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Coat of many colours

coatingsOne of the more interesting uses of coatings is to apply solid lubricants as a substitute for liquid lubricants. The solid lubricant that is used is normally a complex mixture that can include silver, molybdenum and various binding compounds.

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.

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