Ultra-mild wear mechanisms of Al-12.6 wt.% Si alloys at elevated temperature

An internal combustion engine made of Al-Si alloys should operate in ultra-mild wear (UMW) regime. The objective of this work was to understand the wear mechanisms operating in Al-12.6 wt.% Si alloys tested at 100 degrees C under boundary lubricated condition simulating UMW regime. The sliding tests were conducted on surfaces etched to protrude silicon above the aluminum surface and optical profilometery was used to analyze changes in Si morphology during sliding. Three different stages of UMW were identified. During UMW-I, formation of a discontinuous island-like tribofilm primarily consisting of zinc sulphide from lubricating oil on top of silicon particles was observed and silicon particles progressively became embedded in the matrix. A criterion for transition between UMW-I and UMW-II was developed in terms of the ratio of pile-up height to silicon height. In UMW-II, the piled-up aluminum started to wear and an approximately 100-150 nm thick continuous oil-residue layer (ORL) formed on the worn surface primarily consisting of smeared island-like tribofilm mixed with aluminum. The ORL was also supported by a sliding induced ultrafine grain aluminum layer, and consequently microstructure evolution led to a stabilized surface with lower wear loss in UMW-III compared to UMW-II. UMW-III wear rates at 100 degrees C were similar to those at 25 degrees C. (C) 2011 Elsevier B.V. All rights reserved.