Finding a material that fulfils all of the performance demands especially in a race engine is an almost impossible task. The result has been the growth in the use of coatings, used either to protect the load bearing core or to reduce frictional losses or both. Perhaps the best known are the various carbon and graphite coatings (often called DLC for diamond like coating) where a super thin layer of as little as 5 microns is deposited onto a finished surface using some form of vapour deposition. The results are dramatically reduced friction and extended part life, allowing the use in many cases of even difficult materials such as titanium with its tendency to gall.
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.