Micromechanisms of low load wear in an Al–18.5% Si alloy

Hypereutectic aluminum–silicon (Al–Si) alloys are being used for engine block applications. Wear mechanisms at low loads in these alloys require further study to establish correlations between microstructure and wear resistance. Previous work showed that at 0.5 N the A390 (Al–18.5% Si) alloy operated in the ultra-mild wear (UMW) regime, with no damage to the aluminum matrix and very limited damage on top of the silicon particles. This work extends the testing to an order of magnitude higher load, 5.0 N, to assess the upper limit of the UMW regime. Pin-on-disk type wear tests were conducted on chemically etched Al–18.5% Si alloy samples against steel for different sliding cycles up to 2 × 106 cycles under boundary lubrication. UMW and the transition to mild wear (MW) were characterized using optical profilometry and electron microscopy. It was shown that damage occurred in different stages. Initially, silicon particles fractured, some of which became embedded in the matrix, as the contact stress increased. Abrasive wear on silicon particles was also observed. Once the silicon particles were completely worn out, wear of aluminum matrix became the dominant wear mechanism and a transition to MW occurred. Formation of an oil-residue layer reduced wear for sliding at higher cycles.