Solid Lubricant Coatings for Automotive Engine Pistons

The demand for more powerful internal combustion engines with lower weight, reduced fuel consumption, and favorable environmental properties has led to engine piston designs having lower weight, reduced thermal expansion, and good thermal conductivity. These improvements were needed without sacrificing high thermal strength, reduced operating friction, and increased wear resistance. The need for reduced frictional properties resulted in pistons with shorter skirt lengths, which led to a reduction in effective hydrodynamic surface area perpendicular to the pin bore. At the same time, complex cold start cycles and fuel injection systems were developed that brought enriched fuel mixtures into the engine, causing a dramatic reduction in the viscosity of the lubricant film and thereby its capabilities to protect the sliding surfaces from wear. These higher mechanical stresses on the piston combined with the reduced lubricating ability of the oil film led to increased friction and wear rates, which often resulted in decreased customer satisfaction due to high noise (i.e., piston “slap”) or complete engine failure. To counteract these tribological problems, a variety of piston coatings were developed, but the composition of these coatings often brought a new set of environmental concerns, e.g., the use of heavy metals. The current solution for this problem is to utilize Anti-friction Coatings (AFC’s) on specific areas of the piston. These coatings are based on solid lubricant technology combined with that of high performance resins. The coatings are resistant to fuels and engine oils over the normal operating temperature range of the engine. They also provide a long-term lubricant coating with exceptional capability to handle intermittent excursions outside an engine’s normal duty cycle.