Tribological behaviour of alternate hypereutectic Al–Si alloys with different antiwear additives

Abstract

Two alternate hypereutectic Al–Si alloys, varying in composition and processing route, were investigated alongside their commercial equivalents, with two different lubricant antiwear chemistries. A reciprocating sliding wear test was used with a steel nitrided piston ring on substrate configuration. The conditions (average load and speed) replicated those seen in a fired gasoline engine. The oil containing zinc dialky dithiophosphate (ZDDP) antiwear gave the best antiwear protection, determined from the lower specific wear rate and friction coefficient with all alloy types. A zinc–phosphorus–sulphur based protective film was found in the wear track. The poor performance of a zinc free active phosphate ester (ashless antiwear additive) was thought to be due to no phosphorus based film forming. The presence of a calcium based film, originating from the detergent, suggests that it had a higher film forming affinity in the competitive surface interactions. The commercially termed Alusil (A390, Al–18·5Si alloy) is currently used as cylinder material in various gasoline engines. This reference material had a different microstructure to that produced by rheo-die casting. The high shear rate during the rheo-die casting process produced a finer microstructure with smaller but more numerous primary and secondary silicon particles. The increased surface area was envisaged to increase the wear resistance and evenly distribute the applied load. However, the reference Alusil gave considerably better wear performance with all oil types tested. The wear track microstructures showed material loss, void formation with fracture, fragmentation and sinking of the primary silicon particles into the aluminium matrix. The damage was controlled to an extent by use of the ZDDP oil with an organic friction modifier. An uneven wear pattern termed stick slip was occasionally observed. It was found not to be dependent on substrate alloy or oil type. The only differences were greater fluctuation in friction coefficient during the running-in period, higher overall wear track hardness and striated lines on a considerable number of fractured primary silicon particles.