Applications and design considerations of anodising

Although anodising can be applied to a few other metals, the vast majority of applications in motorsport will be on aluminium. This is a reflection of the popularity of aluminium in the construction of race engines and transmissions, and the fact that anodising is not a process that is commonly applied to other materials such as titanium.

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

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

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

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

Reference

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

Written by Wayne Ward