The idea behind them is a very attractive one. For every unit of fuel burnt in the engine, less heat escapes directly to the components next to the combustion chamber, so there is a reduced cooling requirement. This reduction is very attractive to engineers in charge of cooling systems and aerodynamics, as lower heat rejection is associated with smaller coolers, lower cooling air flows and reduced aerodynamic drag. It also means more energy is retained in the combustion chamber, where it can be used for producing extra torque. Good news all round, or so you may think.

There are two problems though that need to be overcome before the coatings become common in race engines. One is the issue of longevity. The aim will be to take maximum advantage of such coatings, and the reduced heat transfer will naturally lead design engineers to redesign components to work at lower temperatures, but if the coating fails and heat transfer is locally increased, it will cause component failure. People who stand to gain the most from the technology also need to be convinced the most of the reliability of the coatings. The coating process also needs to be carried out at a temperature that does not harm the component – especially important for pistons which are very highly stressed but which can stand only relatively modest temperatures before their mechanical properties are degraded.

The other problem is the management of heat and temperature. If volumetric efficiency is to be maintained, so that the amount of air drawn into the engine is not adversely affected by increased component wall temperatures, the coatings’ surfaces need to be able to cool quickly. If the effect of the coating is to increase component surface temperatures such that volumetric efficiency is impaired, the result could be a reduction in performance. To present an overall advantage in terms of performance, any increased fuel conversion efficiency – that is, the ratio of mechanical energy extracted divided by the chemical energy in the fuel – needs to outweigh any loss in volumetric efficiency.

Having worked with a group of engineers who had used low thermal conductivity pistons for extensive trials and found large gains in fuel conversion efficiency, combined with large losses in volumetric efficiency, this can be a real concern. The conductivity of the coating and its thickness need to be managed to give an overall advantage in terms of performance for most racing applications. However, with racing increasingly rewarding fuel efficiency, it might be that thermal barriers which lead to an overall reduction in performance but do this while achieving increased fuel economy may prove useful in certain types of racing, such as categories where a loss of 5% of performance might be tolerated if the car can go one more lap before entering the pits for refuelling.

Written by Wayne Ward

I have had some involvement in motorsport hybrid systems over the past 15 years, some of it on the Panoz Q9, some of it on the KERS systems in Formula One, and some on the early development phases of the current Formula One energy recovery systems. The march of technology has been relentless and impressive, with each iteration showing an increase in performance or a decrease in size; it’s just a shame that those ‘outside the box’ aren’t able to get an appreciation for the dramatic progress which has taken place. As with personal computing, for example, hybrid systems benefit from the miniaturisation and tighter packaging of components.

The problem with the increasingly tight packaging of high-voltage components though comes when we have components working at different voltages getting too close together. The effects of creepage and clearance have been discussed previously in respect of fasteners, but there are other components that absolutely must be metallic if they are to function correctly: one example here might be shields to prevent the effects of electromagnetic interference. In the same way that the ‘Faraday cage’ effect that shields the occupants of a car from lightning, EMC (electro-magnetic compatibility) shielding protects electronics from the effects of interference which might, for instance, be the result of having electronics packaged close to powerful electric motors, where fields are being created and then collapsing rapidly as the motor is used to generate torque or harvest energy.

Plasma and thermal spray processes can be used to help create effective shields. One way to do this is to spray an electrically conducting shield (commonly sheet metal) with an insulating material. The insulation means the shield can then be placed very close to conductors without providing a path to earth or between two other components working at different voltages.

Another approach, which can work out much lighter, is to use an electrical insulator as the substrate for a shield, with the shield being formed by a sprayed coating of a metal – typically something like aluminium, which itself can be over-sprayed with an insulating ceramic or polymer. In order to provide the necessary shielding, very little thickness of aluminium is required, and the substrate can easily be made stiff, especially if it is a moulded composite component. If we try to produce the same effect with a metallic shield, we need to resort to expensive press tooling to form a stiff shape, whereas a simple machined mould for a composite tool can be made pretty quickly these days.

It is possible to incorporate EMC shielding based on thermal/plasma sprayed coatings into composite outer cases rather than using separate shields. Again, a thin metallic coating is sprayed directly onto the casing, and this may or may not be over-sprayed with an insulator.

Written by Wayne Ward

A large part of the revolution in economy and efficiency is energy recovery, and most car companies and race teams have chosen the electric hybrid route. Also, electric racing is now with us, with some big names – teams as well as drivers – signed up to Formula E.

High-powered electric motorsport might prove exciting to watch, if not to listen to. However, for engineers there are challenges with moving from designing complex engine parts to assemblies with very few moving parts. Many adjacent components need to be separated by large gaps or electrical insulation. The problem is possibly most acute in the propulsion motors. The individual turns of the coils in the motor windings need to be separated from each other electrically; the same applies to the coils in each phase, and all the windings need to be insulated from the stator ‘iron’ and the motor’s casing.

In motorsport we want everything to be compact, so we try to make the windings small. We pull the windings around relatively sharp corners, and during winding the coils rub hard against each other, so it is very easy for the insulation on the winding wire to become damaged. A lot of electrical winding wire is coated in a polymer, and several layers of insulating coating may be used. There is a compromise here: more insulation is more robust and reliable, but it also increases the size of the bundle of conductors. There may also be additional insulation applied after winding, in the form of transformer varnish or lacquer.

Elsewhere in the stator of the electric motor, it is possible that insulating ceramic or polymer coatings could be applied to other components. The problem with coatings compared to much thicker separate insulating components is that coatings are far more easily damaged, and at the end of winding a motor a minor short can mean that the whole motor is scrap. The skill of the people winding the motor and assembling the stator into the case is as much a factor here as the integrity of the coating in many cases. The designer also needs to consider the lack of damage tolerance in any coating selected, and balance this against his or her desire to make everything as small and light as possible.

Written by Wayne Ward

Metallic coatings of any description have struggled to be newsworthy though, as they are unable to come close to the new engineering coatings in terms of hardness, friction or wear resistance. However, there is a wide range of soft metallic coatings that remain popular and to which the new hard coatings pose no threat. In this article we will look at some uses of these coatings.

Gold plating is something we might associate with cheap jewellery or cutlery that is more for show than use. However, it is used in motor racing, especially in electrical contacts, where its very low strength and stiffness allow it to conform under low loads. Gold-plated components brought into contact with each other quickly deform under light pressure to give large contact areas. This leads to a lower contact resistance and therefore an increase in efficiency. Gold-plated connectors are used for a range of purposes, from electrical power transmission to sensor wiring and earth terminals.

Silver plating, which is again something we might normally associate with jewellery or trinkets, is widely used on fasteners in industry. Such coatings are not the shiny, chemically brightened type used for decorative products but are quite dull, which may or may not disappoint you depending on whether you like your racing powertrains to look like they were made by Harley-Davidson.

Silver-plated fasteners are used widely to prevent seizure, either due to material compatibility problems (stainless bolts installed in stainless tapped holes are a particular problem) or where thread lubricants can’t stand the high temperatures in service. In this latter case, we find silver-plated fasteners used on exhaust manifolds, especially on boosted engines. A number of suppliers offer silver-plated nuts in various styles for this purpose.

The most common use of soft metal plating is on plain bearings. The coatings on bearing shells may consist of one or more extremely soft coatings, commonly based on lead. These very low shear-strength coatings allow the engine to run with little damage during start-up, and they are used in most forms of motorsport for crankshaft main bearings, con rod big-end bearings and sometimes for camshaft bearings. Their combination of coating thickness and softness allows hard particles to become embedded in them, preventing the particles from going any further and causing damage. This ‘embeddability’ can prove crucial in terms of engine durability, preventing critical bearing surfaces from becoming scored.

The cages of rolling element bearings are also often coated with soft metal plating. Copper is widely used for needle roller bearing cages, and silver is also found in this application too. Such bearings are often used to support the gears in cam drives. Silver plating is sometimes found on valve collets as well, helping them to conform to the valve and retainer geometry.

Written by Wayne Ward

DLC is not a single coating but a family of coatings based on carbon, which has a proportion of bonds that are the same as that found in diamond. The proportion of diamond-like bonds in the coating depends on the specific process; there are also wide variations in the percentage of hydrogen in the coatings, and there are other elements which are added in order to tailor a coating’s properties.

The application of DLC to aluminium automatically reduces the number of coatings that can be considered, as many of the processes are carried out at temperatures that would cause a significant loss of strength and durability in the aluminium itself. From this smaller pool of coatings applied at low temperatures, there are others that are unsuited for use with automotive lubricants and some that suffer from fundamentally poor adhesion. Furthermore, the ‘pre-processing’ of the parts in terms of surface preparation is critical. There are only a few companies manufacturing pistons who apply a DLC coating to them and, as a consequence of the difficulties in getting the coating to work properly, only a few companies to whom the task of coating the components is entrusted.

There are fundamental differences in the design of a piston where a DLC coating is to be specified, and the machining of the skirt and lands is also markedly different, owing to the hardness of the coating. All DLC coatings are very hard and wear-resistant; they are widely used for coating metal-cutting tools, and if the piston is not to become an effective metal-cutting tool within the engine, its surface topology needs to be considered carefully. Those who have managed to develop DLC coatings on pistons have arrived at their chosen surface topology via a great deal of testing, which probably involved a lot of mechanical damage along the way.

The piston skirt and land profiles need to be better optimised than for an uncoated piston or a piston with a softer coating. In the case of the soft-coated or uncoated piston skirt, there is generally some expectation of minor wear or at least some minor displacement of material, but this process of the skirt ‘wearing in’ during use will not happen with a hard coating such as DLC, so any excessive contact pressure is likely to result in wear of the adjacent part.

For a number of years DLC-coated pistons were used only on very high budget engines, and were not available commercially. That has now changed, however, and at least one piston manufacturer will supply DLC-coated pistons to its customers. There is though a significant cost penalty to the use of such pistons, but with reduced frictional losses and increased durability as the advantages, there are a lot of racing customers who would take the opportunity to use such pistons.

Written by Wayne Ward

In motorsport engines, superalloys have found a great many uses, both for elevated temperature use and many applications at much more modest temperatures, where their ability to be processed during manufacture at increased temperature can give them an advantage over more conventional high-strength steels.

However, superalloys on their own have not been wholly responsible for the increase in surface temperatures in aero engines. The increase in the maximum surface temperature has been much greater than that allowed by improved materials, and the remainder has been possible due to some very clever internal cooling and coatings.

The three high-temperature applications of superalloys in race engines are exhaust valves, exhaust systems and turbocharger components. For most of these race engine components, it would not possible to use the same cooling strategies that the aero engine makers use.

However, we could use the coatings that find success on gas turbines, especially thermal barrier coatings. These are plasma sprayed, but the longevity of the components lies in the oxidation-resistant CVD bond coat applied to the component before the actual thermal barrier. Thermal barriers are generally composed of zirconium oxide, normally called zirconia, but zirconia is almost ‘transparent’ as far as oxygen is concerned, and at elevated temperatures oxidation of superalloys can be a serious problem.

The CVD bond coat is impervious to and does not react with oxygen. The bond coats are generally aluminium-based coatings, where the aim is to produce an intermetallic compound with the substrate; most nickel alloys contain a lot of nickel, and NiAl (nickel aluminide) is highly resistant to oxidation. It may be necessary to apply further thermal treatments to fully form the aluminide coating, as it relies on diffusion (a process whose rate is controlled by temperature) of aluminium and nickel.

The actual composition of the coating, namely other elements added to the aluminium, controls how reactive it is. ‘Low-activity’ coatings rely on the outward diffusion of nickel into the coating, while high-activity coatings rely on the inward diffusion of aluminium into the substrate. After the diffusion stage the coating is extremely tenacious; it is no longer a discrete layer on the component, but is now an integral part of it.

The only problem with the CVD coating of superalloys is that the composition of the coating is difficult to control, as the various elements deposit at different rates.

A successful thermal barrier coating applied to components such as turbine wheels could allow them to run cooler for a given gas temperature, improving reliability or allowing the designer to produce a component that is lighter/has lower inertia. A lower-mass exhaust valve might be very much appreciated by development engineers in Formula One wishing to explore new valve lift curves.

Written by Wayne Ward

As with many of the processes used to make a successful and highly optimised component for a race engine or transmission, there are a number of prerequisites to the application of a coating if it is to prove as effective as the customer might expect. One important aspect to consider is the preparation of the parts before coating, especially cleanliness. The difference between a component whose coating is a success and one whose nominally identical coating is unsuccessful may be down to identical preparation processes being carried out in different environments. It is not only the state of the as-machined surfaces that is important, but also the issue of surface contamination.

For the most demanding applications, some component suppliers and coatings companies prepare the parts before coating in a ‘clean room’. These are not simply rooms where no ‘dirty’ processes are carried out – the fact that you set aside a room where no grinding or fettling takes place does not make it a clean room as such – it simply means it is not as dirty as the rest of the factory.

Proper clean rooms are supplied with filtered air, and the rating of the clean room depends on the amount of particles of a certain size there are per unit volume of air. Typically ‘room air’ in an urban environment has around 35 million particles of 0.5 microns or greater per cubic metre. The first level of clean room has 10 times fewer particles of this size. Some of the standards governing the cleanest clean rooms also place maximum values on the numbers of particles down to 0.1 microns.

Clean rooms are entered by an air lock in order to control air quality, and the people who work in them wear gowns, face masks and hair nets. As a reference to how ‘dirty’ it is to have people in clean rooms, a typical person generates 100,000 particles greater than 0.3 microns in size per minute when motionless. When walking at 2 mph (3 kph), that number rises to 5 million.

Components prepared for coating in a clean room will have less surface contamination in terms of particulates on the surface, as such particles can prevent good adhesion of the coating. In many circumstances, there would be no problem with the ‘usual’ level of cleanliness, but where components operate under very demanding conditions of high instantaneous temperatures and high contact stresses, or under marginal lubrication, high levels of cleanliness improve the performance of the coatings. In race engines, such components might include valvetrain components such as valves and finger followers.

Written by Wayne Ward

However, some applications are sufficiently lightly loaded to allow components to be directly coated. Con rods present two such applications. On crank-guided rods – those whose axial position is controlled by thrust faces on the crankshaft – the big-end thrust faces are commonly coated. Traditionally this was done by thermal spraying using metallic molybdenum as the coating material. It is still used, but has been supplanted to a large extent by hard, thin engineering coatings such as chromium nitride.

At the opposite end of the rod, where piston-guided rods are used (rods whose axial position is controlled by thrust faces provided on the piston) the small-end bushes often incorporate flanges that provide the thrust faces. Some people use hard engineering coatings on these bearing faces. Of course, the machined faces of the small end can also be coated.

In oil pumps, some people have found success running shafts directly in coated bores in the pump housings. The housings are often made from cast aluminium, and the machined bores are coated with a thin layer of a relatively soft resin-bonded polymer coating. Such coatings can be very durable if applied and cured properly; this is an important aspect to consider, as the temptation with aluminium housings is to cure the coatings at a temperature that is too low to fully harden the coating.

Using polymer coatings in pumps is possible owing to a number of factors that we don’t find elsewhere in the engine. There is a good supply of lubricant, temperatures are fairly steady and the sliding speed is modest owing to the rotational speed of the pumps and the small diameter of the pump shafts. It is not unusual to find that the pump shaft surface speed is a factor of ten lower than the main bearing surface speed.

Care needs to be taken though when deciding on the position of any oil-feed grooves crossing such bearings. If a soft coating is used on a relatively soft housing then any mistake in the angular position of these groove can lead to high pressures, coating wear and excessive clearances. Unless mistakes have been made in the design or manufacture of the components involved, the main period when wear will take place is during start-up, where the components are essentially running dry until sufficient oil is available.

For more highly loaded components, one option is to apply metallic bearing coatings directly to components. This has been tried in the past with con rods, and it is technically feasible with cylinder blocks and so on, but it is very difficult in practice. It would also require an improvement in the precision with which we are able to machine both the coated components and the components that run in them.

Written by Wayne Ward

Ask any engineer who has worked with the very best cars and drivers what factors are most potent in terms of race performance, and almost all will put the driver further towards the top of his or her list than the engine. As someone who has spent most of his career as a racing powertrain design engineer, it slightly pains me to write this. It should therefore be a priority to look after the driver and, as far as is reasonably possible without costing overall performance, we should consider his or her comfort during the race.

Drivers of closed-cockpit cars are particularly vulnerable to fatigue when the cabin temperature is high. The problem can be so bad that some series mandate the use of driver cooling if the temperature exceeds a certain limit. Even in the relatively mild British summer, driving in the day with no air-conditioning system can sometimes be uncomfortable, even in a T-shirt and shorts. I dread to think what it must be like at Le Mans in a very much hotter car with full fire-proofs and a helmet.

The way to minimise the penalty of having to carry an air-conditioning system on a car is to minimise the amount of heat that needs to be removed from the driver compartment, and this is done by limiting the amount of heat that is transferred to it. By shielding the driver compartment from sources of conducted or radiated heat, we can keep the warm areas of the car warm, and the driver a little cooler. Thermal barrier coatings can be used in this situation and can be applied directly to hot components such as exhausts, heat-shields or onto the chassis on the outer surfaces of the driver compartment.

This includes the obvious areas such as the engine compartment bulkheads and exhaust tunnels, but also the less obvious ones, such as where the transmission cases are close to the driver compartment or where coolant pipes are routed along the underside of the chassis or through the sills. If it is not practical to coat the chassis directly or to fit insulated hard heat shields, thermal barrier coatings have been successfully applied to flexible foils that can easily be formed to fit into tunnels and so on or around certain components.

Where the inevitable compromise lies with the application of thermal barriers to closed-cockpit cars isn’t obvious – we aren’t looking at one-lap performance because adding mass to a car is rarely a cause of lap time reduction. The real benefit comes as the driver tires; the driver can give a better performance for a longer period if he or she feels fresh, comfortable and properly hydrated.

Written by Wayne Ward

The heat transfer from the valve to the water jacket (or cooling air) is improved greatly by bringing the valve and the water jacket (or cooling air flow) closer together. If the head itself were able to fulfil the requirements of deformation and wear resistance, race engine design and development engineers might be pleased to dispense with the valve seat inserts. Not only do the components offer a thermal resistance, so does any contact between components, and in order to get really good contact between components that are forced together, we need to approach the yield stress of the weaker material. As the head is usually the weaker out of the head and valve seat combination, we would have the head stressed in tension close to or beyond its yield point. So, in order to guarantee reliability, the head-to-seat contact offers more thermal resistance than it ideally would.

It is in this context, and for more reasons besides, that a sprayed valve seat looks very attractive. The sprayed seat can be very thin, and it requires no machined recesses with sharp internal corners into which the seats fit. The internal machined corners act as a significant stress concentration from which fatigue cracks can initiate. The distance between the valve-to-seat contact area and the water jacket can be minimised, and the high thermal resistance of the contact between head and valve is also minimised as there is a metallic bond between the seat material and the valve. Furthermore, as an interference fit is eliminated along with the consequential tensile stresses in the cylinder head, the fatigue life of the head is improved.

One of the limiting factors in the proximity of valves to each other in four-valve chambers is the stress in the area between adjacent valves. With a large part of the tensile stress eliminated, the designer might choose to use the lower stress to reposition and maybe resize the valves. Whatever he or she chooses, sprayed valve seats offer great latitude in design and development, or greater reliability.

The disadvantage of the sprayed valve seat though is manufacturing complexity and the cost for small production quantities. Their use has been a common practice for some time in production engines, and heads manufactured with thermal-sprayed seats are economical when produced in large quantities. However, for a race engine the cost can only be justified if the engineering reasons are good and the rewards for success are great. We should not be surprised to find that it is not widely used in racing at present, and those that have used it have generally been engine suppliers run by large car manufacturers – that is, those involved in Formula One. Even with their large budgets, this technique is not used universally.

Written by Wayne Ward

What will generally be seen where fretting has taken place is some roughening of the surface, and in many cases there will also be some oxidation of the fretted surface. In severe cases there can be a significant transfer of material between one surface and the other. The worst case I have seen was between two steel components on a dyno mount, where large chunks of material were transferred from an engine mount bush to the mating part on the dyno over the course of only a few dyno runs.

Between aluminium engine components, even where only one part is aluminium, the result can be anything from roughening of the surface to severe oxidation and material transfer. Due to the roughening of the surface, the stress concentration factor is increased, and a crack can very easily start from a fretted area on an aluminium component, whether this is machined from a casting or wrought bar.

There are a number of solutions to the problem of fretting. Material substitution is one which, but it is often impossible simply to swap an aluminium casting for one of another type of material; a change to a subtly different alloy will almost certainly not make the problem go away. Surface treatments can often make a difference though. In steels this can often be a good solution, but for aluminium the option to hard-anodise locally can be very expensive and time-consuming. Hard anodising is also known to reduce the fatigue strength of aluminium, so we have to be careful that we don’t swap one fatigue problem for another.

What we can do is apply a thin film of another material to the surface of one of the components (or both). There are various options for this, but polymer coatings can be used  quite effectively to prevent or reduce the problem. While metallic plating might seem to be the more obvious option, there are some drawbacks with this, especially the fact that it is very difficult indeed to mask complex parts so that only the required surfaces are plated. Many polymer coatings are sprayed and some very simple masking can be used to restrict the coating to the areas of the component to which it needs to be applied. While polymer coatings do not gave very high strength, the fact that there are different materials in contact can prevent fretting, and even if the coating wears, it will have been effective in delaying its onset.

Written by Wayne Ward

Cylinder bores were once commonly chromium plated, although this has been largely supplanted by a composite electroless nickel/ceramic coating or by one of a number of thermal spray coatings. There are though some specialist processes used for chromium plating of cylinder bores in motorsport.

Fasteners are one component that we can often find with metallic plating. This is commonly for corrosion resistance, but we can often find fasteners that have been plated to modify the friction coefficient or provide some lubricity in high-temperature applications where a grease will simply not withstand the heat applied. Cadmium and silver are commonly applied to high-temperature nuts for this reason.

Running surfaces on shafts are commonly plated with hard chromium, which can subsequently be ground to the correct size. This means that a thick, hard layer can be applied to the surface of a shaft that is not necessarily as hard as the seal or bearing requires. This is also often used as a repair scheme for damaged shafts that have been worn by seals.

Case-hardened components are often subject to a copper-plating process where areas need to be masked to prevent them becoming hardened. Such areas might require further machining, have features that require some toughness, or thin areas that might otherwise be ‘through-carburised’, rendering them brittle and prone to overload failure.

With the advent of hybrid systems and their growing use in motorsport, metallic platings have found another area of use in the racing powertrain. Again they play a number of different roles here in electrical and electronic applications. Soft metallic platings, commonly silver and gold, are used in order to provide a layer that conforms to provide good electrical contact, even under modest loads. For a typical metallic joint, the two counterparts will only reach 100% apparent contact when the yield strength of the weaker surface has been reached. This can be difficult to achieve over anything more than a very small area. However, if we use a very low-strength coating on the surface of a metallic part requiring good electrical contact, we can get much better contact than with unplated parts. This leads to lower contact resistance and thus lower electrical losses through ohmic heating.

Where contacts in electrical connections require a high-strength surface, especially where some sliding is involved in their engagement, a hard metallic plating may be applied to an otherwise low-strength material such as copper. The copper is selected for its electrical conductivity, but this may not prove durable enough for repeated connection/disconnection.

Written by Wayne Ward

There are two types of bonds in carbon coatings, known as sp2 and sp3 bonds. Carbon composed solely of sp2 bonds is what we would called graphite; carbon composed of sp3 bonds is what we know as diamond. Graphite is soft and lubricious, whereas diamond is one of the hardest substances known. Commercially available DLC coatings have a mixture of sp2 and sp3 bonds, and control over the ratio of these has a strong effect on the properties of the coating. You may see some of them referred to as a-C coatings, which stands for amorphous carbon.

Many coatings are not solely carbon, but have a large proportion of hydrogen, as high as 50% in some cases. You will often see these referred to as a-C:H, which signifies that they are amorphous hydrogenated carbon coatings. If you see ta-C:H, the ‘t’ signifies tetrahedral, showing that the carbon bonding is predominantly tetrahedral sp3 bonds.

It is also common to tailor the coatings’ composition and properties further by ‘doping’ with metals or ceramics, and there are at least a dozen metallic doping agents in use. These agents modify various factors, from wear and friction to coating adhesion, and can play an important role in making the coating suitable for use in certain environments. For example, the coefficient of friction for many DLC coatings is very sensitive to relative humidity, and some of the doped coatings make the coating less sensitive in this respect. A Czech study (1) of both ‘plain’ and hydrogenated zirconium-doped DLC coatings showed a marked difference in frictional behaviour, with the doped coatings having a coefficient of friction about 50% lower than their undoped counterparts when running against an unlubricated steel ball.

With such a huge number of available coatings, the difficulty in making the correct choice is not easy, and is further complicated by the fact that the coatings, when used in an engine or transmission, are generally used in an oil-lubricated environment. A specific study (2) into the application of DLC coatings in automotive applications found that different DLC formulations can affect the interaction of lubricant additives with any uncoated parts in a sliding contact. One common example of this might be the use of an uncoated camshaft with a DLC-coated cam follower. The way each coating affected the wear behaviour was not constant, but also depended largely on the composition of the lubricant, in particular detergent and anti-wear additives.

References

1. Vitu, T., Pimentel, B., Escudeiro, A., Cavaleiro, A., and Polcar, T., “Zr-DLC coatings – analysis of the friction and wear mechanisms”

2. Renondeau, H., Papke, B., Pozebanchukz, M., and Parthasarathy, P., “Tribological properties of diamond-like carbon coatings in lubricated automotive applications”, Proc. of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 2009

Written by Wayne Ward

Two groups of coatings then came along that offered a good combination of low friction and wear resistance – PVD (physical vapour deposition) and CVD (chemical vapour deposition). CVD coatings first became industrially available about 40 years ago, but their high process temperatures (in the region of 800-1200 C) limited their applications to substrates whose strength was unaffected by such temperatures and to components that would not suffer excessive distortion; their application to precision components is also impractical.

There are several variations on the basic CVD process, although the one used to deposit the kind of hard surface coatings we would like for our engine and transmission components requires those components to be hot. The heat promotes chemical reactions between the vapour and the component surface, and the process is essentially to pump reactants into a chamber holding the heated components. The combination of temperature and pressure promotes a reaction at the surface of the component, with one of the products of the reaction being deposited onto the surface.

The process can be used to deposit various substances onto surfaces, including metals and ceramics, in thin layers. The gases used to transport the reagents to the surface need to be specially selected, and often more than one gas is required to produce the correct reactions. For example, tungsten can be deposited at quite low temperatures by using a mixture of a tungsten hexafluoride and hydrogen gas, with a reduction reaction taking place, while nickel can be deposited using a single gaseous reagent through thermal decomposition. To produce such metallic coatings by processes such as PVD, where the source coating material has to be evaporated, might prove very difficult.

Where the process has traditionally required high substrate temperatures, for example titanium carbide (TiC) coatings, recent advances in CVD have reduced processing temperatures significantly. Plasma-assisted CVD (PACVD) and plasma-enhanced CVD (PECVD) are techniques that increase the energy of the gas from which the coatings are deposited. This means that comparatively less heat energy is needed at the component’s surface, allowing lower substrate temperatures. For a coating such as TiC, typical PACVD process temperatures are around 700 C, compared to 1200 C for the process without the assistance of plasma.

The low-temperature CVD processes in particular make the technique attractive for depositing metals onto materials such as aluminium, titanium or steels with low tempering temperatures.

Written by Wayne Ward

After a relatively short period of time, a range of PVD coatings have passed from the race engine sector through to the mainstream production passenger car market. PVD-coated engine components are not the preserve of high-tech niche sportscar makers; the large automotive players see the advantages in terms of reduced friction and wear to benefit both the fuel economy and reliability of their cars.

Titanium nitride is probably the first of this family of coatings that we would have noticed. Pre-dating other well-known coatings such as DLC, TiN was widely used as a cutting tool coating. Its relatively low friction and wear resistance also made it a firm favourite for high-performance suspension, especially on racing motorcycles. In terms of engine components, it was applied to some titanium racing rods, with Yamaha’s OW-01 benefiting from it in the late 1980s. It has also found widespread use on titanium valves. Chromium nitride (CrN) is another commonly used coating, again finding use on con rod thrust faces and titanium valves. PVD coatings are also widely used on piston rings to reduce friction and wear.

DLC is a widely used term, but this is not a single coating; it is used to describe a wide range of carbon-rich coatings that often have a high metallic or ceramic content. More than any other type, the DLC ‘family’ of coatings has been used to great effect to reduce friction, and improve the performance and reliability of components. It has allowed already aggressively engineered valvetrains to be run to even higher levels of stress with remarkable reliability, and has been used everywhere from piston pins, piston rings, piston pins, cams, cranks and especially cam followers.

PVD coatings are not however a ‘silver bullet’ that will solve all of our engineering woes and bring improved performance. If they are badly planned – and if we don’t take the process parameters, requirements and the coating properties into consideration – we can easily do more harm than good. For instance, if we are aiming to coat materials that lose mechanical strength at temperatures lower than the PVD process temperature, then many of those coatings will not be suitable, as they will soften the material. Trying to coat materials which are too soft and which are loaded in contact, or coating parts which lack stiffness, can lead to damage and delamination of the coating.

The danger of selecting the wrong coating can be mitigated though by talking to the coatings experts, who might recommend a different coating or a ‘functionally graded’ coating – a term used to describe a multi-layer coating, engineered to provide good adhesion and support for the thin layer of coating which forms the actual working surface. Coatings suppliers will also counsel you in the correct surface finish prior to coating, which is a key element in the successful use of PVD-coated components. This is an important consideration given that PVD coatings are very thin and so will tend to reproduce the surface finish of the substrate.

Fig. 1 - Motorcycle fork tubes are commonly coated with titanium nitride (TiN). More than 20 years after its introduction, it remains a popular coating

Fig. 2 - Cam followers have been a real PVD success story; widely used in racing, followers coated with low-friction coatings are now found in a range of roadcars

Written by Wayne Ward

A few months ago, in an article on plasma and thermally sprayed ceramic coatings, I wrote about the use of electrically insulating coatings being applied to the outer races of rolling element bearings in electric motors. With the increasing adoption of hybrid power units in race and roadcars, the effective electrical insulation of rolling bearings will be an important factor for reliability. There are several ways to achieve this, not all of which involve changes to a standard bearing, but electrically insulating coatings are one method which may be considered.

There are also reasons why one might consider the use of low-friction coatings on rolling element bearings. A rolling element bearing might seem an odd application for a low-friction coating, but these are in common use. There are two types of low-friction coating - those that are soft and which rely to some extent on shearing to give low-friction behaviour, and modern low-friction coatings which are extremely hard.

Soft metallic platings encompass the first group, of which silver plating is the most common. The application that you might be familiar with is the silver plating of steel bearing cages, which is especially common in needle roller bearings, as used widely for small-end bearings on two-stroke engines and for the support of camshaft drive gears on four-stroke race engines.

The modern, hard, low-friction engineering coatings also find use in rolling element bearings. Diamond-like carbon (DLC) is not a single coating but a ‘family’ of coatings of different composition and differing properties. Several bearing manufacturers supply rolling element bearings with the races coated with DLC.

When a bearing is functioning and lubricated properly, there might seem little reason to use a low-friction coating - after all, when adequate lubrication is available, there should be no contact between the rolling elements and the races. However, when the lubrication is marginal, or where loading conditions allow ’skidding’ of the rolling elements on the races, there is a case for hard coatings, especially where the races and rolling elements are made of the same or very similar material.

Where identical or similar materials are placed in moving and heavily loaded contacts, adhesive wear may occur. Minute high points on the contacting surfaces may become welded together, and material can be pulled off one surface. This damages both parts involved in the contact, and once roughened, the surfaces become much harder to separate satisfactorily through lubrication. By changing the surface so that we separate identical or similar materials, adhesive wear is very much less likely to occur.

Where loading is such that the rolling elements are allowed to skid across the races, and where lubrication is again marginal, the tangential component of the surface load can significantly increase the maximum subsurface stresses. As these are cyclic, wear may occur due to subsurface fatigue, shown as pitting. Having a low-friction coating helps in two ways: the tangential component of loading is minimised, as is the generation of heat in the contact. The temperature at the point of contact can be substantially higher than the surrounding material, and this can itself lead to increased stresses.

Written by Wayne Ward

In recent years, aerodynamicists have tried hard in Formula One to take advantage of the energy contained in the exhaust flow. In 2011, there was a lot of controversy over the use of blown diffusers and blown floors, which gave extra downforce. For 2012, such devices were banned, and the regulations stated that the exhaust exits had to be on the top surface of the bodywork, with the direction of the exhaust pointing slightly upwards at an angle of 10º from the horizontal. There is, of course, the option of simply using the thrust coming from the momentum of the exhaust gas to increase the motive force. The flow could be directed toward the rear wing of the car; increased mass flow over the rear wing should increase downforce.

The solution chosen by a number of the teams is to use a 'Coanda Effect' exhaust system. There is a lot of material published in the press about which teams are using this technique, but the basic idea behind such systems is to cause the exhaust flow to attach itself to the top of the bodywork, from where it sweeps down toward the rear of the car. Experts surmise that the exhaust flow is directed toward the gap between the diffuser at the rear of the car and the rear tyre.

As clever as all of this undoubtedly is, it makes life very difficult for a number of components. These new exhausts, adopted by an increasing number of teams as the season wears on, rely on the interaction between the relatively solid bodywork and the hot, fast-flowing exhaust plume. Therefore the bodywork is the first to suffer from the effects of heat. Further downstream, other items of bodywork and suspension components may be affected.

Thermal barrier coatings are used to protect sensitive areas of the car from the damaging effects of heat transferred from the hot exhaust flow. While there are composite materials which are resistant to the effects of extreme heat, teams often prefer to use materials that they have a lot of experience with and to protect these from damage from hot gases. Sprayed ceramic thermal barrier coatings can be applied directly to both metallic and carbon composite components. This gives the race teams the chance to coat bodywork, floor and suspension components where protection from exhaust heat is required. The last thing engineers want to do with their clever exhaust systems is to torch some bodywork!

For 2014, and the change to turbocharged Formula One engines with low mass flow through the exhaust, and with much of the exhaust energy removed by the combined turbo-generator, there will be less that can be usefully done in terms of aerodynamics with the exhaust flow.

Written by Wayne Ward

Trials of ceramic thermally sprayed bore coatings have been successfully undertaken in race engines; this was done by a major coating company in order to prove the process before transferring it to passenger cars. The coating was part of a trial involving both ceramic and metallic sprayed coatings in a NASCAR engine, and the trials were successful in proving both the durability of the coatings and the potential for increased engine performance due to decreased friction.

NASCAR engines don't usually use a bore coating, one of the reasons being that engines are routinely re-bored when required and the block can be used for a number of builds. However, this strategy means keeping a large inventory of pistons and piston rings. Sprayed ceramic coatings on bores might prove to be an equally economical way to run an engine, but without the need to stock varying sizes of piston, rings and so on.

With the increasing adoption of hybrid systems in motorsport comes a proliferation of electric motors designed to exploit the new regulations allowing energy recovery. It is necessary to electrically insulate the rotor shaft from the stator assembly. If the rotor is electrically connected to the stator, a circuit is formed and the bearings become damaged very quickly, essentially due to 'spark erosion' of the races. There are various reasons why a voltage is induced on the rotor shaft which are far beyond the scope of this article.

The characteristic pattern of damage resulting from the arcing across the gap between the races and the balls is referred to as 'fluting' or 'washboarding'. This quickly leads to notchy bearings and increased clearance in them. As with many other wear problems, once there is clearance and loss of precision, wear can quickly accelerate.

There are a number of ways to counter this, but one is to use a ceramic coating on either the bearing outer race or to the bore of the bearing housing. Some bearing companies have offered such coated races on steel bearings for electric motor applications, although there are other materials for rolling element bearing construction that can achieve the same effect.

The methods for applying ceramics to the outside surfaces of the outer races are generally thermal or plasma spraying. I have seen high velocity oxy-fuel (HVOF) methods used to coat bearing bores on end caps of bespoke high-performance electric motors, although this is not as easy as buying a bearing that is already coated by the manufacturer.

The one advantage of developing a relationship with a thermal/plasma-spraying company is that you can tailor the coating to your requirements. Some ceramics that aren't offered by bearing companies offer attractive combinations of properties for the design engineer.

Written by Wayne Ward

While the use of polymers in race engines for the manufacture of entire components has been sporadic and pretty limited, the use of polymer coatings on components has remained reasonably popular over a the past 20 years or more. Much of their popularity as a coating comes from two main properties - low coefficient of friction and high wear rate.

The attraction of a low coefficient of friction is easy to understand: wherever loaded sliding contacts occur, less energy is converted to heat and lost to atmosphere. Thin polymer coatings on metallic components can reduce the coefficient of friction in sliding contacts. Such coatings remain popular on racing pistons for many applications, having been used widely for series such as Formula One and IndyCar racing in the past. While DLC is now sometimes preferred by some engine builders, polymer piston skirt coatings are well known and are more forgiving. As we shall see below, it is not only for friction that polymer coatings are used.

Whether high wear rate is an attractive trait in a race engine is an interesting debate, and the answer depends on the application to a certain extent, but even more relevant is the portion of a component's life during which the wear occurs. Polymer coatings tend to wear heavily where contact pressures are high and sliding is present; where contact pressures are low, wear rates are minimal. The fact that material is easily lost at points of high contact pressure means that polymer coated components easily 'wear in' to an optimal profile.

Again, the main application is piston skirts. The ideal skirt profile for pistons in identical engines can be different based on how and where the engine is used. A skirt profile that can wear to form a profile which is close to ideal is very attractive. Polymer coatings have actively been used to determine piston skirt profiles for metallic pistons in the past.

The same principle of components wearing in means that polymer coatings find widespread use in oil pumps on pump housings and on some pump elements. Polymer coatings also allow small particles to become harmlessly embedded in the coating, where the same situation with close-running pump components can lead to damage when small, hard particles pass through the pump.

In recent years there has also been a trend towards giving shell bearings a polymer coating. There are mixed views on the effectiveness of this; some people are convinced of the benefits while others cope without the coatings.

Some engine builders equate low friction to an ability to 'shed' oil from the surfaces of components. Occasionally I have seen engines where all manner of components, from con rods to the inside of crankcases, have been polymer coated.

Written by Wayne Ward

Basic electroless nickel plating generally contains a high percentage of other elements, most notably phosphorous but sometimes boron. There are a range of phosphorous contents, which vary according to the requirements for corrosion resistance and as-plated hardness. Low-phosphorous coatings are quite hard as plated (>700 HV), medium-phosphorous coatings are around 500 HV as coated but may be heat-treated at a temperature of about 400 C (750 F) to increase hardness to around 1000 HV, while high-phosphorous coatings are the most resistant to corrosion when left as plated, although all compositions may be increased in hardness to above 900 HV by heat treatment.

The composite coatings are of two main types - the first has particulate 'reinforcements', the second is a multi-layer coating. The former type can incorporate a number of different particulate materials, ranging from PTFE to diamond. Electroless nickel/PTFE composites are aimed at low friction with a degree of wear resistance; electroless nickel/diamond and electroless nickel/boron nitride composite plating processes are generally aimed at wear resistance.

However, electroless nickel/diamond coatings are sometimes used in sliding contacts to increase friction and so prevent sliding. This range of properties with nickel/diamond coatings is possible owing to the size of the diamond particles in the coating. Thin coatings used in comparison with large diamond particles allow the diamond pieces to penetrate any surfaces that the coated component is trapped between. This technique is used to prevent slip in critical joints by using a coated shim. Alternatively, such coatings can be applied locally to surfaces, although masking can prove to be very expensive.

Electroless nickel/PTFE composite coatings have been applied successfully to prevent seizure in poorly lubricated piston/pin contacts, where the low friction provided by the PFTE, combined with the hardness of the nickel, has proved to be a good combination.

The multi-layer coating has a standard electroless nickel base layer, a top layer of a low-friction polymer and an intermediate layer to provide improved adhesion between the base layer and the polymer.

Perhaps one of the best-known applications of electroless nickel coatings is on pistons. In engines that are especially prone to combustion knock - that is, where very high compression ratios are used, or where the engine is supercharged or turbocharged - electroless nickel coatings can mitigate knock damage. The coating has been widely used in turbocharged rally engines for a number of years, and the main area of application is the piston crown and top land.

Other than the expense of masking, there is no practical reason why the same approach could not be followed with the cylinder head, which can also suffer severe mechanical damage due to knock. In allowing an engine to run with a certainly level of knock, or allowing the ignition to be changed to run the engine further into knock, turbocharged engines may be able to increase output. According to Towers and Hoekstra*, the nickel plating limits damage, owing to the increased strength and modulus of the coating, rather than preventing knock occurring.

* Towers, J.M., and Hoekstra, R.L., "Engine Knock, A Renewed Concern In Motorsports - A Literature Review", SAE Paper 983026

Fig. 1 - Electroless nickel plating can mitigate knock damage

Written by Wayne Ward

There are a number of reasons for the use of a coating on a piston skirt. The conclusion many people will jump to is that they are primarily to reduce friction, but this isn't always the case.

One important reason to use a skirt coating is to protect the piston during 'running-in'. During the initial period of engine running, either using a new piston or in a completely new engine, there is an excess of friction compared to normal running, owing to asperity contact. A low-friction or lubricious coating can help in this initial period. The coating doesn't have to be especially durable, and coatings based on phosphates, graphite and resin-bound polymers have all been used for this purpose.

Where the piston design will be used by a range of drivers of differing abilities, and cars with varying cooling efficiency and duty cycles, the ideal piston skirt profile can be difficult to produce. With a coating of substantial thickness, the piston can develop its own profile to a certain extent, as the piston coating 'wears in' to a shape based on they way the engine is used and driven. This practice has been used to quickly develop skirt profiles, and is documented in SAE Paper 820769*. The process described in the paper is one where the engine is run for a while under real operating conditions, and the 'worn-in' skirt profile is then copied to a real piston. Where coated pistons are run continuously with the aim of wearing in to an ideal shape, the underlying piston skirt profile should be reasonably close to the end result.

Of course, one area of interest to all engineers concerned with producing competition engines is that of friction reduction. Whether your competition is NASCAR Sprint Cup, Formula One or Shell Mileage Marathon, you want as little as possible of the chemical energy contained in the fuel to be wasted in frictional losses. Some of the coatings applied for improving run-in or skirt-profile compliance will have an effect on friction, but there are also piston coatings that are applied specifically to reduce friction.

A number of companies now have DLC-coated pistons commercially available for racing customers. As mentioned in the Race Engine Technology articles on pistons (RET 43, November 2009 and RET 61, March/April 2012), the adoption of DLC is far from simple. Not only is the coating very specialised owing to application temperature and other parameters, the machining of the piston also markedly differs from a conventional, non-coated item. If you try to apply any old DLC to your pistons, you will probably get a very different result from that desired!

With the desire to produce ever-more reliable and increasingly efficient engines, piston producers and users will continue to turn to coatings to help them achieve their goals.

Fig. 1 - Piston skirt coatings are specified for a number of reasons

* Yagi, T., and Yamagata, I., "Experimental Method of Determining Piston Profile by Use of Composite Materials," SAE Technical Paper 820769

Written by Wayne Ward

They are not though something that can be applied without due thought to any situation that the designer feels may merit such a coating. The geometry of the part and the underlying material must also be considered when selecting the correct coating. The stiffness and strength of the underlying part have an important impact on the likely success of DLC, and there are a wide range of applications where we can expect DLC not to work.

Where the problem to be solved is one of excess wear, a hard, thin, low-friction coating may prove to be ideal. However, if the wear problem is one caused by subsurface stresses exceeding the material's limitations, putting something black and shiny on the surface is only going to make it look pretty initially; it will do very little to alter the basic problem apart from changing the subsurface stress field owing to lower friction. In situations where full hydrodynamic lubrication generally prevails, DLC may provide only limited protection against damage at start-up.

In situations where the substrate material is insufficiently stiff - either through its modulus, or by the design of the part - the DLC may simply crack as the substrate deforms elastically. Where sharp edges run across a surface, problems are especially likely. There are other situations where the adhesion of DLC can be insufficient to prevent flaking of the coating.

So, is there anything that can be done in such situations? Unsurprisingly, the answer is yes. Our friendly coatings suppliers have developed solutions for many of the situations that might previously have rendered DLC unsuitable.

A number of suppliers have developed multilayer coatings that are aimed at solving the problems above. In terms of adhesion, a 'base layer' is selected which has sufficient adhesion to the substrate, and to which the DLC adheres adequately. Such processes are similar to techniques used in plating, where a thin base coating improves adhesion. Sometimes more than one different type of coating is used for the base layer, common ones being metallic titanium, chromium nitride (CrN), titanium carbide (TiC), titanium nitride (TiN) and titanium carbo-nitride (TiCN). An example of a multiple layer coating is one that is applied to a titanium substrate: TiN may be applied first, followed by TiCN and then finally the DLC.

Such multi-layer coatings, (also known as functionally graded coatings) can be used to prevent brittle flaking of DLC by providing a stiffer substrate, thereby reducing compliance to a level that the limited ductility of the DLC coating can cope with.

With such a huge array of different coatings and coating systems, one should not expect the designer to know which coating should be applied in every situation. Thankfully coatings companies are usually very helpful in recommending a suitable coating for a given application.

Fig. 1 - Multilayer coatings are commonly applied to cam followers

Written by Wayne Ward

In this article, I'll look into the method in more depth. The method lends itself to a very wide variety of coating materials, providing that the material can be furnished in a suitable solid or liquid form to be placed into the vacuum chamber as a solid target. PVD coatings lend themselves to use on many different substrate materials and are used not only for high-end technology; cutting tools and even our aluminised crisp packets are often PVD coated.

There are a number of methods of turning our solid or liquid 'target' material into a vapour:

• Sputter deposition (also known as sputter ion plating) is used for many motorsport components. In this method a plasma discharge is focused around the target by a magnetic field, often generated by one of more magnetrons. The plasma vaporises the solid target.
• Electron beam deposition relies on bombarding the target material with a beam of electrons in a vacuum chamber.
• Liquid or solid targets can be evaporated by a number of conventional heating methods - resistive heating or inductive heating, depending on the nature of the material and so on - thus producing a vapour in a vacuum chamber.
• Lasers are used to evaporate material from the surface of a target by a process of ablation. The laser ablation process is also used to good effect in laser peening, which has been covered previously in a RET-Monitor article.
• Electric arc evaporation of electrically conductive targets is also used, and this employs the same principles as the spark erosion methods used in industrial manufacture.

Given the number of methods of vaporising the target, we can use PVD to apply everything from ceramics to metals and carbon coatings such as DLC.

Of special interest to the designer of a race engine, where parts are often subject to very severe cyclic stresses, is the fact that metallic coatings can be applied without the danger of hydrogen embrittlement that comes from electroplating and other coating processes, owing to the fact that the coatings are deposited in near-vacuum conditions. Of course, in order to prevent embrittlement completely, pre-treatment processes need to be carefully selected so that they don't cause hydrogen to be diffused into the surface of metallic components. High-strength steels are well known for suffering hydrogen embrittlement. Although there are processes to mitigate the effect of hydrogen embrittlement, some people are adamant that it can't be fully reversed. PVD coatings appear to be one way to preclude it.

There are a number of ways of depositing coatings that differ from the target material. For instance, the same titanium target material can be used to deposit not only a titanium coating, but also titanium nitride (TiN) and titanium carbonitride (TiCN), simply by introducing a carrier gas to the vacuum chamber which contains ionised titanium vapour. For instance, introducing nitrogen gives a TiN coating, and the use of acetylene (ethyne) gas produces a TiCN coating.

Fig. 1 - Coated valves are only one application of PVD coatings in race engines and transmissions

Written by Wayne Ward

There are other applications of thermal barrier coatings which have the same basic aim but which are applied to the cylinder head. Of course, all of the exhaust flow must travel through the exhaust port before it reaches the exhaust system. In the same way that insulating the exhaust system with a thermal barrier coating can reduce heat rejection to the atmosphere, a thermal barrier coating applied to the exhaust port of an engine reduces the heat rejection from the exhaust flow to the coolant water. This has the additional benefit of reducing the amount of heat to be rejected at the radiator, which has an additional aerodynamic benefit.

This is not novel thinking. Porsche were thermally insulating exhaust ports in turbocharged production engines 25 years ago. However, they did it by casting a ceramic insert into the head rather than using a coating, the aim being to improve the transient response of the engine by improving the spool-up speed of the turbocharger. Twenty-five years on, and coating technology might have allowed them to do this in a simpler manner. There are a number of companies offering thermal barrier coatings, some of which have been applied to the exhaust ports of race engines.

If you have seen a very high performance engine (especially a turbocharged engine) running at full load on the dyno, you cannot fail to have noticed the dramatic and impressive spectacle of the exhaust system glowing red hot. Irrespective of the amount of heat radiated to the atmosphere, there must be a significant amount of conduction across the joint between the exhaust system and the head. It is likely that there would be some small reduction in heat conducted to the cylinder head by coating either the face of the exhaust flanges or the cylinder head. Even in the case of having an insulating gasket between the two, there would be some conduction through the studs used to mount the exhaust systems to the cylinder head. If the exhaust flange has a thermal barrier coating where it contacts the exhaust stud and nut, there would be some reduction in thermal conduction across the head joint face.

In both of these cases, there is a clear benefit for naturally aspirated engines due to a reduction, however small, of their cooling requirements. Given the number of times we have seen under-cooled Formula One cars rolling out of the garages in pre-season tests with large holes carved into the sidepods, every little must help in this regard.

In the case of turbocharged engines, thermal barrier coatings could also be used to protect the turbine wheel from excess heat. This technology has been used to good effect on high-pressure turbine blades of modern aero engines, helping materials survive for long periods of time where they could not otherwise work satisfactorily.

Fig. 1 - Could turbine blade coating technology used in modern aircraft find application in a racecar turbocharger?

Written by Wayne Ward

There are a number of sprayed coating methods, distinguished not only by the particular method of coating but also by the ratio and absolute levels of thermal and kinetic energy. The more traditional flame spray processes have low thermal and low kinetic energy, while the HVOF (high velocity oxy-fuel) process has very high kinetic energy but low thermal energy. Owing to the high temperature of the plasma 'plume' generated, plasma spraying has much higher thermal energy than either of these processes, and is approximately intermediate in terms of kinetic energy. The high thermal energy makes plasma spraying particularly suited to spraying materials with high melting points, such as ceramics.

There are two main variants of plasma spray processes: one takes place in ambient atmosphere, the other is carried out in 'vacuum' (or very low pressure) conditions. Obviously, it is much easier to carry out 'atmospheric' plasma spraying, but it has the disadvantage that the coating material has an opportunity to oxidise before it is coated onto the substrate, where it then rapidly cools and is therefore less prone to oxidation. Vacuum or low-pressure plasma spraying is much less prone to oxidation of reactive coating materials, and the coatings are said to be of higher quality.

The plasma flame is formed by passing a gas, which is typically nitrogen (N2), hydrogen (H2) or helium (He) or mixtures of these, between an anode and a cathode. There is a high-frequency arc ignited between the electrodes and the gas which passes between them is thus ionised. The resulting plasma plume can reach temperatures of around 16,000 C (about 29,000 F). The powder material to be coated is fed into the plasma plume or flame, and is almost instantly melted. The high kinetic energy of the plume is sufficient to propel the molten particles at the substrate, where they deform into a flattened 'pancake' form and then solidify.

The process is carried out following a blasting process to roughen the surface and a cleaning process in order to promote adhesion of the coatings. Blasting the surface not only provides increased surface area ratio compared to the nominal 'presented area', but also increases the surface energy of the substrate; both of these effects increase coating adhesion. Suitable substrate materials are those generally with a surface hardness of 55 HRc or lower, which are easy to roughen by blasting, although substrate materials which are harder than 55 HRc can be coated following a special process to roughen the surface.

Fig. 1 - One application of plasma-sprayed coatings in race engines is the coating of cylinder bore surfaces

Written by Wayne Ward

The Toyota Avensis NGTC (Next Generation Touring Car) campaigned by Frank Wrathall Jnr in the British Touring Car Championship benefits from well planned containment of heat. It's one of the strongest NGTC-specification cars, often getting on the podium ahead of most of the supposedly superior S2000-specification cars. Such is the speed of the NGTC cars that S2000 drivers are calling for them to be slowed by changes to the rules.

The engine in the Avensis was developed by Warwickshire-based X-CTechR, which gave careful thought to the heat radiated from the exhaust system of the turbocharged four-cylinder engine. Technical director Mark Faulkner says, "Turbocharged application demands specific attention to thermal management to achieve optimum performance." In line with this statement, X-CTechR turned to Zircotec to coat both the exhaust manifold and downpipe with its ceramic coating.

The coating appears to work very well, with Faulkner explaining, "On the dyno the ceramic-coated exhaust gave an improvement in turbocharger response, as more energy was retained within the exhaust rather than radiated as heat." The main criticism of many turbocharged engines is a lack of instant response to changes in throttle position. In race series with more liberal regulations than the BTCC, and bigger budgets, specific anti-lag measures are used, which can involve deliberate combustion of fuel within the exhaust system or to manipulate the throttle independently of pedal position in order to maintain a certain mass flow rate through the turbine.

The BTCC rules, sensibly in these current days of financial insecurity, are looking to establish rules that don't require excessive development budgets. In this context, the use of ceramic-coated exhausts is a sensible step in minimising turbo lag.

By keeping the maximum amount of energy within the exhaust flow, the exhaust turbine is able to accelerate the compressor with the minimum of delay.

While improved turbocharger response is a laudable aim in itself, touring car engines have to operate within a small engine bay. Therefore, under-bonnet temperatures can be a problem. The ceramic exhaust coating also paid off in this respect. Faulkner confirms, "In the vehicle a significant reduction in under-bonnet temperatures was achieved with the ceramic-coated exhaust. This contributes to reduced air intake temperatures, releasing more power."

Fig. 1 - The Toyota Avensis NGTC car benefits from ceramic-coated exhausts (Courtesy of Propel Technology/Zircotec)

Written by Wayne Ward

The reality is that, where opportunity exists to engineer a more carefully considered solution to the problem of oil shear, we should see greater gains. Reduction of the quantity of oil, by tackling the quantity of supply and/or improvements to scavenging, combined with an increase in clearances (if possible) will also bring useful results, but clearly with much greater expenditure of time and money.

Some companies sell oil-shedding coatings based on the fact that components will be heavier when coated in oil, and are therefore likely to 'slow the engine down'. How much oil is likely to adhere to a given component under conditions of high acceleration is not clear, but the main gains from such a coating are unlikely to be due to this reason. There are amateur demonstrations of the amount of oil adhering to an engine component, but at working temperature, the oil viscosity is much lower than at room temperature, and the masses of oil added to the reciprocating and rotating components will be lower than these garage experiments demonstrate.

However, this is not to deny the fact that oil-shedding coatings may offer advantages quite separately from decreasing oil-shear losses. There is quite possibly a reasonable amount of oil hanging around in any engine on static components that are not subject to significant oil shear, and this amount can be significant. More efficient drainage of oil back to the sump/lower crankcase might allow the amount of oil on the car or bike to be reduced.

In designing a new engine, or modifying an existing production engine for racing, much consideration is given to providing easy egress to oil from the top end of the engine, where large quantities are apt to become 'lost'. Cam covers, cam carriers and cylinder heads can all have significant wetted areas where oil is likely to adhere, and where there is nothing physical to sweep the oil away, other than more oil. Here, oil-shedding coatings work by changing the 'surface energy' of the metal surfaces, such that oil will not spread into a thin film but will collect into droplets which then run away under the action of gravity.

The 'non-wetting' action we are aiming for here is similar to that we see on a freshly waxed car after rain, with liquid forming broadly spherical droplets. Making a smooth surface is not necessarily the way to go though; a dusty car can also show non-wetting tendencies.

The best examples in nature of non-wetting surfaces are petals and lotus leaves. Both are microscopically very rough, but in different ways. While water will 'wet' neither surface, lotus leaves will shed a water drop of almost any size. Water forming small droplets on a rose will not run off even if the flower is tipped upside down.

In an oil-shedding coating, we are looking not only for the surface not to 'wet' but also that the surface can shed the oil. With liquids showing lower surface tensions at high temperature, as might be found in an engine, the job of preventing wetting and shedding becomes more difficult. Work continues in trying to create better oil-shedding coatings and surfaces (called super-oleophobic surfaces) on many fronts, of which motorsport is only one. Creating lotus-like liquid-shedding behaviour for both water and oil has benefits for many applications outside of motor racing.

Fig. 1 - We can learn much about wetting of surfaces and shedding of liquids from studying nature. This is a lotus leaf.

Written by Wayne Ward

The manufacture of bearings, through the application of better manufacturing methods and better material combinations for the bearings and their platings/overlays, is now better than ever. Bearings can withstand higher pressures with better reliability than ever before.

There are a number of companies who supply 'coated' bearings, and who will procure your desired bearing and add their coating before delivering them to you. The claims for these bearings are impressive, but there seems to be little scientific data to back them up. Some companies make more impressive claims than others, but the common claim is that bearing life is increased. Some of the outlandish claims regarding huge power gains or being able to run smaller coolers due to bearing coatings seem implausible. If the lubrication system is working, then the power should be insensitive to changes in the bearing surface material. The fact that some bearing manufacturing companies have now decided to supply coated bearings means they either see the technical benefits or have decided to reap the financial benefits of producing what the customers want.

To try to get to the bottom of this, I spoke to a company that is successfully marketing coated bearings, and also to the representative of a company whose job is the design, manufacture and supply of bespoke bearings for the race business, covering all levels of motorsport.

The coatings applied are said to be 'solid lubricant' or 'dry lubricant' films, and are basically polymer coatings with a low coefficient of friction. They are said to help when lubrication conditions are particularly poor, with engine start-up or oil pump problems being cited as examples. The claim is made that Alan Kulwicki's 1992 NASCAR Cup championship was won due to coated bearings, allowing an engine with an intermittent oil pump problem to finish a race at Martinsville, running several laps after having thrown a pump belt. It is difficult to know if the coating did actually prevent an engine failure.

The make-up of an uncoated bearing should also be able to cope with a temporary loss of oil pressure, although it is obviously desirable if that doesn't happen. The innermost layer of the bearing is a very thin layer of a material with a very low shear stress. This is supported on a much stronger bearing shell. The low shear stress of the overlay means the material can shear when metal-to-metal contact is made, with very little frictional heat being generated. The depth of overlay is important - if it is too thick, it will deform, giving a large contact area and higher friction in conditions of marginal lubrication.

So, we have two approaches to the same problem, one relying on a metallic alloy with low-shear strength to reduce frictional losses and heating in conditions of marginal lubrication, and another using a stronger but low-friction polymer-based coating to achieve the same thing. Once the polymer wears through, however, don't we have the original bearing underneath? Well, probably not as it was intended. By coating a lower shear-strength material, the failure mode of the polymer may not be wear but bulk shear of the layer underneath. If this happens, very little of the original bearing overlay will be left. Pre-coating treatments may also remove some of the original bearing overlay.

There are companies and engine builders who remain adamant that coated bearings are a worthwhile technology, and others who find no improvement and are equally adamant that such coatings offer no gain.

Fig. 1 - Coated bearings are popular, but the debate over the value of coating a bearing is not settled (Picture Courtesy of SwainTech Coatings)

Written by Wayne Ward

In SAE paper 911230, for example, the authors Tsuchida and Tsuzuku study the effects of various design features on piston friction losses for high-speed engines (up to 16,000 rpm). At the time - the late 1980s and early '90s - the Japanese were selling production motorcycles with engine speeds higher than even Formula One engines were capable of revving to at that time. The first graph in their report showed that, for the high-speed engine being studied, crank and piston friction was by some margin the largest source of friction in the engine and transmission assembly.

The reduction of piston assembly friction still remains a constant target of research projects in the mainstream automotive field. Coating pistons has been common in motorsport for many years, and the coating of piston skirts in particular has proven successful, with many companies offering these products. Such coatings found widespread success through many top-end race series including CART (Champ Car) and Formula One.

There are a few main types of piston skirt coating in general racing use; there are a number based on polymer formulations, with others based on graphite or a composite coating with a resin binder with fillers. The polymers are commonly spray coatings with a build-up of between 10 and 25 microns per surface (0.0004-0.001 in) and are based on PTFE, although screen printing methods provide for a more accurate method of application.

Sprayed coatings require masking to prevent overspray, according to Martin Keswick of Capricorn, who says that about 70% of the pistons Capricorn produces are coated. The polymer coatings are relatively soft, and piston manufacturers expect these to have significant loss of polymer in use, effectively wearing-in during use. This is a similar principle to that described by Yagi and Yamagata in SAE paper 820769, where they used a loaded resin composite coating as a way of rapidly determining optimum skirt profiles for automotive pistons.

The graphite coatings are applied by screen printing, and these are more durable than the polymer type. Graphite is a material renowned for its low friction.

The metallic-loaded resin coatings are specifically recommended to be run in hypereutectic bore materials.

A number of piston companies also phosphate pistons all over, giving them a grey colouration.

The reasons for coating a piston skirt are not necessarily to do with friction, but can be to combat wear and to make the piston less sensitive to skirt profile design in cases where the coating is expected to 'wear into' an optimum profile. The optimum skirt profile can be different for two seemingly identical engines, depending on how they are used, and the wearing polymer coatings means that we effectively have a piston surface that can adapt to the particular loads imposed on it.

The latest development, slowly trickling down to the wider racing market from Formula One, is DLC coating. This requires special piston machining and a special coating process so that the strength of the piston material is not compromised by the temperatures involved in the coating process.

Fig. 1 - This piston skirt is coated with a sprayed PTFE-based polymer (Courtesy of Capricorn)

Fig. 2 - All these pistons have run in engines. Some, such as the green PTFE coated item, show signs of wear, with others showing no obvious wear (Courtesy of Capricorn)

Written by Wayne Ward

Students of the internal combustion engine, and those among them with a particular interest in the frictional losses they suffer, will know that the piston assembly is responsible for a large proportion of these losses. In these days, when fuel economy is increasingly important to race engine suppliers and everyday drivers alike, it is little surprise that we find much attention lavished upon the piston in order to reduce friction.

Of all the hard, thin engineering coatings developed in recent years, DLC (or variations on it) have made the greatest impact in applications where friction is a serious problem. Once the sole preserve of the big-budget Formula One teams, DLC-coated pistons have been made available to the wider racing world recently.

However, DLC-coated pistons were not used for quite a long time, even in Formula One, compared to the other DLC-coated parts in use. The reason is that the early DLC processes required high processing temperatures compared to the temperature at which piston materials are aged. The heat treatment of aluminium alloys involves a 'solution treatment' at a high temperature, and then an ageing process at a lower temperature. This ageing process develops strength in the material, and if this temperature is exceeded, loss of strength and fatigue life is likely.

With the early DLC processes, these temperatures were certainly enough to severely overage the aluminium piston, and ruin it for the purposes of racing. While outwardly little damage was probably visible, hardness testing would reveal that the component had softened, and its use in an engine would quickly lead to failure. It's known that in the past more than one person tried DLC with disastrous consequences before the process was properly understood.

Special processes have been developed in recent years which maintain good adhesion of the coating to the substrate material without the process temperatures exceeding the ageing temperature of the piston. According to those who use DLC pistons, the results of the process are increased wear resistance and lower friction.

One point to be aware of though is that, besides being very careful to select the correct process and the right coating subcontractor, the design and manufacture of the piston needs to change if DLC is being used. Owing to the unique characteristics of such hard coatings, their application to standard pistons is likely to damage the piston and the engine in which they are installed. Hard, low-friction coatings were initially developed for cutting tool applications. Putting super-hard coatings onto a standard piston can make quite an effective reciprocating cutting tool.

Fig. 1 - Hard coatings are popular on cutting tools, and care must be used before using them for coating race pistons

Written by Wayne Ward

PVD (physical vapour deposition) coatings were among the first to market many years ago, and were developed initially for coating tools, specifically milling cutter and machining inserts. Having seen the difference in cutter performance afforded by coatings, even in their infancy, it is little surprise that coated machining cutters and inserts are so popular.

The PVD process of is one of condensing a vapour onto the surface of a component after evaporation of a 'target' in partial vacuum conditions, whereas CVD (chemical vapour deposition) involves a chemical reaction at the surface. The technology is not new, although the term physical vapour deposition was coined only relatively recently. Scientist Michael Faraday was using a PVD method to deposit coatings as early as 1838 - although not for motor racing!

The motorsports sector was a little slow to catch on to the potential benefits of coatings. The applications were limited initially to suspension components, where users were looking to minimise the effect known as 'stiction', where a high friction coefficient needed to be overcome before a much lower coefficient prevails once movement ensues.

Manufacturers of titanium valves saw real benefit earlier than most other engine applications. Titanium's notably poor surface wear characteristics are transformed by the use of PVD TiN (titanium nitride) and CrN (chromium nitride) coatings. To quite a large extent, TiN has been replaced for valve applications by CrN.

These are two traditional 'tool' coatings that have enjoyed success in motorsports due to low friction. Some other tool coatings aren't suitable, owing to higher friction, unsuitable processing temperatures and so on. Tool coatings that are developed to work at high temperatures during metal cutting often require high deposition temperatures.

The process temperatures required for many PVD coatings are greater than the temperatures that can be withstood by some materials used for race engine components, and motorsports have played a key role in developing lower temperature processes to suit steel materials in particular, which have low-temperature final tempers. Such materials can be 'let down' or softened by some coating process temperatures, and are ruined as a consequence, so beware of blindly specifying a process if you are unsure about process temperatures.

However, the automotive industry has been instrumental in developing new coatings that suit our needs rather than those of the machine tool industry. Low-temperature, low-friction coatings, which are perfect for most automotive applications (including many engine applications), are suitable for racing. New-generation PVD coatings include some amorphous carbon coatings and low-friction coatings involving molybdenum disulphide (MoS2). MoS2 can be applied by other means but has very poor adhesion, but the coatings applied by PVD have much-improved adhesion.

Fig. 1 - A typical application of PVD coatings for race engine applications is TiN on poppet valves

Written by Wayne Ward

The job of those working with internal combustion engines is made simpler by the fact that the service temperatures for most fasteners are moderate. Moreover, the environment in which the fasteners operate - certainly in terms of the chemicals present - is quite benign. Most coated fasteners are zinc-plated or chromium-plated, or to a much lesser extent recently, cadmium. Cadmium is generally frowned upon nowadays owing to environmental concerns, and processes involving chromium are increasingly under the spotlight, as certain forms of chromium are also judged to be bad for the environment.

There are certain applications however where temperature is a real concern. Exhaust systems are the main example, especially where turbochargers are employed. The use of standard steel fasteners can lead to seizure and breakage, as was discussed in a previous RET monitor article. It is also common to use a fastener in a high-temperature material that also employs a coating material which will not degrade at elevated temperature.

It is normal to use a stud-and-nut combination for exhausts. There is less imperative to use a high-temperature material for the stud as it is generally well cooled at one end, being installed in a comparatively cool cylinder head. The nut material needs to be chosen such that it will not weaken in use due to high temperature.

The plating chosen is commonly silver. Silver-plated fasteners are widely used in industry where lubricant cannot be used or where fasteners are assembled into materials where galling is a serious risk. Silver plating is very soft and has very low shear strength, and so acts as a solid lubricant, lowering the coefficient of friction where it is used dry. This means that less torque is required to produce a given level of preload, and this means that a stress is induced in the fastener during tightening. Silver also has the advantage in that it doesn't oxidise significantly at temperature, making it ideal for exhaust use.

Silver-plated nuts are also used in some applications where high loads make it difficult for lubricants to remain in highly loaded parts of the contact. In these instances a low-shear strength coating such as silver can be beneficial. Such is the effectiveness of silver as a lubricant that there are a number of silver-doped pastes and greases available commercially.

Silver-plated nuts are widely available commercially from motorsport and aerospace outlets, in free-running and self-locking styles, but if these aren't available or you need something special, industrial silver plating is also widely available from surface treatment specialists. Industrial silver plating is not however the same process as the treatment you might see on cheap jewellery, as you can see from the accompanying picture.

Fig. 1 - These 12-point nuts are silver plated

Written by Wayne Ward

The aims of this are generally to produce an improved coating in terms of one or more parameters. It would be considered a worthwhile step forward in coating technology to improve adhesion, reduce the coefficient of friction, minimise wear rates or raise the upper temperature limit at which such coatings will satisfactorily operate without degradation.

In the previous article on surface treatments, ion implantation was discussed. Besides the benefits that this might bestow on a component by virtue of altered composition of the surface or the compressive residual stress it can impart, an important development has been the improved adhesion of coatings which has been observed when followed by 'ion mixing' treatments.

One company has taken the process a stage further and uses an ion implantation process before DLC coating in order to achieve better results. Systec has developed a new family of DLC coatings which offer improvements over other DLC coatings, and I spoke to the company's Val Lieberman about the processes involved and the results.

I asked how the ion implantation stage affected the performance of the subsequent DLC coating, and Lieberman's reply was, "Primarily, the implantation provides better adhesion", but he added, "A side effect is that it increases the load-bearing capacity of surface layers due to the 'hardening' effect of the implantation."

This improved adhesion is borne out by test results on each of Systec's coatings, all of which have improved adhesion compared to competitors' DLC coatings as measured by LC2 critical load testing. LC2 results show the load at which coatings show delamination.

Lieberman pointed to the coating process as being critical in this improvement, saying that the HIPIMS (High Power Impulse Magnetron Sputtering) process produces coatings virtually free from pinhole defects which, he says, are "local coating delaminations" typical in many DLC processes. He suggested that these defects "may act as islands, where local increase in contact temperature may trigger thermal degradation of the DLC".

The three coatings from Systec offer a range of properties, with the hardest and most wear-resistant showing good frictional behaviour (with equal or lower friction than a competitor's DLC) combined with a wear rate as little as 1% of that of another DLC.

Surprisingly, these improved coatings are produced at a lower cost compared to competitor coatings "due to higher processing yields and scaled-up processing," says Lieberman. The coatings are still a step above something like CrN in terms of cost though.

These coatings are being used successfully in motorsport valvetrains and other race engine applications.

The processing temperature is quite low, being in the 200-230 C range, and is suited to any material that is stable at these temperatures, which would include most steels (apart from those tempered at lower temperatures than here, such as most carburising steels), titanium alloys and possibly some aluminium alloys aimed at high-temperature service.

Fig. 1 - This NASCAR camshaft has been coated with a new generation of DLC; the process incorporates ion implantation (Courtesy of Systec SVS Vacuum Coatings)

Written by Wayne Ward

Many years ago, engines were usually equipped with cast-iron bores, and aluminium cylinder blocks were likewise equipped with cast-iron liners, either of the dry or wet type (referring to whether the outside of the liner is in contact with the water in the cooling system). Nitride-hardened steel or iron liners were used for a while but, more recently, and especially with the widespread use of the linerless aluminium cylinder block in series production roadcars, it has become common to use coated bores. Liners can thus be produced in aluminium, offering a weight saving for race engines still fitted with liners.

There are a number of suitable candidate coatings here, some of which are commonly used in race engines.

The most traditional of these, though by no means the most popular, is 'hard' chromium. As the name suggests, hard chrome is a chrome-plating process which has a very hard surface, in the region of 65-70 Rockwell 'C'. The process is electrolytic, allowing reasonably thick deposits to be applied, certainly much thicker than is common with decorative chrome-plating processes. After plating, the surface is honed to produce the correct surface finish.

There are a number of proprietary variations on the basic chrome-plating process, with some coatings specifically developed for bores to retain oil. These have been in common use for more than 40 years, and were among the first to allow aluminium liners to be used in place of cast iron or steel.

For race purposes, the most common family of coatings are 'composites' based on nickel, with hard particles of ceramic (generally silicon carbide) embedded in the metallic matrix. These were also introduced more than 40 years ago, originally in response to the poor wear characteristics of rotary engines which were more popular at the time for automotive use. The honed surface finish leaves hard particles of ceramic at the surface that help with wear resistance.

Although these processes are electrolytic they can, with appropriate measures, be applied to linerless cylinder blocks for race purposes.

The alternative is to use a sprayed coatings, and in 2008 these were reported in RET magazine (Issue 31) as being used at the time in both Formula One and sportscar racing. Again based on a composite metal coating with ceramic particles in a metal matrix, this process is thought to offer more opportunity to tailor the coating to the exact application.

Fig. 1 - The Bugatti Veyron has plasma-sprayed cylinder bores (Courtesy of Sulzer Metco)

Written by Wayne Ward

In terms of the eliminating surface hot spots, there are obvious advantages with being able to do this. There are many materials used in or around a race engine that experience a significant drop in mechanical and fatigue properties with increasing temperature. If we can achieve a better temperature distribution over the surface, we could avoid there being a premature failure at the hot spot.

Any steel with a low tempering temperature is an example of a material where a local hot spot could start serious damage. Some tool steels and most carburised steels temper below 200 C (392 F). Many of the aluminium alloys commonly used in race engines are aged in the 130-200 C (266-392 F) temperature range, so these may also benefit from the mitigation of local areas of high surface temperature.

It is on cooling systems, however, that these thermal dispersion coatings are most often used. In the case of the major elements of a cooling system, such as a water radiator, oil cooler or charge-temperature cooler (intercooler), there are efficiency gains to be made from having a more even distribution of cooling across the whole area of the cooler.

The net effect for a given area of cooler is that the maximum temperature in the system is lowered. In the case of the water system, a lower temperature can mean that the engine produces more power.
There can be unintended consequences of lowering water temperature in an engine, especially in bespoke engines where the piston-to-cylinder clearances may be tight. But we can use this increased cooling capacity to the benefit of the car and to make lap-time improvements.

In one case of a team using this coating, they recorded a significant water temperature decrease - 10-12 F for a water temperature of about 230F. The team subsequently blocked the face of the radiator so that the cooling airflow was reduced, and the previous water temperature was restored. This has an aerodynamic benefit that improves lap time.

The full re-optimisation of the system would include reducing the radiator area (with attendant reduction of mass) and smaller cooling ducts. The requirement for smaller sidepods on a single-seat car would be gladly welcomed by the aero department!

These coatings can also be used to good effect on oil pans (sumps).
One point to consider is the requirement for the coating to be cured at a temperature of 150 C (300 F). This may mean that the ageing or tempering temperatures of some materials are exceeded.

Fig. 1 - This water radiator has been treated with a thermal dispersion coating (Courtesy of HM Elliott)

Written by Wayne Ward

In wet-sump engines the control of oil has traditionally been looked after by devices such as gauzes, scraper plates and windage trays and, if we needed further proof that racing engines improve road engines, many modern roadcar engines incorporate such devices to minimise friction. We can therefore say these devices are effective.

But there have also been other 'tricks' used to lessen the effects of friction, and one of these is to use oil-shedding coatings. These have been around in one form or another for a long time. Many years ago, they came in the form of polymer coatings which I have seen liberally applied to the insides of some racing engines. Now, there are specific oil-shedding coatings (also referred to as oleophobic coatings) that are applied to the internals of the engine, specifically crankcases, crankshafts and con rods, although there are certainly other applications where we might find a real gain.

If we are to ignore the adverse frictional effect of splash lubrication due to a crank skimming through a reservoir of oil, where else might we find frictional losses due to oil? The answer lies in the effect of oil shearing in confined spaces as high-speed surfaces sweep past. If we imagine oil trapped in a gap between two rotating plates - for example, the thrust faces on the crankshaft - there is a certain amount of force needed to shear the oil, and this is proportional to the viscosity of the oil and the area of oil being sheared, and inversely proportional to the gap between the surfaces.

So, from this knowledge, we can easily work out where our losses due to oil shear are going to come from, namely parts of the engine where there are small gaps between components with high relative speeds. The crankshaft is the focus of most of our attention - the crank counterweights sweep past the stationary crankcase walls within a few millimetres, and past the con rods, also in close proximity. In designing an engine we can try to provide greater clearance in order to lessen the shearing rate, but for fixed designs we are limited to our attempts to keep oil away from these sensitive areas.

Oil-shedding coatings are a way of doing this, and the results seem impressive. I recently spoke to Jack McInnis from Dart Machinery, a company providing a coating service to the motorsport industry. I asked McInnis what sort of gains could be expected due to oil-shedding coatings and his answer was, "We often see increases of nine to 15 horsepower above 5000 rpm when both the crank and rods are coated in a performance engine."
These results alone suggest that the coatings may be worthy of investigation, especially in an engine where the oil flow rate in the crankcase is high.

Fig. 1 - Crankshafts and con rods are among those components considered most suitable for the use of oil-shedding coatings

Written by Wayne Ward

This month we shall look at some of the past and present applications of polymer coatings in racing engines besides piston skirts, although there isn't room here to cover all the applications.

Staying with pistons for the moment, however, polymers were used briefly by some manufacturers when DLC-coated gudgeon pins (piston pins/wrist pins) started to become popular. Some early coated pins tended to wear the pin bore in the soft aluminium piston. Whether coating technology or preparation was to blame at the time is not certain, but one successful solution was a surface treatment that involved a polymeric component. An electroless nickel/PTFE 'composite' coating was one company's way out of this problem. DLC-coated gudgeon pins nowadays run happily in many racing engines.

Polymer coatings were once seen as a cure-all by some engine builders, and you might still find polymer-coated connecting rods in some engines that have not been rebuilt for many years. The idea behind this is to promote oil shedding in the hope of lower friction, but the practice seems to have fallen out of use. In a similar vein, I have seen inside dry-sump racing engines originally put together in the 1970s to find the internal cavities of crankcases, especially the lower crankcase (sump), coated with the a green polymer compound.

An example of where we might find such 'hopeful' applications of polymer coatings by privateer builders is the widely used Ford/Cosworth DFV, as shown in the picture here.

Coating the outside of aluminium (and sometimes steel) cylinder liners with polymers has been popular for many years, and many companies continue to do this. The reasons are to prevent damage to the liner or cylinder block during fitting and to minimise the effects of fretting damage between the components, possibly by eliminating cold-welding between similar metals. In any case, even if the polymer coating's wear rate is relatively fast, replacing the coating is generally less expensive than replacing the cylinder liner.

In the case of a steel or cast-iron liner in a water-cooled engine, a polymer coating can prevent corrosion of the part. It can also prevent galvanic corrosion, where dissimilar metals are in contact in the presence of an electrolyte, becoming a problem.

Coating pump components - both rotating and static - with polymers has been common for many years.

Engine builders have also used polymer coatings as a tentative first step toward material substitution, to convince themselves that by swapping from a metal to a polymer component, the wear of the polymer won't be an immediate problem.

Fig. 1 - The Ford/Cosworth DFV has seen privateer builders using polymer coatings in the most unlikely places

Written by Wayne Ward

Looking at the thrust faces of the big end of the rod, many people choose to run without any form of coating. If the lubrication regime is such that wear is not a problem, then this solution is a good one, and certainly economical. However, for those who perhaps want to run the absolute minimum thrust face width on their connecting rods, or those who need their rods to have the best endurance in terms of wear, it has been common practice to coat the thrust faces of the con rod and this has traditionally been a metal-sprayed coating of a molybdenum alloy, although other materials have been used for the purpose. Although the metal-spraying process is still in use, it has, to some extent, been overtaken by the rise in the modern hard coatings. Chromium Nitride (CrN) is a popular choice for rod thrust faces, and this is applied only a few microns thick. We should note that care has to be taken with the surface finish of hard coatings as they can, if too rough, act as very effective cutting tools, damaging the components which they slide against.

Remaining with the big end, some companies who I spoke to reported that they thought that it would be possible to dispense with the big end bearing and run using coatings applied to the bore of the con rod. DLC was mentioned for this purpose, but we don't yet know of any successful applications of this technology. It would offer some significant advantages in terms of con rod design, and would also give the fastener an easier life too, as the bending stresses associated with the off-centre loading would be reduced as the fastener moves toward the big-end axis.

Looking at the small end of the con rod, there are a number of applications where a coating is applied. Where the con rod is piston-guided as opposed to being crank-guided, the same CrN coating could be applied to the thrust faces. Where two bronze 'top-hat' bushes have been used in these circumstances, the single-piece plain bush can be used, with the coating taking the thrust loads. In some cases, bronze 'top-hat' bushes have been replaced with coated steel items. In the case where piston-guided con rods have managed to dispense with the bush altogether, the modern hard coatings are ideal for taking thrust loads.

For titanium con rods which use a pressed-in small end bush, there exists the problem of galling when the bush is pressed into place. The resulting surface damage may not be detected until the con rod has failed, and for these reasons people have tried coating both the small end bush and the small end bore in the con rod to prevent this problem.

Fig. 1 - Connecting rods are often coated on thrust faces to prevent premature wear (Courtesy Arrow Precision Ltd)

Written by Wayne Ward

Phosphate is itself not a substance, but a negatively charged ion which may be found in solutions in a number of forms. The form which we might remember from chemistry (if we haven't used selective recall to blank chemistry from our memory) is PO4, which comprises one phosphorous atom and four oxygen atoms surrounding it. It has a charge of 3-, and will therefore combine with three positively charged ions with a charge of 1+, as with hydrogen to form phosphoric acid. Therefore, when we talk of phosphate coatings, we are really talking of a compound involving one or more phosphate ions and a positively charged ion, which in terms of engineering coatings is a metal ion. Zinc, manganese, iron and calcium phosphates are used as engineering coatings for a number of applications, and here we will discuss those that are relevant to engines.

The coatings are conversion coatings, which involve a chemical reaction of the component surface in a solution containing salts of the phosphate compound in question.

Iron Phosphate is widely used as an undercoat or primer for painting and so is unlikely to be extensively used for racing engine components, unless as a pre-treatment for paint where external surfaces need to be protected. An example of a racing application might be a cylinder block on an engine which we don't expect to regularly remove for service.

The combination of oil and zinc phosphate can give a very useful degree of corrosion protection to components, and in this case the phosphate coating is used as an undercoat for oil. As we are aware, oil will run and drip off a component, and so any protection against corrosion is generally temporary and short-lived. Zinc phosphate itself is not a corrosion inhibitor, but its ability to absorb and retain oils is what gives this combination good corrosion inhibiting properties. For racing components it is also used in some circumstances as an undercoat for other surface coatings, including organic and polymeric coatings which might not have sufficient adhesion to the metal on their own. Fasteners are a common application of this finish (see picture).

Manganese Phosphate often used as a surface treatment to aid running-in is quite commonly applied to cam followers and other components for this precise purpose. Pistons are another application of these coatings and a number of suppliers of racing pistons specify these coatings, where they feel that, in addition to improved running-in behaviour, they offer some protection against wear damage at start-up, especially in the piston pin bore and piston ring grooves. Many of the grey-coloured pistons that we see in the cabinets of piston manufacturers at trade shows have this phosphate coating applied. Some companies supplying pistons with coated skirts first apply a phosphate coating to promote adhesion of the skirt coating.

Fig. 1 - A couple of examples of phosphated fasteners.

Written by Wayne Ward

Quite often, most especially for high-strength steels, there is a serious disadvantage to electroplating, and that is the phenomenon of hydrogen embrittlement. Hydrogen gas is liberated at the surface of the cathode, i.e. the part to be plated, and this rapidly diffuses into the surface of the part, causing it to become brittle. This can be overcome by the use of a low-temperature post-plating heat treatment process which reverses the process of hydrogen diffusion into the surface layers. As an unqualified engineer some years ago, I was unaware of hydrogen embrittlement, and decided to send some motorcycle parts to be made pretty with zinc. I was soon to discover the error of my ways when my side-stand spring broke very soon afterwards. The same mistake is very often made on a much more grand scale.

The second method of plating with nickel is known as electroless nickel plating, and as the name suggests, it does not rely on an applied electric current, but is a chemical reaction. It has an advantage in that the parts to be plated do not need to be electrically conductive, and so the process can be used on polymers, composites and ceramics etc. It is quite widely used to produce prototype representations of metallic parts by plating something made from polymer castings or parts produced by rapid prototype methods. Parts treated with electroless nickel platings have been known to suffer from hydrogen embrittlement and so for cyclically stressed parts, we need to take the same precautions as we would for parts using an electroplating process. The post-plate heat treatment is considered to improve adhesion of electroless nickel; the reason for this is felt to be reduction of internal stresses within the deposited nickel. It is possible to heat-treat the nickel plating to produce a very hard surface, but you must be careful that the heat-treatment does not otherwise compromise the existing material, i.e. by temper-softening it.

There are a number of variations on electroless nickel, and some coatings which are described as 'composite' coatings. These don't refer to the substrate, but to the face that it is possible to have embedded in the nickel matrix particles of other materials. There are a number of electroless nickel treatments which have additions of PTFE (Teflon) added for lower friction and several with ceramic particulate reinforcements, such as zirconia and silicon carbide. Although probably very expensive, electroless nickel composite platings containing industrial diamonds show good promise as wear resistant coatings. The accompanying picture shows the microstructure of an electroless nickel composite containing diamond.

Fig. 1 - Microstructure of an electroless nickel composite containing diamond.

Written by Wayne Ward

In terms of coating ‘hard’ components, the new generation of coatings including TiN, TiCN and DLC are now very popular for racing use, and the application has begun to spread to high-volume series production road cars with a view to friction reduction and higher levels of efficiency. A number of people have been experimenting with DLC coatings on piston skirts for some years, with Formula One applications leading the way. Those Formula One engine manufacturers who had brought the technology to a sufficient level of maturity before having to homologate their engines are now able to reap the benefit of their early adoption of this technology. As with many new technologies, development of DLC coatings for pistons relied on these early adopters to bring about the possibility for more widespread use.

One of those companies who we spoke to is now in a position to offer DLC coated pistons to their customers, having developed the coating over a number of years. Cosworth, based in England, are involved in the supply of pistons for a wide variety of applications from tuned series production vehicles to Formula One. Discussing the matter of coatings with Cosworth’s Andy Pascoe and James Simester, they stated that the premium for DLC coatings is likely to be significant initially and that only part of the extra cost is due directly to the coating process. Development of the piston machining to accompany the coating has been a necessary part of the process, and the machining time on each piston is increased as a consequence. Having been able to examine a number of DLC coated pistons in recent years; the difference in machining is very noticeable in comparison with uncoated pistons.

However, the benefits are also significant, despite the extra cost. Asked about the increased efficiency of DLC coated pistons, Pascoe said, “Its coefficient of friction is excellent and outperforms everything else available currently,” before going on to state that, “it also achieves notable increases in piston life and peak power.” He further suggested that the coating enjoyed “unparalleled” wear resistance, presumably being part of the reason for increased piston life. Cosworth already plan to supply this to a number of teams racing in various championships in 2010 in both car and motorcycle applications.

So, the DLC coated piston is now available (albeit at a premium) to us all, and despite this extra price, may well work out less expensive due to increased piston life.

Written by Wayne Ward.

Polymer coatings are much more tolerant of deformation of the underlying material and, owing to their low elastic modulus, are capable of withstanding a significant amount of deformation themselves without failure. When compared to the new generation of hard coatings though, they are not capable of withstanding a high level of contact stress, especially where there is relative movement between components, i.e. sliding. Where there is significant sliding, even at quite modest levels of contact stress, wear ensues causing loss of material from the coating. The debris thus produced is not very damaging, being softer than almost any other component in the engine. Providing that such debris is small and isn’t produced in volumes significant enough to block filters, it is unlikely to cause substantial damage to the engine.

So, is this combination of low stiffness and low resistance to contact stresses a useful combination? Well, yes it is and this is the reason why it is still widely used in racing engines today. One application of polymer coatings is on piston skirts, to determine the optimum profile. The 1982 SAE paper on the subject of piston profile design details a method by which development time is cut by producing a piston skirt coated in a polymer-epoxy composite and running this in an engine. The coating material wears away where the contact stress is high, and remains in good condition where the contact stress is low. From examination of uncoated aluminium pistons, we know that the surface stresses are reasonably low. The SAE paper says that the worn profile of the epoxy-polymer composite coating can be reproduced on an aluminium piston and that further development of the piston skirt profile is markedly reduced in terms of time and expense. These days the computer-based simulation involved in piston design that is available can replace the trial and error development of skirt profiles or even the accelerated development of the method described above. However, not everyone developing a new piston design has the resources or capability to undertake this kind of simulation. Clearly, for a production engine, using a piston designed to wear into the optimum profile is not acceptable. However, for racing engines, where the engine is stripped and examined regularly, pistons with skirts coated with a compliant coating which can be used, and a number of racing piston manufacturers offer pistons with such compliant coatings, and these are used in many formulae.

Written by Wayne Ward.

Corrosion
- Material incompatibility
- Friction
- Lack of suitable lubrication

In terms of addressing the problem of corrosion, the main candidates come from a list of metallic platings, although polymeric alternatives exist. The common types of plating aimed at limiting or delaying corrosion are ones that we are probably all familiar with, such as zinc or cadmium. Cadmium plating has, mainly for environmental reasons become less prevalent in recent years. Zinc plating, (often called galvanising) is still very common and can be done by your local plating shop, although you must be very cautious before considering this. There is a thermal process which must be undertaken after any electroplating process (discussed in more detail in the Fasteners Focus article in Race Engine Technology issue 41) immediately after plating if brittleness and early failure is to be avoided, particularly with high-strength steel fasteners. ‘Mechanical Zinc’ coatings do not suffer in the same way with this problem of embrittlement. There are sprayed metallic coatings and some which are either water-borne or solvent-based which again will not be subject to concerns over embrittlement. Plated fasteners offered by reputable manufacturers of fasteners will have had the thermal treatment done (the reputation of the fastener manufacturer relies on this process having been done) and you can rest assured that embrittlement should not be a problem to you. A number of polymeric alternatives exist and these are becoming more popular but are often an expensive option compared to more established metallic plating methods. ‘Black Oxide’ or ‘Chemical Blacking’ as applied to steel fasteners gives only temporary protection against corrosion, and phosphating may be a better alternative here.

Material incompatibility problems occur where reactive fastener materials are used in combination with identical or similar materials. The microscopic cause is the cold-welding of material in the threads between the male and female parts. Stainless steel is a good example of a material where this is a serious concern. A stainless bolt wound finger-tight into a tapped hole in the same or similar material may become immediately and permanently seized without any significant load ever being applied. Titanium is also afflicted by this problem known as galling. In the case of galling, there are many options available as far as coatings are concerned. Metallic platings are often used, and industrial silver plating is often used in this application. Other solvent and water-based spray coatings are also popular, which are often based on traditional lubricating substances such as molybdenum disulphide or PTFE. The new generation of PVD and CVD coatings are also popular for these applications

In the theoretical relationship between torque and pre-load, we find that friction plays a large part, and reducing this can have some advantages. Clearly we generally apply a thread lubricant of some description, but sometimes this may not be possible or may give inconsistent results. In this case we may want to apply a low-friction coating, and we find that most of the aforementioned platings and coatings can be used for this reason. Electroplated zinc fasteners for example are quoted to have a coefficient of friction around 25% lower than otherwise identical steel fasteners. We should be careful to re-evaluate tightening procedures if using low-friction coatings. Failure to do so may result in yielded fasteners or stripped threads.

Lack of suitable lubrication is often a problem where fasteners are used in hot environments. The obvious example here for us are exhaust manifold studs, where we will often find silver-plated nuts being used, and these are commercially available from various sources for this and other applications in industry.

Written by Wayne Ward.

One area where ‘traditional’ DLC coatings have struggled is in applications where temperatures are significant. The DLC coating begins to degrade at quite a low temperature in comparison with other coatings, such as titanium nitride (TiN) for example. Many of the more common and traditional coatings were developed years before DLC and were used initially as tool coatings for the manufacturing industry. Common examples of this are coated shear and press tools for sheet metal manufacturing and, of course, the almost ubiquitous coated drill bits which we now find. Milling tools are another popular application of coatings, both on traditional cutters and those with interchangeable machining inserts.

The types of coatings that we find on milling cutters are suitable, in some cases, for applications where temperatures exceed 1000 degrees C (1832 degrees F). Widely available data from coating suppliers shows us that those coatings based on aluminium chromium nitride (AlCrN) can withstand temperatures of 1100 degrees C. However, data from the same source states a maximum temperature of 300 – 350 degrees C (approximately 570 – 660 F) for the carbon based coatings such as DLC and tungsten carbide/carbon (WC/C). Unfortunately, with those coatings such as AlCrN, we find that the coefficient of friction is not as low as the DLC coatings.

You might ask what applications, besides those in the exhaust flow or combustion chamber, experience temperatures in excess of 300 degrees C in an engine. Where very high contact pressures are present, in combination with high sliding velocities, the temperature developed in the contact can become very high, and this can lead to degradation and failure of the coating. These ‘flash temperatures’, if excessive, can lead to tempering and consequent decreases in mechanical properties of the underlying material. In addition to the very welcome decrease in measured friction, the avoidance of high flash temperatures in heavily-loaded sliding contacts is one of the reasons why coatings were used in these applications in the first instance. For example, finger followers, used with the extreme valve opening accelerations in Formula One, will not survive without low-friction coatings, and the limitation on the coating is due to flash temperatures in the contact area. So, there is pressure on coating suppliers to develop low-friction coatings which have a higher service temperature limit.

Of course, there are applications where we can expect temperatures to exceed 300 degrees C. Valves are an obvious example, and on these components we might find coatings such as TiN and chromium nitride (CrN) much more commonly that we would find DLC, although some coating suppliers now coat valves with DLC. The area of valves which are most commonly coated, if they are not coated all over, are the valve seat surface and the stem. Valve seat coatings can help prevent wear of both the valve and the seat insert. We should note that, for coated valves, we should not lap the valves to the head in the traditional method, because we can easily destroy the coating that we have just paid for.

Written by Wayne Ward.

The next most popular application, and the one that the automotive manufacturers seem to be very keen on, is on valvetrain components. Where engines are used mainly at part throttle, the friction from the valvetrain can be very considerable and the mitigation of frictional losses is seen as a key area of research. In racing the frictional losses are equally important, possibly even more so. I doubt that there is an engine manufacturer in Formula One who does not use DLC coatings on the valvetrain components. Certainly its application to finger followers is one of the reasons that people have been able use the very high contact pressures and velocities which have been common recently. In formulae not using finger followers, the application of DLC to other types of cam followers is now very common, and the gains in reduced friction, especially when the components are new, is easily measurable. A more expensive option, that some manufacturers choose to use, is to coat both the camshaft and the follower. This is supposed to offer further advantages over simply coating the follower alone.

The application of low-friction hard coatings to gears is not new, and it was pioneered in non-automotive applications, specifically helicopter gearboxes, where the extra seconds of component life in an oil-out situation is the main gain rather than the slightly reduced friction. Some people have experimented with coated gears in motorsport, although it is not a popular application.

The application of DLC to crankshafts in Formula One has been tried by several engine manufacturers, although it has not been taken up by all with equal enthusiasm.

One note of caution about DLC coatings for those with no experience who might be tempted to try them – speak to your coating supplier and seek their advice on the correct application, substrate and steps (possibly including other coatings underneath the DLC) to get the best results. Its application to unsuitably prepared parts, unsuitable materials, or correct materials in the wrong condition can lead to poor results and damage.

Written by Wayne Ward.

In the 1980’s and 1990’s US defence budgets encouraged the development of a range of coatings of all sorts and the solid lubricant found a use for example in a NASA project for a fully recirculating, non-air breathing engine.

A Wankel unit was constructed and ran successfully: rumour has it that it ran on a continuous cycle for several days with no liquid lubricant at all. Because it was intended for use at very high altitude, cooling wasn’t a problem, but the components weren’t exotic materials.

Eliminating the windage losses caused by an oil/air mixture can have significant benefits especially in small engines but these pale into insignificance at the gains realised by eliminating pressure and scavenge pumps and the weight reduction of no longer having to carry lubricant sloshing around in a high performance engine.

Solid lubricants do allow engines to run hotter, since the concern of lubricant breakdown at raised temperatures no longer applies. Recent improvements in low friction coatings and the use of nano-platelet ceramic coatings might allow serious improvements in MotoGP engines, for example. With rotational parts using solid lubricant coatings, zero co-efficient of thermal expansion liners and pistons using metal matrix composites and with very few parasitic power losses, percentage power gains could be useful. By applying similar technology to transmissions, further gains could be found.

The technical challenges of lubricant coatings are considerable: the main direction has been to use various deposition techniques for low friction materials, notably diamond like and tungsten derived materials. The solid lubricant technology ranges for the relatively simple application of molybdenum disulfide where the surface to be coated is machined and the molybdenum applied by spray deposition. The application of solid lubricants has been achieved by electro-deposition although the original work has often also been applied through a spray deposition process.

Solid lubricant coatings include molybdenum disulphide, tungsten diselinide or niobium diselinide or silicon nitrides. Nickel boron nitride remains a staple lubricant coating (as does metal bonded chrome oxide) and is used as a simple aerosol spray in die casting, but most coatings use plasma sprays or ivd techniques.

Whether the halcyon materials development days of star wars will ever return is debatable; in some respects that era was a real ‘swords into plough shares’ exercise that is unlikely to return. Coatings generally were one of the beneficiaries, but ultra-high fibre volume metal matrix composites, engineered ceramics and similar materials science work was equally encouraged. Much, if not all, is in the public domain but it remains a little known and little used resource. With the emphasis now moving towards smaller engines delivering high efficiency, those archives may merit close attention.

Written by David MacDonald.

Although depositing amorphous carbon (DLC) has been available now for many years, the improvement in deposition techniques has been more recent and this has reduced the coating temperatures very considerably. The benefits in terms of reduced frictional losses even on valvetrains have been dramatic; gains of up to 8 bhp on Moto-GP engines as a result of coatings on cams and followers have been achieved and the use of these coatings on race engine parts generally as well as in transmissions is now normal.

The range of materials available for vapour deposition onto finished substrates is now quite wide. Where the objective is simply a reduction in friction, some of the nano-platelt techniques using tungsten for example give excellent results, while amorphous carbon have additional properties that can be useful: electrical insulation for example. Coatings make possible continual shaft torque analysis that can be a significant input in engine and transmission control technology. But coatings have a much longer and wider history of use than advanced technologies.

For many years molybdenum disulphide coatings have been used to reduce or eliminate frictional damage. The side plates of the Norton rotary motor cycle and aircraft engines have molybdenuim coatings to allow the side seals to operate efficiently. These engines (which powered the TT winning motor cycles on the early nineties) have exceptional power to weight ratios but exhibit complex and difficult wear patterns that the MS coatings resolve satisfactorily.

However, coatings are used for quite different reasons than simply to reduce friction. Sprayed ceramic coatings or various sorts have been in use to reduce exhaust manifold temperatures (Sperex was a well known name that was almost universally used on primary exhaust pipes in the 1960’s and 70’s although it became perhaps better known for the cars it sponsored) and in some cases ceramic coatings protect composites from very high temperatures.

Costs for the various DLC-type coatings have fallen dramatically as the techniques become widespread: in industry, coatings for extrusion dies are now pretty well standard and this has reduced overall costs throughout the industry.

In terms of improved efficiencies, the use of coatings has had a disproportionate impact on engine technology. The provision of smoother surfaces can improve gas velocities and reduce boundary layer turbulence effects. Reduced frictional losses in themselves are important but the concurrent tribology gains have additive effects: lower friction means less oil having to be pumped through the engine, the ability to withstand higher temperatures reducing cooling requirements especially for oil systems. Even oil pumps benefit from modern coating technology, with reduced clearances where the ability to specify micron dimensions of deposition give significant performance gains at low cost.

Written by David Macdonald.

A lingering skepticism among hard-nosed engine builders often brought into question the actual value of something like a ceramic coating on a piston crown, or the inside of a combustion chamber. Moreover, ceramic thermal barrier coatings frequently were said to peel or flake off from the surfaces on which they were applied, further arousing suspicion. In recent years, however, several racing and motoring publications have conducted convincing “before and after” tests of performance coatings, and there is also a general perception that the most advanced teams at the highest level of the sport are now regularly using advanced coating technologies.In the area of friction reduction, for example, Teflon coated piston skirts have become very common. In addition increased lubricity, TFE also provides increased component survival time should oil starvation occur. Dry film lubrication is not limited to fluoropolymer coatings, and “wash on” thin film lubricants are also available for bearings, valve stems, buckets, and the like.Many of today’s manufacturers of high-performance surface coatings trace their origins to an aerospace background. They point to their extensive research and development programs. Today’s engine coatings are said to be easier to use than before, with some companies even offering water-based materials designed to spray on using standard HLVP equipment. All manufacturers are clear that users must adhere to proper safety precautions, particularly in regard to filters and respirators, as well as proper waste material removal.Manufacturers point out that most coating failures occur because of improper application. In particular, incomplete surface preparation appears to be the biggest culprit. Materials to be coated need to be completely decreased with a non petroleum-based solvent that will not leave even the slightest amount of residue. Aluminum surfaces, for example, should normally be blasted at low pressure with a sharp, angular media. Glass beads, or any media used for cleaning should be completely avoided, since these will close the pores on the surface of the object, thereby rendering incomplete adhesion. Sprayed thickness should normally not exceed .0015”; typically this occurs as soon as an opaque color coat is achieved. Manufacturer guidelines for baking out the coating should be followed with care, and it is wise to use a separate, calibrated, thermometer to validate oven temperature. (Baking or curing time refers to the time that the coated object is at baking temperature, not the oven cycle time.) Manufacturers say that adhering to good procedures almost invariably produces a successful outcome.

For individual engine builders, this is good news. Tight rebuild schedules, especially during the racing season, often preclude the possibility of sending out components for processing. Even if the vendor has a quick turnaround capability, there is still shipping both ways, which often makes the service non-viable. Frequently this inconvenience factor is more significant than cost when a decision to coat or not coat must be made. With these new materials and processes, the choice of applying an advanced surface coat can be made on the technical merits, without regard to schedules.