TRIBOLOGICAL STUDIES OF COATED PISTONS AGAINST CYLINDER LINERS IN LABORATORY TEST CONDITIONS

The presence of coatings and surface topography play an important role in the tribological performance of sliding components. Depending on the coating used, it is possible to reduce friction and/or prevent premature wear. However, while there may be low friction and wear resistant coatings suitable for use in pistons, some coatings may have high wear rates hindering the tribological performance of the parts by changing the lubrication regime from boundary to mixed, or by preventing additives from their intended function through chemical mechanisms. In this work, piston skirt segments extracted from a commercial aluminum alloy piston, were coated with a nickel-polytetrafluoroethylene (Ni-PTFE) coating, a graphite-resin coating and a diamond-like carbon (DLC) coating and were tribologically tested using a reciprocating laboratory test rig against gray cast iron liner segments also extracted from a commercial system. The tribological tests were conducted in commercial synthetic motor oils and basestocks at temperatures up to 120 C with a 2 cm-stroke length at a reciprocating frequency of 2 Hz. Results showed that the graphite/resin coating is a good break-in coating when used in formulated oil, but wears quickly, leading to morphological changes and subsequent changes in tribological performance. The Ni-PTFE coating showed a friction reduction in formulated oil, while the DLC coating showed little wear. However, in the basestock oil, while friction reduction was achieved through morphological changes, excessive wear of the graphite-resin and DLC coatings was observed.