Polymer Coatings in Racing Engines
The use of polymer coatings in racing engines has been widespread for many years, but is to some extent being displaced by the newer generation of coatings such as titanium nitride, chromium nitride and DLC as examples. In a lot of circumstances the newer coatings are an improvement, but the coatings rely on the fact that there will be no significant deformation of the coated surface, otherwise the thin coating, which itself is brittle in comparison to the underlying substrate, can craze and flake off. Debris thus caused can, in some circumstances, go on to cause more serious failures elsewhere in the engine. Hard coatings are also capable of withstanding very high levels of contact stress.
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.