Polymer coatings to reduce fretting fatigue
The widespread phenomenon of fretting is a form of wear found between pairs of components that are normally designed to have no relative movement or at least very limited amounts. We often find evidence of fretting on splines, where the degree of axial movement is small, and in race engines we also find evidence of it on mating faces between large structural components. Race engines are commonly constructed using aluminium for the structural components, and where both components are of the same type of material, fretting can tend to be worse.
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