine open, so it is very easy to see when the parts have been polished to the desired finish, in the case where the finishing is done for cosmetic reasons.

The media is constantly cycled around the machine, and as it becomes ground to ever finer sizes, the media which is too fine to work properly is screened out.

There is a wide range of media sizes available, and other process variables are vibration amplitude, frequency and processing time. Large amplitudes and frequencies imply high energy, and unsurprisingly the higher energy processes remove material more quickly and can also remove sharp edges too, which may not be what you are looking for. As ever, it is important to work with a finishing company in order to get the correct result. So, if you want to retain sharp edges, you will probably need a different process from someone who wants high rates of material removal, even for the same material.

* Shigley, J.E., and Mischke, C., “Mechanical Engineering Design”, McGraw Hill, 2001, ISBN 0-0736-5939-8 

Written by Wayne Ward

These processes cannot be applied to any material though. In particular they are applied to steels, although outside of motorsport, nitrocarburising is sometimes used on cast irons. In terms of the carburising process, the materials suited to this process are low-carbon steels.

The rate of diffusion of carbon into the surface is not only a function of the carburising medium and the temperature, but also the composition of the steel in question. The difference between the ‘carbon potential’ of the carburising medium and the percentage of carbon in the steel dictates the speed at which carbon is taken up by the component; the diffusion rate is fastest at the start of the process, when the difference is greatest.

A carburising steel contains elements such as chromium, vanadium and molybdenum, which are strong carbide formers; nickel is usually also present in larger percentages but this is to aid ‘hardenability’ in thicker sections. The tempering temperature of the steels in question also places limits on the type of components that are suited to carburising. With typical tempering temperatures in the 150-180 C region, we have to ensure that carburised components are kept cool enough not to be softened during operation.

Nitrocarburising takes place after hardening and tempering, so is reserved for materials whose tempering temperatures are higher than the nitrocarburising temperature. This ensures that the component’s hardness and strength are not diminished by the nitrocarburising process. Typically the process takes place at about 480-600 C, depending on the grade of steel being treated. This is in the same region as nitriding takes place, and the materials normally used are nitriding steels.

The elements that take part in the chemical reaction to form nitrides in the surface are again chromium, molybdenum and vanadium, but aluminium is also a strong nitride former and can be found in a number of steels suitable for nitrocarburising. Aluminium-bearing steels show slightly higher surface hardness after nitrocarburising than those without, owing to the formation of aluminium nitrides. The surface hardness increases with increasing aluminium content up to 1% aluminium, after which further additions have no real effect. Tungsten is also used in steels suited to nitrocarburising for use at temperatures approaching 600 C. Nitrocarburising can be applied to typical nitriding steels, many tool steels and stainless steels. For example, many austenitic stainless steel valves are nitrocarburised.

Written by Wayne Ward

There is a wealth of other thermal and surface treatment processes that may be successfully combined with nitriding treatments to real advantage. For example, hard engineering coatings can be successfully applied to all manner of steels, but might suffer premature failure for a number of reasons. Depending on the load applied, the substrate may not be sufficiently strong to resist a small amount of permanent deformation, which would prove too much for a coating to withstand. Coatings again might offer a friction or corrosion resistance advantage, but the substrate might fail owing to subsurface fatigue stresses through repeated contact loads, or possibly cavitation. In both these cases, a nitrided case below the coating has been proven to help.

Nitriding can be carried out simultaneously with some age-hardening processes on certain special steels. I’ve seen cases where such simultaneous treatments have been shown to increase the wear resistance of these special steels without having to resort to multiple processes at different suppliers. This meant that components could be very quickly produced from solution-annealed bar stock and simultaneously aged and nitrided in a short nitriding process. The parts could be machined, heat-treated and delivered for use within a couple of days. Although the steels suitable for such processes are expensive, the fast turnaround of components, combined with a very simple heat treatment, can actually offer very cost-effective components.

Mechanical surface treatments whose aim is to produce residual compressive stresses can be enhanced by being applied to a nitrided surface. The magnitude of the residual stress can be much greater after the shot-peening treatment if the nitriding treatment has increased the surface hardness and strength of the component relative to the non-nitrided material. Racing valve springs are commonly both nitride hardened and subsequently shot-peened.

SAE Paper 2001-01-0834, among others, details one such process and the improvements it can bring for springs of many types. The paper gives an example for a spring, nitrided and then peened twice, whose fatigue strength has been enhanced significantly. The increased allowable stresses made it possible to save more than 50% on the spring mass, although the example given is not a valve spring. However, the allowable stresses for valve springs treated by nitriding and peening are enhanced compared to springs subjected to either nitriding or peening in isolation, and lower-mass valve springs are the result. Low-mass valve springs are valued for allowing more latitude in valve-lift profile design, or for lower friction owing to lower contact stresses between cam and follower.

Written by Wayne Ward

Many engineers are aware of the benefits of controlled shot-peening, but there has been a lack of in-depth understanding of where the benefits come from, and how the process can be altered to suit certain materials and applications. For many years, engineers have been used to specifying shot-peening on critical areas of a component, relying on the fact that the compressive stress is likely to bring a benefit. Very few however have looked into the process in enough depth to have known what is possible, or been in a position to ask for anything other than to have the surfaces peened. This passive attitude towards the application of shot-peening is beginning to be replaced though with a more proactive approach to the process.

The process of shot-peening puts the surface of a component into a state of compression, but the level of compressive stress occurring at the surface can be manipulated to an extent, as can the depth at which a certain level of compressive stress exists. Within limits, a shot-peening supplier may be able to help you achieve something very close to what you want in terms of a compressive stress field. Whatever your level of analytical capability, there is a certain amount you can do to predict the state of stress in a critical section of your component, whether this is via pencil and paper, spreadsheet or finite element analysis. You can also make some assumptions about the compressive stress field that can be achieved and, through superposition of the compressive stress field and the applied stress, arrive at a more realistic state of stress in the component.

So, the compressive stress requirements may not only depend on the material and its level of strength, but also on the loads it is expected to experience. For example, a component whose maximum stress occurs 1 mm below the surface is not going to benefit greatly from a compressive stress in the top 0.1 mm of the surface, but which has no residual compressive stress at the critical depth below the surface.

Some shot-peening companies have predictive tools that allow them to manipulate the compressive stress field to match that which the customer has requested. The desired stress field may require a complex multi-step peening process, with different media and peening intensities used to produce the desired result. In some cases the maximum compressive stress may be a small distance below the surface following initial peening, and a second step may be required to put the outermost surface layers into the greatest compression.

There are practical, though destructive, techniques to verify the magnitude and depth of the compressive stress field resulting from the peening process. Such verification tests can be carried out on relatively simple samples, however, so that expensive parts such as gears or con rods don’t need to be sacrificed.

Written by Wayne Ward

There is however some debate as to the effectiveness of phosphating as an aid to running in; the use of phosphated camshafts and cam followers remains common, so we should investigate this further. Fortunately there are some well-researched technical papers to help guide us. Those by Devlin et al (1) and Chen et al (2) give us some insight into the effect of phosphate coatings on friction and wear. Farrara and Ritter (3) acknowledge the deleterious effect that the hydrogen evolved during phosphating has on the fatigue strength of high-strength steels, so where we are using high-strength steels and are considering phosphating, we need to be wary of the potential disadvantages and how we might mitigate them.

The debate about the effectiveness of phosphating is more to do with the mechanism by which it is achieved rather than whether it has proven effective. In some people’s opinion, the phosphate conversion coating acts as a fine abrasive, effectively lapping the cam and follower surfaces. However, phosphate coatings are also known to have a porous structure that promotes the retention of oil at the component surface. If the component is well-oiled before installation, it is likely that there will be more lubricant present during the initial start-up, and it’s been postulated that it is this lubricant availability at start-up which aids running-in. If there is an extended period between the engine being built and it being started, however, there is a danger that the cams and followers will have corroded, and oxidised surfaces can be abrasive. Phosphating, especially when oiled, is known to inhibit corrosion.

One area where we need to exercise caution is where fatigue properties are concerned, as there is more than one effect to consider. Phosphating can reduce friction, and in an environment where the cyclic stress is affected by friction (for example, conditions of dry sliding or boundary/mixed lubrication), this can reduce the stress amplitude, which should increase fatigue life.

As noted in the previous article, hydrogen is evolved at the component surface during the phosphating process. Hydrogen diffusion into the surface can result in embrittlement and a marked reduction in fatigue life, particularly in the case of high-strength steels. This is the same mechanism that makes electroplating of high-strength steels something to be careful of. There are ‘baking’ processes that mitigate the loss of ductility and strength due to hydrogen embrittlement, but there is no guarantee that hydrogen embrittlement can be completely eliminated.

A further mechanism by which phosphating has been shown to decrease fatigue life is where the acid reagents can attack the metal surface, corroding it and leaving ‘etch pits’ that act as local stress concentrations.

To summarise, one has to choose phosphating for the applications for which it has proven to be of benefit, and to consider the possible fatigue effects on certain materials – especially high-strength steels.

References:

1. Devlin, M.T., et al, “Effect of Phosphate Coatings on Fatigue and Wear”, paper no 722, presented at the National Lubricating Grease Institute’s 74th annual meeting, 2007

2. Chen, Y., “Improvement of Contact Fatigue Strength of  Gears by Tooth Surface Modification Processing”, Proc. 12th World Congress of the International Federation for the Promotion of Mechanism and Machine Science, Besançon (France), June18-21, 2007

3. Farrara, R.A., and Ritter, J.C., “The Effect of Phosphate Coatings on Fatigue Crack Initiation in Quenched and Tempered Low Alloy Steel”, US Army Armament Research Technical Report MRL-TR-90-14, 1990

Written by Wayne Ward

The main materials which are directly subjected to chromate conversion processes are aluminium and magnesium alloys. Magnesium is a very reactive material, and the chromating treatment on these materials is not generally held to give any useful degree of protection against corrosion; it is generally used as a pre-treatment for a further process such as painting. Chromate processes are also used as a post-plating treatment on steels. When you hear people talk about ‘yellow zinc plating’, this is actually a zinc-plated component that has subsequently been chromated.

On aluminium alloys, which aren’t as reactive as magnesium, chromate treatments can offer a degree of protection against corrosion. There are various different processes, but a number of these also go under different trade names. Alodine, Iridite and Alocrom are some common examples, and some of the processes marketed under these names are actually equivalent and thus interchangeable. The chromate processes are often used in conjunction with other surface treatment procedures, with the chromate process forming a base layer on the material for subsequent processes such as painting and powder coating. In a racing powertrain, such duplex processes are rarely required, but the chromate conversion processes are often used alone to provide a degree of corrosion protection for vulnerable parts.

Although chromate conversion processes are generally undertaken commercially, the process is one which can be done relatively easily in any workshop. The chromating chemicals are based on chromic acid. There are a number of ways to carry out the process, with a simple dipping treatment being one of the most common, although there are application methods which suit in-situ processing or surface repair, including brushing on the reagents or using special fibre-tipped ‘pens’ that dispense a pre-mixed chromating chemical.

The application of chromate treatments to plated steel parts is to prevent corrosion of the plating. In this instance it is known as a chromate passivation treatment. As we might guess from the word ‘passivation’, the aim is to make the surface of the plating passive, or less reactive, protecting the surface from atmospheric corrosion.

Zinc is a sacrificial coating, and as we can often see from zinc roofing sheet - which is actually corrugated steel which has been coated with zinc - it soon loses its lustrous sheen as the zinc is attacked. The principle of sacrificial corrosion prevention is that the substrate is not attacked while it is in electrical contact with a more reactive element. Once the sacrificial material has been ‘used up’, the substrate will corrode. So, in preventing the rapid oxidation of the zinc plating, we extend the life of the plating and the substrate.

Written by Wayne Ward

The anodising of titanium is certainly less widespread than anodising aluminium, but it can fulfil an important role in the successful use of some titanium components. Titanium is a strange material, but has quite a lot of desirable properties for certain applications. For example, it is quite strong in tension, has low density, has low modulus and low thermal conductivity.

However, what sets titanium back for many uses is its poor surface behaviour. In any sliding application, the surface of titanium is very quickly damaged. It is an excellent material to use for threaded fasteners, but there is a risk of the surface becoming damaged at extremely low levels of stress, especially when the female thread is also titanium.

This is a behaviour that titanium shares with austenitic stainless steels, and the phenomenon is referred to as 'galling'. Where stress levels are modest, an anodic surface treatment changes the behaviour of the surface enough to allow it to be used successfully. The anodising treatment affects only a very thin layer, and is often combined with further surface treatments which are aimed at providing a low-friction surface. If these further treatments wear or otherwise degrade during installation or use, then the anodising prevents any damage to the titanium substrate. For surface protection purposes, the anodising process takes place in a caustic electrolyte.

Another use of titanium anodising is for decorative purposes. In the decorative anodising of aluminium, the anodic film is colourless, but is quite porous and readily accepts dyes, of which a wide range exists, in hues of varying degrees of tastefulness. Titanium anodising has an infinite number of shades available, but these rely on the phenomenon of interference rather than any chemical dye process following the anodic treatment.

In the same way that steel components change colour at around the 200 C mark and continue to exhibit changing colours until around 340 C, owing to the changing thickness of the oxide layer, the same applies to titanium. Where a coloured decorative film is produced, the anodising process takes place in a weakly acidic electrolyte.

The final colour of the oxide film produced depends on the terminal voltage during processing, and it is the adjustability of this voltage which allows the shade to be 'tuned'. However, the exact shade can't be guaranteed from piece to piece, and while the number of shades is theoretically infinite, the range of colours is not. Decorative anodic films on titanium range from magenta to blue. It is possible to produce the same effect, although with less control, by heating the surface in air.

Written by Wayne Ward

The essence of the process is to dissolve the substrate material using a chemical reagent, such as an acid or alkali. Of course, immersing a workpiece in a strong reactive liquid will simply tend to remove material from all surfaces, so masking the component allows selective material removal, and this is an important aspect of any chemical machining process.

Chemical machining is well suited to the manufacture of very complex geometry on thin sheet-metal parts, although we seldom have much call for such pieces in a race engine. One exception could be the production of bespoke flat filter elements for fluid filtration, which can easily be made by the process of photochemical machining, where precision masking techniques using light-sensitive masking materials give very accurate and fine geometry.

Anyone who studied chemistry at school might recall that the rate of reaction between a liquid and a solid is not only a function of the reagents but also the surface area. A finely divided solid reacts more quickly than a single block, and there is a direct analogy in an engine, where well dispersed fuel sprays burn more quickly. Where this affects chemical machining is in areas with a large or small surface area ratio. External sharp corners have a higher ratio of surface area to material volume compared to an internal corner and therefore material removal will tend to be higher on external corners. Perhaps the best example of a high ratio of surface area to volume is a burr; these will be rapidly removed by chemical machining, and the same is true of surface asperities. So, we can see that one ideal application of chemical material removal is deburring and improving surface finish.

Since the material removal rate can be accurately predicted, chemical machining can be used to reduce the thickness of metallic components. This is sometimes used where there is a limit to what can be fabricated in terms of thin metallic sheet metalwork; the finished fabrication can be chemically machined to produce a component that is thinner than can be manufactured practically.

In a similar way, castings can be reduced in thickness to below the practical limit for the usual production method. Titanium castings are very often chemically machined as a matter of course, owing to the propensity for titanium to react with the moulding material, normally causing the casting to have a brittle case. Such brittle material has poor mechanical properties, and the resulting parts are not suited to cyclic loading. Chemical machining removes this brittle case, leaving the casting with the desired amount of ductility.

Written by Wayne Ward

The surface treatment may be applied generally to all surfaces of the component, as is common with some surface hardening techniques such as nitriding or nitrocarburising. Equally, the surface treatments may be applied locally, only to critical surfaces. In many race engine components, fillets are a weak point. Those with mechanical design textbooks to hand can browse tables and charts to find stress concentration factors for fillets in all manner of components. Books such as Peterson's "Stress Concentration Factors"* even deal with specific cases such as fasteners and crankshafts.

There are a number of mechanical surface treatments that can be used to improve the endurance limit of a component. Not only can we increase fatigue life for a given component by using the correct treatment, we can also optimise the component to be lighter, but still have acceptable service life. Fillets, as mentioned, cause us particular problems. Many components that have failed due to fatigue will have cracks that started in a fillet. Race engine components are usually generously filleted, but space constraints often prevent us from reaching a level of stress in the part that will give us the operating life we want.

Where we have components with critical fillets at the junction of a cylindrical surface and a perpendicular surface, or undercut fillet, we can use fillet rolling to increase fatigue life in these areas of high stress concentration. The process cold-works the surface of the material, plastically deforming it and placing the top layer of the component into a state of compressive stress. When the compressive stress is superimposed on the service stress, the actual state of stress at the surface - where most fatigue cracks begin - is lowered, thereby increasing fatigue life.

Fillet rolling has some advantages over other mechanical or thermal processes in that it is a 'clean' process that can be done in a conventional machine shop, albeit usually in a special machine. By contrast, peening using small media requires a special booth, although many people will send such work out to a specialist company.

The fillet-rolling process is particularly suited to parts such as critical fasteners or crankshafts. Con rod bolts, for example, often have the underhead fillet rolled in racing applications. In a well-designed bolt, this part of the shank is often the point of lowest fatigue life, as it experiences high nominal stresses that are then subject to a high stress concentration factor. When discussing surface treatments with racing fastener manufacturers in the past, a number used fillet rolling for their most highly stressed products.

Reference
Peterson, R.E., "Stress Concentration Factors", Wiley, 1974, ISBN 0-4716-8329-9

Written by Wayne Ward

One fact that we are aware of is that poor surface finish can lead to premature component failure due to fatigue. Machining marks or other blemishes are often found to be the point from which fatigue cracks initiate. Our complex machining means that we are often not able to see all machined surfaces, and we may also be unable to reach them with tools in order to improve surface finish or remedy blemishes. There are some kinds of manufacturing processes where we simply want to remove the top layer of the surface. Cast-in curved pipes and tubes are examples of component geometry where the internal surfaces cannot be reached.

Abrasive flow machining, also known as 'extrude honing', offers the opportunity to improve surface finish in complex internal geometry. As the name suggests, the technique relies on a flow of an abrasive substance through, over or around a surface. As it flows over the surface, the viscous liquid, which contains abrasive particles, removes material preferentially where the ratio of surface area to volume is high. On the visible scale, burrs and sharp edges are removed, while on a much smaller scale, high spots are removed from the surface.

What we end up with is a smoother surface finish. When the surface finish is at a consistent and high level, further material removal will be at an even rate where the flow velocity is even. In practice, the abrasive may flow continuously through a bore (for example) or it may be pumped backwards and forwards. While the technique is most easily applied to bores and other internal geometry, it is possible to use the process on more 'exposed' geometry by constructing tooling to essentially turn the geometry to be processed into an internal surface, forcing the abrasive fluid to flow past the surfaces of concern.

The abrasive media is classified as a liquid, and behaves as such, but it is not necessarily something that is wet to the touch, and some very viscous media might be better described as a putty. The abrasives contained in such media are often oxide or carbide ceramics, such as aluminium oxide or silicon carbide; diamond is used for some applications. The harder abrasive particles, such as boron carbide and diamond, are used for flow machining of hard or difficult-to-machine metals, including tool steels, superalloys and titanium.

It is possible to apply the process to a wide range of geometries, from very small to large parts. It is often used to de-burr and improve the surface finish of spray holes on fuel injectors, for example.

Fig. 1 - Cast intake manifold which has been abrasive flow machined

Written by Wayne Ward

Where there is sliding involved in a loaded non-conformal contact, it is common to find parts with a very fine surface finish. This is not simply for decoration; there are very good reasons for achieving a good finish, and the cam-to-follower contact is an excellent example here. While gears are a non-conformal contact pair, there is a lot of rolling contact in a gear, while the cam-to-follower contact is predominantly one of sliding.

The lubrication of these parts is very important to ensure sufficient service life and to mitigate the effects of friction. The lubrication regime is fundamentally hydrodynamic, and relies on oil being swept into the contact. There are two points during the opening and closing cycle of the valve where the entrainment velocity - a measure of the speed at which oil is swept into the contact - becomes zero. Where entrainment velocities are zero, or close to it, the oil film will tend to become much thinner. We can sometimes see the point at which the entrainment velocity is low by examining the cam lobe, as it can show signs of distress before any other points. If you go through the calculations, which are beyond the scope of this article, you can often correlate this point against real cam lobe damage.

As the oil film becomes very thin, the height of the asperities (high points) on the surfaces of the contacting components becomes significant. At the point where these can begin to touch because the oil film isn't thick enough to keep them apart, friction starts to increase rapidly. In this situation, the lubrication regime is called 'mixed' or 'boundary' lubrication, where the applicable friction coefficient is a function of both the lubricated friction coefficient and the dynamic dry coefficient of friction. As the film thickness ratio - the ratio of oil film thickness to the 'combined' surface finish of the pairs of surfaces in contact - decreases, a greater proportion of the contact area comes into solid contact.

By improving surface finish, the oil film can be much thinner before boundary lubrication applies. This mitigates frictional losses and can prevent wear from occurring.

There are several possible wear mechanisms in such a contact. Even if adhesive or abrasive wear does not apply, subsurface fatigue may become a problem, and another effect of rising friction coefficient is to increase the level of subsurface stress.

This is why it is common to find very highly polished camshafts and followers.

Fig. 1 - Cams are often polished, and there are very good reasons for doing so

Written by Wayne Ward

We should not though expect to see benefits from throwing components into a bucket of something very cold. The details of the process are much more complex than this, and just as problems can arise with poorly executed heat treatment, so we can expect to find problems arising with poor cryogenic processing.

There are various temperatures at which different phenomena occur. For the transformation of retained austenite in steels to martensite, the requirement is to take the material to a temperature below the martensite finish (Mf) point. For most steels, this is somewhere in the region of -70 to -120 C (-94 F to -184 F). Carbon content has a marked effect on the Mf point of a steel, with increasing carbon content leading to lower Mf temperatures. For this purpose, frozen carbon dioxide (dry ice) can be used. Transforming retained austenite into martensite is known to increase material hardness and improve dimensional stability.

Beyond this level of cold, further benefits can be realised, and at lower processing temperatures than can be achieved with carbon dioxide, precipitation of fine carbides in steels has been noted, leading to improved wear resistance.

At what stage in the thermal processing of steel components cryogenic processing takes place depends on the alloy concerned. For maximum transformation of retained austenite, it is recommended to carry out cryogenic processing after quenching. However, some materials are damaged unless tempered immediately after quenching. Tempering is known to render the retained austenite more difficult to transform to martensite. In these circumstances a low-temperature temper to stabilise the material is used before cryogenic treatment.

Owing to the very low temperatures being considered here, the choice of coolant used in the process is between liquefied gases. Common sense dictates that we ignore oxygen, so we will sensibly use nitrogen. Given the proportion of nitrogen present in air, we are not restricted by the availability of the raw material from which the liquid is derived. There are a lot of industrial processes that use liquid nitrogen, and indeed you will find it used in some race engine shops for cooling components prior to shrink-fitting.

The temperature at which liquid nitrogen boils at atmospheric pressure is -196 C (-346 F). Some cryogenic processing companies now offer treatments using liquid helium, which like nitrogen is inert but boils at around -268 C (-452 F), only a few degrees above absolute zero. Liquid helium is more expensive to produce and store than liquid nitrogen.

It is not good practice to immerse expensive engine parts in liquid nitrogen or helium, as the extreme cold can induce some very steep thermal gradients which can lead to distortion, just as immersion into high-temperature salt baths can.

The process will generally see the parts suspended in an insulated cryogenic chamber or supported on racks while liquid nitrogen is introduced to the bottom of the chamber. The result of the evaporation of the nitrogen cools the contents of the chamber, and the rate of evaporation and the flow rate of liquid nitrogen into the chamber control the temperature of the chamber. As with heat treatments, a controlled cryogenic process will often specify a period over which the temperature of the chamber or its contents are reduced or returned to room temperature.

Fig. 1 - High-strength steel components in particular are known to benefit from cryogenic processing

Written by Wayne Ward

The process itself involves the diffusion of carbon into the surface of a low or medium carbon steel. Raising the 'carbon' potential of the carburising medium (which may be a gaseous atmosphere, liquid or even solid) allows carbon to diffuse into the surface. Carburising offers a hard, deep case in combination with a tough substrate. Compared to a process such as nitriding, deep cases may be formed in a relatively short time.

So why don't we carburise more components? There are a number of reasons, most of which are less valid now than a decade or two ago.

One problem conventionally associated with carburising has been distortion. This can come from a number of sources. Two of the main contributors are the stresses that are built up from manufacturing processes being relieved in heat treatment, and careless heat treatment. This second factor is far less likely to occur if you use the services of a reputable heat treatment company that is used to treating components with a mixture of thick and thin sections, slender components and so on.

Another contributor to distortion comes from the process itself. Areas where more carbon has diffused into the surface of the component tend to 'grow' more than the average for the whole component. Even where distortion due to manufacturing stresses was very small, the effect of variability in carbon content and case depth has often meant that grinding is needed. The problem is most pronounced on sharp edges and corners - gear teeth, whose function is impaired by inaccuracies in the profile of the flanks, are particularly affected where the top flank meets the top of the tooth. Conversely, on 'inside' corners of gear teeth roots, there is a lower than average case depth.

A relatively new process, known as low-pressure carburising (LPC) or vacuum carburising, aims to minimise distortion by more careful control of the concentration of carbon in the case layer. In conventional carburising processes, the carbon-rich medium surrounds the parts while they are heating, and because the rate of diffusion depends on the temperature, those parts that heat up quickest start to take on carbon earlier and at a more rapid rate than average, while those that heat up slowest have a depleted case depth. In the LPC process, the carburising medium is a gas that is introduced only when the part is up to an even temperature.

LPC has a number of other benefits: compared to conventional processes, the rate of diffusion is higher, leading to faster processing or a greater case depth. Higher compressive stresses are also said to be possible - the value of compressive stress in improving fatigue life is well known. The depth to which carburising is effective in small bores is also improved.

It is likely that we will see this process used more widely for motorsport engines and transmissions in the years to come.

Fig. 1 - LPC might have been a godsend to the engineers working on the Bristol Hercules engine!

Written by Wayne Ward

The process itself is generally carried out either by immersion or spraying, with the parts being treated using a combination of phosphoric acid and various phosphate salts. There are variations on the coatings, with iron phosphate, zinc phosphate and manganese phosphate being used.

The zinc and manganese variants are most commonly used where improved lubricity is sought. There is the possibility, however, of hydrogen embrittlement through the use of phosphate conversion coatings: the metal-acid reaction liberates hydrogen at the surface, which can be absorbed by the surface, causing it to become brittle and lowering the fatigue limit. The problem is more serious in electroplating processes, and should be borne in mind if specifying phosphate coatings on high-strength steels, which are known to be most affected by hydrogen embrittlement. This hydrogen embrittlement can be minimised by adding an oxidising inhibitor to the solution, which converts any hydrogen to water.

While often specified for corrosion resistance, the phosphate conversion of the surface itself offers only a small improvement, as the phosphate layer is porous. The coating is therefore sealed using oil, which fills in the pores. Both manganese and zinc phosphate treatments are used where corrosion resistance is required.

The more useful property of phosphate treatments to engineers involved in race powertrains is the tendency to improve lubricity and prevent galling during periods of oil starvation, especially at first build and start-up. Again, some of this lubricity comes from the fact that the phosphate coating is sealed with a lubricant, which can be oils or greases.

Where this property of lubricity is required, manganese phosphate coatings are generally specified. The treatment is used on cam followers as an aid to break-in, and is also used on pistons for the same reason. Other notable engine applications are camshafts and piston rings, although gears and cylinder liners have also been treated with success. Owing to the fact that the coatings are porous and hold oil, even as they wear, they present a lubricating surface because there is oil present throughout the structure.

Both manganese phosphate and zinc phosphate treatments can show a range of colours, between black and grey, with the manganese salts showing a generally darker colour.

As mentioned above, one point to be wary of with the phosphating of high-strength steels is the possibility of hydrogen embrittlement. Normally associated with metallic plating processes (most especially with electroplating), this problem occurs where hydrogen is formed at the surface of a steel part. In the case of phosphating, this can happen during 'pre-cleaning' processes, where any oxide layer is removed by acid before phosphating, and during the phosphate process itself. Both involve the interaction of acids and metals; from school we might remember that salts and hydrogen gas are formed. For high-strength steel components, some standards recommend a 'baking' treatment as used with electroplated parts.

Fig. 1 - Phosphating is widely used to treat engine components, including piston rings

Written by Wayne Ward

Aluminium has a number of properties that make it an attractive material for use in race engines. For example, it has low density, and some alloys possess good specific strength (strength divided by density) and high thermal conductivity. However, there are other properties that make it unsuitable for some uses, many of which are concerned with the behaviour of the surface of the material.

Aluminium alloys are reactive, and very easily form an oxide film in air. Over a short period this isn't noticeable, but in the presence of moisture the formation of oxides is accelerated and can quickly become unsightly. One only has to look in the engine bay of a relatively new car to see the effect of using untreated castings.

Another problem is the tendency of aluminium surfaces in sliding contact to 'pick up' at relatively low pressures, especially where two aluminium parts are in contact. Fretting between aluminium parts that are bolted together can also present serious problems. And when connected electrically to a less reactive metal - not necessarily in a circuit, but just a metal-to-metal joint - in the presence of an electrolyte such as water, aluminium will corrode very rapidly.

All these problems are the consequence of 'failings' in the surface of the aluminium component. Fortunately, there are a wide range of surface treatments available based on the principle of controlled oxidation, which can solve (or partly solve) these problems. Lumped together, these treatments are widely known as anodising. However, there are two main variations on this theme, and a number of variants in each group.

To deal swiftly with the two main groups, these are based on the thickness of the oxide film produced. The thin oxide films are generally used to prevent corrosion and pick-up, and can also be dyed to produce attractive colours (depending on your taste for such things). An earlier article dealt briefly with these thin anodic surface treatments.

The thick films, which are commonly 25 microns (0.001 in) or more in thickness, are also very hard (>1000 HV) and can be used as functional surfaces, with useful wear resistance. The hard anodised coatings can also be sealed with solid lubricants such as PTFE to produce low-friction running surfaces. Hard anodised films can be dyed, but only really successfully accept black dyes.

Anodising, particularly hard anodising, is linked to a loss of fatigue strength, but there is no rule of thumb here. The method of anodising, the sealing procedure and the anodised material all have a significant effect on the loss of fatigue strength occurring due to the anodising process. Shot-peening before anodising can mitigate the effect of the anodising itself.

If you are considering hard anodising your race engine components, you would be well advised to consult your anodising supplier. They can advise on the anodising response of the material in question and on the growth you will find due to the hard anodised oxide film produced. They may also be able to advise on the method of anodising that will least affect your component in terms of fatigue, if you choose to anodise the component all over. While mechanical masking of some fatigue-sensitive areas can be expensive, it can prevent any loss of fatigue life while taking advantage of the benefits of hard anodised surface treatments.

Fig. 1 - Hard anodising of piston ring grooves has proven successful in some applications where ring 'sticking' has been a problem

Written by Wayne Ward

The effect of reducing roughness on component reliability is well known, and has been quantified for a range of surface finishes from rough-cast, through machined and ground finishes to 'mirror' polishing. In all these cases, reducing roughness has an attendant increase in endurance limit.

As has been discussed previously, in the article posted in May 2011 on the subject of 'Superfinishing', reduced surface roughness helps to mitigate friction and wear in sliding contacts. In situations where oil films are very thin, the roughness of the surfaces in contact are important in determining if there will be metal-to-metal contact, how much there is and how this affects friction.

Polishing can be a purely mechanical process, either carried out in a purpose-designed machine or semi-manually, as is the case with tape-polishing of crankshafts.

However, many superfinishing processes are chemically accelerated and, as such, their success depends on how chemically reactive the parts being processed are. The processes known as ISF (Isotropic Surface Finishing) are an example of chemically accelerated superfinishing, which combine the action of chemical agents with an automated mechanical surface finishing process such as vibratory polishing.

The chemical part of the process forms a film or 'conversion' layer on the part; this process converts the very top layer of the surface to a soft state that is easily removed by the vibratory finishing process. Any asperities (peaks) have a greater surface area:volume ratio. As the surface area dictates the volume of conversion layer, we can see that this allows the vibratory finishing process to remove material preferentially from peaks. The conversion layer in the 'valleys' in the surface finish is not removed, as the size and form of the polishing media is such that they do not 'fit' in the valleys and can therefore not remove any material. As the process is repeated many times, the surface becomes increasingly smooth.

The final part of the process is called 'burnishing', which removes any remaining conversion coating in the valleys of the surface. This process is undertaken once the initial cycles of surface conversion and conversion layer removal have produced a satisfactory level of finish. After burnishing, the surface is rendered bright and shiny, as we would expect from a highly finished component.

Such isotropic finishing processes find common use in highly loaded contacts where sliding is present. Very popular applications for such processes are gears, where the process has been found to have a beneficial effect in mitigating scuffing and micro-pitting, and also in terms of friction reduction. In this case, the benefits are not only applicable to engine gears, but to transmissions. Racing transmissions have been tested and proven to benefit from such processes (see Reference). The contact between a cam and follower is also highly stressed, and is known to benefit from improved surface finish in terms of reduced friction.

Reference : Niebolo, W.P., "Harnessing the Horsepower Thief", Gear Solutions magazine, August 2006

Fig. 1 - Gears are an excellent application for chemically assisted processes to improve surface finish (Courtesy of REMChem)

Written by Wayne Ward

There are a number of ways of introducing such stresses during manufacture, including specialist machining methods, heat treatment and mechanical treatments. Mechanical treatments work the surface to produce compressive stress, and the main methods used are surface rolling and peening.

Peening is basically working the surface of a component by impact, and hammer peening of very large components is a visible example of what is happening on a very small scale in the shot-peening treatment. Essentially, the surface of a component is mechanically worked into the 'plastic' state through impact. The deformations are very small and the worked region is limited to a layer close to the surface.

Shot peening is a controlled version of shot blasting, where the size, form and hardness of the blasting media is closely controlled, as is the intensity and coverage of the component. It is popular for all manner of race engine components, being widely used in applications from reciprocating parts such as pistons and con rods, valvetrain components, gears and even on areas of castings that show a tendency to fail in fatigue.

The parameters of the shot-peening process are tailored to suit the type and hardness of material, and the particular geometry of the component. As we know from experience, fatigue failures are particularly apt to occur in tight machined radii, and this radius can be the limiting factor in the choice of media; there is no way for a spherical particle 0.5 mm in diameter to contact the surface of an inside corner with a 0.1 mm radius.

There are some more complex shot-peening treatments, called duplex treatments which peen the component, or certain areas of it, at two different intensities, in an attempt to have high stress at the surface and also treat the component to an increased depth below the surface. This is important in components such as gears, where the maximum stresses due to contact occur some way below the surface.

In addition to controlled shot peening, which can be carried out using 'guns' or 'lances' fed with compressed gas, or a machine where centrifugal action propels the shot towards the workpiece, there are various alternatives open to us. Other methods include needle peening, which is a miniature version of hammer peening using handheld equipment, and a rotary method where hard media attached to flexible wires are spun using a handheld tool. Neither of these methods can be considered to be as repeatable as controlled shot peening. Of course, shot blasting is an uncontrolled version of shot peening, and can be carried out using readily available equipment.

Laser peening is a new method that involves no mechanical working of the component with a tool. Compared to shot peening it is very clean, but is also very specialised and comparatively expensive. Widely used in modern aero engines, it is not thought to be widely used in motorsport.

Fig. 1 - Peening is very widely used in race engines to improve fatigue life. Gears are a popular application for duplex peening processes

Written by Wayne Ward

Nitriding is a type of surface hardening that is beloved of many engineers, in the racing world and outside. It is a popular form of surface treatment, not only for its ability to create a hard and wear-resistant surface, but also finds use on components that have no requirement at all for a hard surface. This is due to residual compressive stresses. Any component that is subjected to a cyclic stress which is not purely axial in nature (and few things come into this category of pure axial loading) is likely to benefit from having its surface treated such that compressive residual stresses exist at the surface. Nitriding is one way of achieving this aim on steel components, although not all steels are suitable for nitride hardening.

The nitriding process works best on materials that have additions of nitride-forming elements, most commonly chromium but also aluminium and, less commonly, molybdenum and titanium. There are a number of engineering steels that are suited to nitriding, and others that are specially manufactured with nitriding in mind. Chromium is a strong nitride former, but in steels with large percentages of chromium, special surface pre-treatments are sometimes required to remove a very thin oxide layer that disrupts the efficient diffusion of nitrogen into the surface during the nitriding process.

The process generally takes place in the region of 450-600 C (about 840-1110 F), depending on the type of process. The aim of the nitriding process is to cause nitrogen to diffuse into the outer surface of the material, and this is quite a slow process, with 'case depths' of 1 mm (0.040 in) requiring very lengthy processes that are expensive as a consequence. The process temperatures make it unsuited to steels whose final tempering temperature is lower or equal to the nitriding temperature, as the material core hardness will fall.

The levels of compressive stress that can be achieved with nitriding processes can be large, running into several hundred MPa (well over 100 ksi) at or close to the surface of the nitrided part. Some processes have been optimised to produce the greatest compressive stress, such is the potency of nitriding in combating fatigue failures.

For highly stressed parts with large stress concentrations, nitriding can be one way to achieve a good compromise between low component mass and adequate component life. The most common use in race engines is for crankshafts, where the fortunate combination of wear resistance and improvement of fatigue life are highly valued. It is important to ensure that the nitriding is present at the regions of greatest stress concentration; where crankshafts are ground after nitriding, care needs to be taken that the pre-nitriding surfaces of fillet radii and oil holes are an offset of the final surfaces after grinding, lapping and polishing. Removal of the nitride layer will lead to reduced component life, as has been confirmed many times by race engine manufacturers.

Fig. 1 - Crankshafts take advantage of the wear resistance and residual tensile stress that nitriding offers (Courtesy of Arrow Precision)

Written by Wayne Ward

There are very good reasons though why some parts benefit from having a high level of surface finish. One only has to look at the data presented in engineering textbooks on the subject of fatigue strength and cyclic loading to realise that improving surface finish has a direct and beneficial impact on fatigue life. In the same way that grinding offers an improvement in fatigue strength over conventional machining processes such as milling and turning, polishing offers an improvement on a ground finish.

The polishing process is often described as 'superfinishing', with polishing often considered to be a manual and uncontrolled process, where superfinishing is an automatic process with controllable parameters.

There are other reasons why a very smooth surface finish might be desirable on some engine components, particularly those in lubricated sliding contact. Superfinishing is commonly called for on camshafts to improve wear resistance and to improve the frictional behaviour of the cam-to-follower contact. Owing to the relative motion of the camshaft and follower, an oil film is formed, and the thickness of this film is affected by a number of parameters such as pressure, sliding velocity and certain qualities of the lubricant.

One thing we need to appreciate though is that any surface, however smooth it appears to be visually, has peaks and troughs - high points and low points - and it is the high points of surfaces that are most likely to come into contact with each other and which might therefore cause wear damage. Where the 'combined' height of the peaks on each surface is of a greater thickness than the oil film, metal-to-metal contact ensues, and the lubrication regime is said to be 'mixed', with the friction coefficient being higher than a contact in a fully hydrodynamic or elasto-hydrodynamic regime.

Reducing the height of the peaks, by improving the surface finish through superfinishing, allows us to decrease both the coefficient of friction and the probability of damage being caused to one or both of the components in the contact. Both friction and wear are equally unwelcome in a race engine. If we imagine a cam follower wearing, the surface finish is roughened, and we can understand that this becomes a self-sustaining cycle with accelerated wear and ever-increasing friction.

An automated polishing process is often achieved by placing the parts in a machine containing media in constant motion. This might be anything from a vibratory finishing machine using crushed walnut shell as a polishing media to others where carefully shaped ceramics and polymer media are used, and where the motion of the media might be more carefully controlled. Such ceramic and polymer media might be conical, cylindrical or tetrahedral in form and may be used with an abrasive paste or liquid depending on the application. Examples of race engine parts that are commonly superfinished are crankshafts, camshafts, cam followers, valves, valve springs and gears, although this is by no means an exhaustive list.

Fig. 1 - Crankshafts are one of a number of race engine components that are commonly superfinished (Courtesy of Winberg Crankshafts)

Written by Wayne Ward

All these processes involve raising the temperature of the material more than once. However, there is a process in which the temperature of the material is lowered markedly and again there is an improvement in its properties. Cryogenic treatments are commonly specified as part of the 'harden and temper' treatments for special tool steels. When a steel is raised to high temperature (the austenitising temperature) before quenching, the material is in a form known as 'austenite', and on quenching, this is transformed to another 'phase' called martensite. Tempering improves the ductility of martensite.

However, within many highly alloyed steels, there is an amount of austenite that isn't transformed to martensite. Austenite is, in most cases (with the exception of special austenitic steels), not a stable phase at the temperatures at which we will use the steel and will, over time, transform to other phases. By cryogenic treatment, we can transform nearly all of this 'retained austenite' to martensite. This is only necessary with steels where the proportion of retained austenite is significant.

There is good evidence though to suggest that cryogenic treatments have a number of other benefits, including improving the wear resistance and fatigue behaviour of steels. Metallographic examination of cryogenically treated steels reveals a network of finely dispersed carbides within the steel matrix. The cryogenic treatment is responsible for the precipitation of these carbides, which have been shown to improve wear resistance.

Cryogenic treatments are also known to provide a degree of stress relief; this might seem counter-intuitive when we are used to 'conventional' thermal stress relief treatments being carried out at elevated temperatures. The technique of 'uphill' or reverse quenching is known to provide effective relief of stresses in aluminium castings. In this process, a casting (or other components with unfavourable residual stresses) is cooled using liquid nitrogen - or, more often, the evaporated vapours of liquid nitrogen - and then moved to a steam chamber where the parts are reverse quenched using high-velocity steam.

Cryogenic treatments - whether they are aimed at changing the structure of the material through more complete transformation of austenite to martensite, carbide precipitation or relief of unfavourable stresses - are known to improve fatigue life/strength.

As John Stowe pointed out in his cryogenic treatment article in Race Engine Technology (issue 28, February 2008), there is an increasing acceptance of cryogenic methods in racing, and this is right across the field of motorsport, from 'grassroots' clubmen to the top racing teams. Some people dismiss cryogenic treatments as hocus-pocus or voodoo, but they are generally ignorant of the facts, many of which are widely available in peer-reviewed journals.

Equally, however, there are a lot of unsubstantiated claims for improvements which, when queried, aren't backed up by any data. If these claims aren't based on data, then the basic idea is quite likely to be true - for example, a gain in fatigue life or wear resistance - but you will need to judge the effectiveness for yourselves.

Fig. 1 - Carburised gears are a popular application for cryogenic treatments

Written by Wayne Ward

One point against nitriding though, in all of its forms, is the amount of time taken to carry out the process. Common gas-nitriding processes can take between 24 and 90 hours, with the less common 'super deep' processes taking more than a week in the furnace. Quite often there isn't the requirement for such deep processes, especially on components with smaller sections.

Nitriding can also result in the very top layers becoming friable and abrasive. For components like crankshafts, where it would compromise the function of the part, a post-nitride grind is necessary. For many parts this would make them very expensive, but they still require some form of hard-wearing treatment or some degree of compressive stress.

Nitrocarburising is a process that is suitable for many of the same materials to which nitriding is applied. In common with the nitriding process, nitrogen is diffused into the surface of the component at temperature. Many nitrocarburising treatments also take place at broadly similar, although generally slightly higher, temperatures to nitriding. However, carbon is also diffused into the surface of the component.

There are variations where other elements are also diffused, sulphur being an example. High concentrations of sulphur are generally frowned upon in the overall composition of steel, as the element forms compounds with other elements - manganese sulphide for example - that can lead to a loss of fatigue strength.

Some metals producers deliberately add sulphur to improve the machinability of certain steels. Sulphur added as a surface diffusion treatment is not likely to cause any damage, and indeed such treatments are known for their damage and seizure resistance in highly loaded sliding contacts .

A number of different nitrocarburising treatments are carried out in salt-bath equipment, where the parts being treated are submerged in baths of molten salts from which the elements diffuse. The active ingredient is a cyanate salt; cyanate ions contain one atom each of oxygen, nitrogen and carbon. Gaseous and plasma nitrocarburising processes are also common. Compounds of these three elements are found in the layer immediately below the surface of the nitrocarburised component; the very top surface layer is a form of iron nitride. Because the surface layers of the part are no longer metallic in nature, the tendency for the part to wear when in sliding contact with metal parts, caused by the mechanism of adhesion, is greatly reduced.

Many processes now also include a post-nitrocarburising oxidation treatment. This converts the very top layer to a form of iron oxide (Fe3O4) which, in addition to being an attractive black colour, also gives improved corrosion resistance. This can be useful for race engines as they are often standing for considerable periods of time immediately after build, or at the end of the season if a strip-down is not required. The oxidation treatment can also contribute to wear resistance.

Fig. 1 - Race camshafts are commonly nitrocarburised (Courtesy of Vitesse Engineering Services)

Written by Wayne Ward

I discussed the process with Dr James Curran, principal materials engineer at Keronite. Much of the initial engine work had come from Formula One applications, and the progress being made in expanding the motorsports business has been slowed by the engine development freeze, although applications in homologated components continue. The main commercial progress is now made outside motorsports in industries as diverse as semiconductor manufacture, racing yachts and bicycles.

As for current motorsports engine applications, Dr Curran says the main applications are turbocharger rotors, valvetrain components and the crowns, rings grooves and skirts of racing pistons. Many of these parts are aluminium, reflecting the fact that little magnesium is used currently in motorsports engines. Where magnesium is treated, Dr Curran says that, in general, magnesium parts are subjected to a further treatment such as a polymer coating or other top coat.

I asked Dr Curran what the advantages of the process are compared with a conventional hard-anodising process. He says, "The main advantages stem from the coating composition. Whereas hard-anodising processes form amorphous alumina, the Keronite process converts this to crystalline alumina". Alumina, or aluminium oxide, (a-Al2O3), known as 'corundum' in mineral terms, and is what constitutes sapphire and ruby gemstones.

"This means that hardnesses of between 1500 and 2000 HV are readily achievable with the Keronite process, making aluminium surfaces harder than steels, sand, glass and most other wear counterparts." He went on to talk of the adhesion of the surface treatment and also its strain tolerance, which is helped by the low elastic modulus of only 30 GPa.

The structure of the Keronite layer, the porosity of which is "significantly finer and more complex than that of a hard anodised layer", gives benefits in corrosion protection compared with anodised layers, according to Dr Curran.

It should be noted that both anodising and Keronite produce an aluminium oxide surface layer, and it is well documented that anodising processes have a deleterious effect on the fatigue strength of aluminium alloys, which I asked Dr Curran about. He explains, "There is a fatigue penalty (simply because metal volume is being replaced with a lower toughness ceramic), but the effect is generally smaller for a Keronite layer than for an equivalent thickness of hard-anodised coating. In practice, a much thinner layer of Keronite can usually outperform thick anodising, and the penalty is thus further minimised - typically less than half that of a hard-anodised finish."

I also asked him about the main applications of the process for titanium, a material that can also be anodised. He says titanium is mainly treated to prevent galling, corrosion/oxidation and for electrical insulation, and that many titanium parts thus treated also benefit from a further application of solid-state lubricants.

Fig. 1 - This dramatic picture shows a piston being treated with Keronite (Courtesy Keronite)

Fig. 2 - This surface-treated piston has a coated crown and skirt. Note that the parts can be masked as shown by the untreated ring groove (Courtesy Keronite)

Written by Wayne Ward

With a density of 1.74g/cm3 compared to 2.7g/cm3 for aluminium, it is easy to see why magnesium is an attractive proposition for car and motorcycle makers. Given magnesium's advantages, the reasons why some race series regulations choose to legislate against its use aren't clear.

In the production car world, BMW for example now uses magnesium not only for lightweight covers but for cylinder blocks too, using a very clever production process that sees all surfaces in contact with the coolant being made from a less easily corroded aluminium alloy. The reason for using aluminium is the behaviour of magnesium when a corrosive medium or electrolyte is present. Without protection, magnesium tarnishes and corrodes rapidly.

This is the main reason why, in the aerospace industry, where magnesium is widely used, it is almost always found in the painted condition. This is the most basic (yet one of the most effective) anti-corrosion treatments. The magnesium parts generally undergo a chromate treatment before painting. The treatment can offer a little protection against corrosion, but its main purpose in this instance is to act as an 'undercoat' for the paint.

With processes based on chromium judged to be bad for the environment, other pre-treatments are available. One such passivation treatment (a treatment which makes the surface less prone to corrode) is based on using the natural behaviour of the surface in terms of oxidation, combined with other metal oxides, particularly manganese oxide, to form a corrosion-resistant layer. As with chromate treatment, this process offers some basic corrosion resistance but this is greatly enhanced by painting or sealing. Epoxy resins are an example of a sealing compound often used for this purpose.

In addition to painting, powder coating is another process widely used for protecting magnesium parts, and is much beloved of those restoring magnesium wheels and so on.

Anodising magnesium for corrosion protection and to act as an undercoat for subsequent processes isn't new, with two widely used processes having been available for more than 60 years. However, these processes leave a porous oxide film which is far from perfect when corrosion prevention is concerned, so parts thus coated will generally require painting or sealing.

In recent years there have been a number of arc-assisted anodising processes that allow the build-up of hard and thick anodic films of magnesium oxide on the surface of components. Whereas painting offers an attractive and corrosion-resistant appearance, we can't depend on the dimensional accuracy of the process, nor can we hope to use the surface in a functional sense - we couldn't rely on the paint layer to support any load, for example where we use threaded fasteners.

These plasma electrolytic oxidation processes produce a surface that is far less porous than the older anodising processes. There is an outer layer of porous oxide, but below this there is a far more tightly packed and amorphous layer. The surfaces thus produced are far more corrosion resistant than the older processes, and because they don't necessarily need to be painted or sealed afterwards, they can be used to support load.

Fig. 1 - These magnesium motorcycle wheels need multiple surface treatments to ensure they remain in good condition

Written by Wayne Ward

There are a number of ways to impart beneficial residual stresses, and many of these have been discussed in the pages of Race Engine Technology magazine and here on RET-Monitor. There are surface hardening treatments such as nitriding and carburising, and mechanical processes such as shot-peening and fillet-rolling that can be used to impart these stresses in critical areas.

In some ways, ion implantation is similar to the surface hardening processes, in that it seeks to implant elements into the surface of components, thereby imparting compressive stresses and changing the composition of the surface. While not a temperature-controlled diffusion process, it is possible to implant a wide range of ions into the surface of a given component, and the range of materials to which the process can be applied is also very diverse - to ceramics and polymers, as well as metals.

The ions are implanted into the surface due to the kinetic energy of the ion beam. There is some heating of the surface due to the energy of the ions bombarding the surface. The same equipment used for ion implantation may be also used to apply PVD coatings. High Power Impulse Magnetron Sputtering (HIPIMS) is an example of a process which can be used for ion implantation and coating.

In terms of metallic components, the most common processes, where steel components are concerned, implant nitrogen ions into the surface to a far shallower depth than is common with nitriding or nitrocarburising treatments. In fact the depth to which the ions are implanted is often less than 0.5 microns (0.00002 in).

These ions form nitrides in the immediate surface layer of the component, giving a hard and wear-resistant surface. Nitrogen ions can also be implanted into the surface of high-temperature alloys to improve wear resistance, as well as into titanium, to form a thin layer of finely dispersed titanium nitride, combined with compressive residual stresses.

It is also possible to implant other ions such as chromium, titanium and boron into the surface of steel components, forming compounds within the surface layers of components based on elements not present within the base material composition. For instance, it is possible to form titanium boride (TiB2) within the surface layer of steel components, and chromium ions can be implanted into steels to improve corrosion resistance.

Titanium ions implanted into the surface of an aluminium alloy have been shown to form surface layers of titanium aluminide. Further studies have shown that implanting nitrogen ions into aluminium form aluminium nitride. The result is dramatically improved frictional behaviour and improved resistance to oxidation.

By using inert gases - argon, for example - the same process of bombarding coated components can improve adhesion by 'knocking' part of the coating into the substrate, in a process known as ion mixing. The process has also been shown to improve the tribological behaviour of polymers.

Fig. 1 - Ion implantation is widely used in other areas of engineering to improve wear behaviour. This is a replacement hip joint

Written by Wayne Ward

There are various surface hardening techniques that also serve to stress the surface of parts in compression very effectively. Among those of note and which are used widely for race engines are carburising and nitriding. There are others such as nitrocarburising (also known as tufftriding) and carbonitriding which offer thinner surface hardened layers but which also stress the surface in compression.

There are mechanical treatments as well, such as shot-peening, which are also very effective and which can be applied to a wider range of materials. Peening puts the surface into compression.

Another technique that appears to be extremely effective compared to shot-peening is the 'WPC' treatment, developed in Japan. The process is similar in principle to peening in that it involves the impact of hard media against the surface of the part to be treated. But it is sufficiently different such that the process has been patented and that the results achieved are different; it alters the nature of the surface material, markedly changing the structure (see Fig. 1).

I discussed some aspects of the process with Izumi Ogawa from WPC Treatment Co. He reveals that it is commonly used on pistons and valvetrains in motor racing, as well as in transmissions, where it has proven more effective than conventional shot-peening in some applications. It is also widely used for series-production car engines.

Concerning piston applications, it is not only the skirt surface that is treated but also piston pins and piston rings. In the case of pistons, the benefits of the treatment are increased fatigue life, and improved tribological behaviour. Fig. 2 shows a treated piston.

The commonly treated valvetrain parts are spring retainers and the valves themselves. The benefit of subjecting valves to peening and similar treatments may not solely be down to the fact that the surface is stressed in compression; there is also likely to be some advantage in having disrupted the pattern of machining marks on the valve stem.

In transmission applications, it is common to treat gears where fatigue failures are a problem, and the greater compressive stress the WPC process can provide compared to conventional peening explains the advantage. Clearly, in terms of engine design, we can use this knowledge to our benefit, especially where gear-driven camshafts are used. Providing that the mesh stiffness of the gears remains sufficient, we could employ narrower gears by using this process, thus making the engine lighter and with slightly lower component inertia.

As an illustration of the effectiveness of the process, I was shown some fatigue data for a high-strength steel. The test piece was a notched specimen in a material similar to 4340. The stress run-out for carbonitrided pieces was about 480 MPa at one million cycles. Conventionally peened specimens showed an improvement to 700-900MPa, but the WPC-treated pieces ran out at more than 1150 MPa.

Fig. 1 - The material structure of WPC-treated steel, which was previously carburised, quenched and tempered

Fig. 2 - A WPC-treated racing piston

Written by Wayne Ward

I spoke recently to the owner of Better Than New, in Tennessee, US, the company which carries out the RF85 surface treatment and which claims to reduce dry friction between metal pairs by around 85% (hence the name RF85).

RF85 say people send the whole disassembled engine for treatment, which is a low-temperature immersion process developed in 1999, and parts to be treated are immersed for an hour. While temperature is involved, the processing temperature is said to be lower than the ageing temperature of any of the metals processed.

The result of the process is the diffusion of elements such as calcium and sulphur into the surface of the material, and there is evidence that while the gains aren't so large with all materials, there are significant reductions in friction for most metals including aluminium and titanium. The process is even applied to coated parts with claims of good results, even when applied to DLC-coated components.

In common with some of the coating processes applied to reduce friction and increase wear resistance, the results with this treatment are claimed to be better when both surfaces in sliding contact are coated, with friction reduction of up to 95% in some circumstances.

In addition to a reduction in friction, there are real gains to be had in the longevity of parts - and, if the impressive results found in initial testing in transmission components and engines in the US can be replicated in engines where there is a minimum lifetime requirement (Formula One and MotoGP being obvious examples) then there could be real cost savings.

In transmission applications, this treatment has allowed racers to run with a substantially less oil, and the reduced shear losses mean temperatures are lower so there is a lower cooling requirement on the car.

One area of particular success is in treating interference-fit nested valve springs. Quite often these need to be changed due to wear in the area of contact between the two (or three) springs. Bearing shells are also reported to be a successful application of this coating, with less wear reported on strip-down.

One notable result reported is that engines treated by this process show less degradation of performance over their lifetime than untreated engines.

The treatment is widely applied in racing in the US, with customers in Late Model racing, ARCA and NASCAR all bringing repeat business for this process. Racing is said to be a 'distant third place' though in terms of sales revenues for this process, with other significant applications being cutting tools and firearms.

One 'problem' with the coating is that it can't be detected by eye, so there is no visual difference between coated and uncoated parts. The accompanying picture shows a treated cylinder block assembly.

Fig. 1 - This cylinder block assembly has been treated using the RF85 process (Courtesy of Better Than New)

Written by Wayne Ward

One disadvantage with aluminium, however, is its propensity to oxidise - anyone who has left their motorcycle outside in a British winter, for example, will attest to this. And take a look underneath a relatively new car driven in winter and it will not be a pretty sight once plastic covers are removed. So in this article I will look at anodising aluminium, specifically the thin anodic coatings used primarily to protect against environmental oxidation and provide a decorative coating. I will cover the process known as hard anodising in a later article.

Anodising is a process whereby a component's surface is deliberately oxidised under controlled conditions, converting the surface layer to a film of aluminium oxide (alumina). The aluminium parts are used as an anode in an electric circuit, with an acid bath as the electrolyte. There are variations on this basic process using different acids, but the same principles apply.

The two common anodising types are based on solutions of chromic acid and sulphuric acid. Chromic acid has been in use for longer, but pressure is growing to phase it out because of environmental concerns about the use of hexavalent chromium in industrial processes, and it is felt to be harder to dye these films to produce attractive coloured surfaces.

Anodising using sulphuric acid is a new process, and requires more careful control of processing parameters such as voltage and electrolyte temperature. This type of anodising is more suited to being dyed, and a wide range of colours is available. Because sulphuric acid anodising is not a subject of environmental concerns, more development is going into this process, with the sulphuric acid being combined with other acids to produce better results.

The choice of which type of anodising is used can affect the fatigue performance of the anodised part. While a general rule of thumb is that thicker films will degrade fatigue strength to a greater extent, we need to take account of the type of anodising used.

For example, Augros et al* show that, for 7050 alloy, samples undergoing chromic acid anodising at a thickness of 1-3 microns (1 micron = 0.000039 in) have a lower fatigue strength than identical samples with a sulphuric acid anodised film thickness of 10-12 microns. The authors also present data showing the positive effect on fatigue when citric acid is added to the sulphuric process.

The fatigue effect for different alloys also varies greatly. Compared with untreated alloys, the Augros paper reports a 2.5% loss of fatigue strength on 2214 alloy when anodised using chromic acid and with a 5-6 micron film, but a similar film of chromic acid anodising on 7050 alloy causes a 43% loss of fatigue strength.

When specifying anodising, it is good practice to consult the anodiser, and ensure they know which grade of aluminium is being used. Not all alloys give the same results, and processing parameters may need to be changed depending on the material.

* M. Augros et al: Innovative Cr-free anodizing & sealing processes for corrosion protection of aerospace aluminium alloys, presented at Surfair 2006

Fig. 1 - Hose ends and fittings are commonly finished with a coloured anodic film

Written by Wayne Ward

The author has recently been discussing surface finishing with a specialist in the field that provides a sub-contract service to improve surface finishes on components for various industries and recently exhibited at the Autosport Engineering show in the UK. Jon Porter of First Surface Limited explained that for the same reason that racing engineers are keen on the prospect of increased component life, it isn't universally popular with companies who make consumable parts for other industries. Cutting tools are an obvious example where revenues have been cut because the recent advantages in tool finishing and coating have made the tools last several times longer. Incidentally, using this process, it is claimed that it is possible to polish the thin-film coatings which are applied both to metal-cutting tools and racing components, many of which are only microns thick.

What should be clear to us, concerning any process of surface finish improvement, is that most of these will require some material removal, however minimal this is and that this must be accounted for in the design of the component, just as we would with a component finished by grinding etc. The amount of material removal is proportional to the level of surface finish provided on the component before undergoing the treatment, with finer pre-treatment surface finishes requiring less material removal to bring them to the desired finish. It was suggested in our discussion that the material removal required in taking a typical 0.8Ra turned finish and reducing it to 0.05Ra would be in the region of 5-10 microns per surface.

The surface finish treatment provided by the company above is claimed to be suitable for all types of metallic material and finish, including hardened and coated parts. Cams and followers would seem an obvious application where this might be employed, but given the advantages that we have seen can be brought about by having improved surface finish, it is likely that such processes could be used to advantage in a number of applications in the racing engine. Valves are another application, and the picture here is of a Formula One valve, treated by this process prior to use in an engine.

Fig. 1 - This F1 valve was polished before use in an engine (Courtesy First Surface)

Written by Wayne Ward

In terms of endurance, we know from reading Race Engine Technology, other magazines, learned papers and textbooks but probably more from experience, that poor attention to surface finish can cause a part to break. The textbooks will help us quantify the effect of surface finish on the fatigue strength of materials by giving us de-rating factors which depend on the process by which the surface is prepared and the level of finish that it achieved. We know that a part with a machined finish should be more durable than one which has a cast finish, that a ground finish should be more durable still, and that a component with a polished surface should prove to have the greatest fatigue resistance of all of these surfaces. A polished bar, such as that shown in the accompanying picture, will have a higher fatigue limit than a machined bar of the same material and dimensions. There are, of course, various levels of ground finish, or machined finish, and often the subtleties of the method of finishing are important, such as the alignment of any machining marks compared to the load path in the part. So, in this case, surface finish has a delaying effect in the initiation of surface cracks.

It is not difficult to imagine or demonstrate the effect of surface finish on wear. One only has to imagine the difference between rubbing one's hand against a smooth piece of metal and a cheese-grater or a file to see that surface finish on a smaller scale can cause wear. This wear mechanism is abrasive wear and the ploughing effect of hard asperities (peaks in the small-scale surface finish of the part) is the main mechanism by which material removal is effected. By providing a smooth surface finish we can reduce wear by limiting the ploughing action of asperities, and by finding it easier to establish a protective lubricant film. Wear not only leads to harmful wear particles being carried to parts of the engine which are vulnerable to damage from such debris, but to loss of precision and early failure. Also, if wear debris remains within the contact area it can cause further damage.

In terms of friction, improvements in the surface finish can lead to lower frictional losses and therefore improved engine performance. This is especially true where lubrication is concerned. When the 'combined' surface finish of the two surfaces in contact is less than the established film thickness of the lubricant, then the coefficient of friction in the contact is determined solely by the action of oil shear. When there is no lubricant present, we have the coefficient of dynamic friction acting and at any point in between these two points we have boundary, or mixed lubrication, where the coefficient of friction is some intermediate value. By having a smooth surface, we need establish only a thin oil film in order to minimise friction and prevent damage.

In the next article we shall begin to consider some processes by which we can achieve a really good surface finish and look at some of the possible applications.

Fig. 1 - Polished bar.

Written by Wayne Ward

However, as those of us who have spent a lot of time designing machined components know, the problem of burrs and sharp edges is a flaw in the manufacturing process. As the process of deburring is normally a manual one, there can be great variation in the quality of the process. This can be the result of someone simply missing a small area of the component, it can be that some people just don't like doing the job; it is a tedious one and can be fiddly. As deburring is undertaken with sharp tools, there is a risk that a component can be scrapped by putting a deep scratch on a critical face. There are also areas of components which are impossible to properly inspect for the presence of burrs and also inaccessible and therefore don't allow a deburring tool to be used.

However, there is an automated process which is able to deburr components thoroughly including areas with poor accessibility. Thermal deburring is a very simple process which was developed several decades ago in the USA and is now more widely available. It is used on a large production basis as a way to improve quality, but is very well suited to small scale manufacture and individual pieces. The process involves filling the chamber containing the components with a mixture of flammable gases and allowing these to permeate all areas of the component to be deburred. Once ignited, the combustion takes only milliseconds, during which the gas reaches around 3000 degrees Celsius. The components themselves, owing to their mass, only increase by tens of degrees and so remain safe from damage. However, owing to the geometry of a burr, which is characterised by an extremely high surface area to volume ratio, these increase in temperature extremely quickly to a point where there is a chemical reaction which oxidises them completely, essentially a secondary combustion process. The process is said to be suitable for many types of metallic components, with the exception of magnesium which is extremely flammable.

Fig. 1 - Combustion can be used to deburr machined components.

Written by Wayne Ward

Literature is littered with accounts of similar benefits attributed to carburising which is a process whereby carbon is diffused into the surface of a steel. The same favourable combination of improved fatigue strength due to the compressive residual stresses in the surface of the steel and greatly enhanced wear resistance are present for carburising as for nitriding. There are significant differences in the two processes and the resultant properties of the surfaces, and this means that their applications within racing engines are not very common – for example most crankshafts are nitride hardened, and most gears are carburised, although there are exceptions, even at the very top levels of motorsport, to these ‘rules of thumb’.

Therefore we might expect that other surface treatments where elements are diffused into surface layers might lead to the happy combination of increased fatigue life and wear resistance. Whilst there certainly are treatments where this is true (more on these at a later date), it is not a universal truth, and certain diffusion treatments have been proven to reduce fatigue life whilst improving wear resistance. One such case is boriding or boronizing. This process is similar to pack carburising in that the parts are surrounded by a boron rich substance such as a fine powder or granulated medium (although there are boriding pastes on the market) before being taken to elevated temperature for a time during which the boron diffuses into the outer layer of the steel. The processing of the parts after this is again very similar to carburising.

Experimental research by different teams has been reported i ii which shows a deterioration in fatigue life and strength due to this process. The paper by Celik et al focuses on AISI 1010 steel and reports very high surface hardness levels (up to approximately 1500 HV), but greatly reduced fatigue life and strength. The severe loss of fatigue strength (between 14% and 55%) was attributed to the microstructure of the boride containing surface layers. Examination of these layers found that cracks existed in the borided layer. Part of the fatigue life of a part is used in forming the crack, and after this initiation, failure can be rapid. Effectively this portion of the fatigue life of the part, i.e. that part leading to the formation of the crack does not exist in this case, the part having cracks before service loads are applied. The conclusion that can be drawn from examining the evidence presented in these papers is that steel parts may benefit from the increased surface hardness resulting from boriding, but if the parts are cyclically stressed in service, and that these parts do not enjoy a high factor of safety against fatigue failure, then boriding may be a risky route to achieving greater wear resistance.

Written by Wayne Ward.

The benefits of introducing residual compressive stress into the surface of metal components is well-known and the excellent book on this matter by Almen and Black, is worth reading. The great benefit is the increase in fatigue strength that compressive stress can bring about. Whilst the in-service loads do not change, and therefore stress amplitude is maintained, these are superimposed on a lower mean stress. John Stowe recently wrote on the subject of CSP treatment which is another method of imparting compressive stresses to metallic components.

Laser peening is similar in principle to CSP in that it relies on pressure waves rather than the physical impact of hard particles to produce the stress. Compared to conventional shot peening, laser peening promises a very much deeper layer of compressive stress. Moreover, claims are made that the compressive stress due to laser peening is much less likely to be removed by the effects of temperature due to the lower levels of cold-working of the surface. These impressive claims are backed-up by the use of the process by the aerospace industry on fan-blades for both commercial and military applications.

The process itself is quite simple. A special tape or paint is applied to the area to be peened, and water is run over the surface. When the laser strikes the surface of the tape, high pressure plasma is produced locally, and the effect of the water layer is to contain the plasma, and acts to ‘focus’ the energy in the form of a pressure wave through the tape and into the metal underneath.

The disadvantages of laser peening compared to shot-peening are the cost and the level of complexity. Whilst it has been explained that the process is, technically-speaking, quite simple, its implementation is complex. It requires a programme to be written to precisely control the robots required to manipulate the laser and the workpiece. Owing to the fact that the laser ‘point’ is small and of a set shape (square), the control of where the laser is fired is critical. It is most definitely not a process capable of being applied manually. It is this complexity, allied to the high cost of the equipment and the programming costs that mean it is little used in motor-racing or motorsport in general. However, there are definite applications for this, owing to the fact that it can be precisely targeted. We can imagine that it would be useful for crankshaft fillet radii and oil hole exits as one example.

I’m certainly not aware yet of any applications of this in motor racing currently but, as costs decrease, it is likely to be examined in Formula One when there are some new engine regulations. Having encouraged one Formula One chassis supplier/engine constructor to look at this technique, I visited a UK laser-peening facility with them to review the technology for future applications.

Written by Wayne Ward.

We saw that induction hardening carries significant potential benefits; the main one being that the crankshaft is only heated locally which means that distortion can be effectively managed.

There is also no maximum limit on case depth with induction hardening, which is potentially a benefit in terms of strength but also a potential risk. Any experienced crankshaft designer will be well aware of the risks of allowing a thin section to go through hard as this can lead to premature failure.

The disadvantages are that the process typically requires expensive bespoke tooling and that it only works well in steels with 0.4% carbon, although steels with a lower carbon content can be carburised prior to induction hardening.

This month we are looking into case hardening. Case hardening is normally used on plain carbon steels with no, or few, alloying elements which are therefore not suitable for hardening via the conventional quenching and tempering process.These steels, which might typically have a carbon content between 0.1% and 0.3% C, are also carburised prior to case hardening. Carburising is a diffusion based process wherein a component is packed in a carbon rich environment and subjected to heat over a period of time until the carbon content of the surface layer is sufficiently high for it to be hardenable.

The carburisation process can be as unsophisticated as flame heating the component and then quenching it in oil. The surface layer can then be hardened in the conventional manner, through being held at a given temperature for a given period of time before being quenched, so as to lock in micro-structural changes.

So let’s return to the original question from last month; which heat treatment process was specified for the straight eight 1930s grand prix crankshaft for which the author has modelled and produced a production drawing?

If the reader recalls, the crankshaft could not be nitrided due to the rolling element bearings used on the mains and crankpins, but the answer is that the crankshaft will be induction hardened. Induction hardening is ideally suited for such a relatively long (over one metre) crankshaft, as the potential for distortion in an ammonia furnace is immense.

Not that induction hardening will be a cheap way forward as five different inductors will be required due to the variety of journals to be hardened, but then scrapping a one metre long crankshaft at the heat treatment stage is not a cheap thing to do either!

As with everything in life practice makes perfect and the plan is to produce a test piece which will allow the induction hardeners to conduct trials on all five journal configurations to ensure that the settings are perfect for the actual crankshaft.

Induction hardening is not widely used in racing crankshafts – although practitioners are keen to point out that it is used – but it has a lot of potential benefits so the next time you are specifying heat treatment on a crankshaft it may pay to think twice.

Written by Tom Sharp.

However nitriding is not suitable for all crankshafts, as the author found recently whilst detailing a crankshaft from a 70-year old historic Grand Prix racing engine. The crankshaft in question ran with rolling element main and crankpin bearings, in which the actual rollers ran directly on the journal surfaces of the crankshaft.

Under these circumstances nitriding should not (generally) be specified. Nitriding forms a very hard but relatively thin layer on the journal surface, which can almost be likened to an egg shell. The Hertzian contact stresses from the individual rollers can overload this hard outer skin and cause it to crack and break up. This will lead to breakdown of the whole surface, which coupled with the hard pieces of debris from the nitrided layer will lead to accelerated wear of the journal and eventual bearing failure.

The two surface treatments which are typically recommended for this situation are induction hardening and case hardening. But which one was finally specified for this application? Induction hardening is a non contact process in which a conductive work piece is placed into a strong alternating magnetic field. This magnetic field induces an electric current in the work piece which heats it due to the I2R losses.

The generated current is concentrated in the outer surface layer; the depth of this layer being determined by the frequency of the alternating field, the surface power density, the permeability of the material, the heat time and the diameter of the bar.

The work piece is then quenched in a conventional manner to modify the microstructure of the outer surface to achieve the desired properties.

For induction hardening to operate successfully the carbon content of the steel should be 0.40%; hence 708M40 / EN19 is a favoured material for an induction hardened crankshaft. There are many advantages to induction hardening. When a crank is nitrided the whole crankshaft is heated which leads to distortion, which must be dealt with either by leaving material stock on or by a straightening operation post nitriding. With induction hardening, only the surfaces which actually need to be hardened are heated, and they can be heated in any order (i.e. you don’t have to start at the front and do the journals in order). This means that it is possible to greatly reduced distortion.

The second advantage is speed. A typical gas nitriding process will involve the crankshaft being in the furnace at temperature for 90 hours, after which the straightening operation must then be performed.

To induction harden a journal takes just several minutes depending on various parameters, making it vastly quicker. This could remove a week from the crankshaft lead time.

So why are all cranks not induction hardened? Well, large production runs of tens of thousands of crankshafts often are.

The primary reason why race engine cranks aren’t, is the high cost of the tooling which makes it uneconomical for small batches. Special inductors are produced which will only suit one journal diameter, they are complex and relatively expensive and can cost thousands of pounds.

The secondary reason would be availability. As induction hardening is relatively uncommon it requires more research and setting up to get it done. Most crankshaft manufacturers do not have in-house induction hardening capability and are only asked to do it infrequently.

Next month we will look at case hardening and we will reveal which option was chosen for the historic crankshaft.

Written by Tom Sharp.

Optimising journal surface finish and cylindricity is key to minimising wear and improving fatigue life. The rigid polishing system works via a precision manufactured shoe, or housing, which contacts the entire bearing journal surface to be polished. Abrasive tape is then fed into the housing and around the journal surface. The crankshaft is then rotated to generate the polishing action, whilst the shoes remain static.

The machine is also set to oscillate axially by 1 mm; this gives the finished journal a fine cross hatched pattern; which can be beneficial in terms of oil retention.

The machines can be configured to automatically feed the abrasive tape through but Arrow have found manual feeding to be entirely satisfactory thus far.

There are two main advantages which a rigid polishing machine like this one gives to a manufacturer; firstly, the ability to improve surface finish. Secondly, and perhaps most importantly, it gives them the opportunity to modify the surface profile during polishing.
For example, where customers specify a crowned journal surface, say three to four microns radially over a 40 mm journal length, then producing precision shoes to suit the task can enhance the surface profile during polishing.

Similarly, they can correct any surface profiling errors. Say for example a journal was lobed after the final grind; a standard polishing process, such as conventional nutcrackers, would follow that lobing and exaggerate it, but the process described above can directly improve the cylindricity by polishing any errors out.

Prior to installation of the new machine the company would grind the journal surfaces in the opposite direction to engine rotation, and then polish in the direction of engine rotation using conventional nutcrackers, which only has contact in two places. Polishing in the opposite direction to grinding breaks off ferrite burrs, which can potentially wipe away the oil film and cause bearing failure.

Simon Osman explained to RET Monitor that the machine does not allow standards to slip on final grind;

“The machine won’t compensate for a poor final grind. If the Ra of the component is too high post grinding the polishing machine won’t be able to correct it.

“We don’t use the polishing machine on oil seal diameters as the surface can become so smooth that the sealing lip can’t operate effectively. The end result is an excellent and consistent surface finish and cylindricity.”

From the perspective of the crankshaft designer it is important to be aware of manufacturing capabilities; surface finish can have a important influence on allowable stress levels, and bearing journal profile can be critical for managing oil film thickness along a journal, or for using rolling element bearings.

Written by Tom Sharp.

, and a great deal of effort goes into designing hydrodynamic devices so that cavitation is avoided.

Over the past few years however, a new surface treatment process that takes advantage of cavitation shock waves to produce beneficial surface compressive stress has been developed and refined. This technology is called Cavitation Shotless Peening (CSP). CSP provides the same benefits that conventional shot peening does: the surface of the material is work hardened and “closed up” so as to prevent destructive surface crack propagation. The difference is that, with CSP, the ‘peening’ is infinitely finer, resulting in a far smoother surface than with conventional shot peening. In fact, sometimes there is no visual evidence that the treatment has been applied!

Initial work on this methodology was done using a cavitating water jet on steel forging dies. The researchers found that varying upstream pressure, nozzle diameter, and nozzle distance from the work piece gave them complete control of the process, and applying the surface treatment created a 50% improvement in forging die life. The improvement to the surface of the forging dies was verified by an X-ray diffraction method, which allowed them to measure the compressive residual stress with great accuracy.

It should be emphasized here that CSP is not the same as ultrahigh-pressure water droplet peening; the surface modification is not accomplished by impact per se, but by the tiny local shock waves created from the collapsing bubbles in the fluid stream as it engages the work piece. As the shockwave impinges the surface of the material, it also creates a momentary, local, very high temperature condition, further improving the plasticity of the surface. Experiments have been performed with both the subject component and the jet stream fully submerged as well as with the component and jet stream set up to operate in open air. Work has also been done with cavitating oil jet streams in addition to water; in both cases, significant material strength improvements have been recorded. Tensile tests have shown up to a 50% improvement in yield strength with some aluminium alloys, and similar results have been reported with stainless steels, carbon steels, nonferrous metals, and titanium.

Test reports generally show surface hardness and material strength improvements at least as great as with conventional shot peening, but, at the same time, with a compressive layer that is usually thinner than the latter.

Some of the first dynamic components to be tested were impellers; it is ironic that Cavitation Shotless Peening was developed in part to prevent the destructive affects of …cavitation!

Written by John Stowe.

Grinding is the traditional final metal finishing operation performed on engineered metal-to-metal contact surfaces such as roller bearings and some gears.

It results in a surface with a unidirectional ridged pattern that corresponds to the direction of the final grinding operation. However, as was quickly discovered, grinding with successively finer grinding wheels is expensive, repetitious and ultimately ineffective. It simply results in a surface that has more, closer-spaced rows of shorter height asperities. When placed into operation for the first time, ground components have a minimal area of initial metal-to-metal contact at asperity peaks where contact stress is concentrated.

In the past various ‘rumbling’ processes have been used to further reduce these asperity peaks. These processes use an abrasive medium carried in a water-based liquid all contained in a vibrating bowl. The agitation caused by the vibrations causes the medium to abrade the surface of parts immersed in the mix. This is a relatively non-discriminatory, uncontrolled process that can change the surface geometry, attacking only the surfaces that can be accessed by the medium itself and often leaving critical areas such as corner radii and cavity features untouched. Acceptable, maybe, for a relatively coarse deburring operation, but achieving little else in the way of a super finished surface.

In a novel approach to super finishing, REM Surface Engineering of Brenham, Texas developed and patented a unique process called Isotropic Superfinish or ISF. The ISF process is a chemically accelerated finishing process. It uses the same vibratory finishing equipment as the rumbling process but replaces the abrasive media with a high-density, non-abrasive media that in combination with specific chemicals “weaken” or oxidize the tips of the asperities.

The process is not a polishing process but rather a controlled surface finishing process. The REM procedure involves two steps. The first step, referred to as the ‘Refinement Process’, involves a chemical interaction on the surface of the part. The active chemicals compounds create a thin, oxide film on the parts surface with a pH value of between 1.6 – 5.5. Due to the vibratory action in the work bowl the ceramic media ‘wipes’ off this oxide film. The chemically induced film re-forms only at the peaks that are interacting with the vibratory media, and the process repeats itself. Over time this action removes the peaks leaving only the valleys. These remain intact and serve as a necessary reservoir for the lubricant.

The process continues – with the constant addition of fresh compound – until the required surface finish of Ra 0.02 µm or Rz 0.2 µm is achieved.

The second step is referred to as the ‘Burnish Process’. After the required micro finish is achieved, a mild alkaline mixture is introduced. After a relatively short period a polished, chrome-like finish is produced. In addition to the polishing effects, this step effectively removes all traces of the film formation from the refinement process.

Because asperities have been removed, parts that have been isotropically prepared have an improved, highly random, metal-to-metal contact pattern. The final surface is smoother, and the contact stress in any one location is diffused over a wider area due to the improved contact pattern. Among its many benefits, an ISF surface reduces friction and wear, increases part durability, and improves corrosion resistance. The process has proven applications in many industries including aerospace, automotive, gearing & bearings, medical, military and has become the super finishing process of choice in all areas of the motorsports world.

Written by David Wood.

The treatment is normally distinguishable by the gold-yellow appearance that is typically imparted to aluminum components, and used almost universally on electronic parts, as well as military and aerospace housings and structural applications. Conversion coating is often specified as the substrate for subsequent painting and coating operations, since it acts much like a primer, and improves adhesion to aluminum surfaces greatly. Finally, for applications such as gearboxes, the improvement in emissivity over the bare aluminum may even allow for slightly cooler operating temperatures.In motorsport applications, wrought materials are usually anodized, but in particular, aluminium drivetrain castings frequently are not, and their surfaces are left as cast. This is particularly true of competition at club level, where individual engine builders may view any coating process as an inconvenient waste of time. However, it is the engines at this level that frequently are not even rebuilt once a year (and sometimes not even that often), and if the competition occurs in rally or marine venues, where water, in some cases saltwater, is frequently encountered, the inexpensive protection from a passivation bath should be doubly welcome. The widespread availability, fast cycle time, and again, low cost, make this an easy option.

The emerging problem is that the common and traditional chromate conversions are not compliant with RoHS and ELV directives. In particular, the hexavalent chromium component present in this material is especially offensive in this respect. It is clear that these treatments will be progressively phased out over time.In the last couple of years, however, new passivation treatments for aluminum have been developed. These are now available through many plating firms, and are said to have significant advantages. The new conversion coatings are not affected by baking as with chromate-based passivation, and no curing or ageing is needed prior to painting or coating. The manufacturers of the non-chromate treatment claim that it performs just as well as traditional conversion coatings, and that it meets the requirements of MIL-C-5541 electrical conductivity, and in ASTM B 117 testing its corrosion resistance has been shown to be comparable to chromate conversion. A wide range of concentration percentages, temperatures, and immersion times can be tailored to a user’s specific needs. Most manufacturers also offer a ‘paint brush’ type repair kit for scratches or other injuries to the treated surface.Conversion coatings are typically applied in a four-step process: initial wash, etch, passivation dip, and rinse, which can be accomplished in four moderate-sized polypropylene tanks. The elimination of the chromium materials makes prototype and low-use ‘convenience’ setups viable for individual companies as the removal of hazardous waste is greatly simplified, with much lower costs. Local environmental and waste officials should be contacted regarding these considerations before setting up a passivation station in-house.