ELEVATED TEMPERATURE LUBRICATED WEAR OF Al-Si ALLOYS
Cast hypereutectic Al-Si alloys made monolithic engine blocks is an outcome of the global drive towards lightweight vehicles. Eutectic alloys provide cost effective alternative to those expensive hypereutectic alloys. However, the tribological performances of the eutectic alloys needed rigorous evaluation and the objectives of this dissertation were to investigate the ultra-mild wear (UMW) mechanisms of eutectic Al-12.6%Si alloys, chemical and mechanical properties of tribolayers and their long term stability at elevated temperatures under boundary lubricated conditions. Sliding experiments were conducted using 5W30 engine oil on three etched Al-Si alloys containing 12-25% silicon at temperatures between 25 °C and 160 °C for sliding cycles up to 6×106 revolutions. Statistical measurements based on optical profilometer were carried out to quantify UMW damage parameters and volumetric loss. A criterion to asses UMW damage was then developed. Numerical models containing single- and multi-particle were developed to conduct a parametric study investigating the effect of different matrix yield strength and strain hardening characteristics, and the tribofilm formation on UMW damage parameters. UMW in Al-18.5%Si alloy lasted up to 104 cycles at 5.0 N load. In the eutectic alloy, three stages of UMW were identified at 25 and 100 °C at 2.0 N load, and UMW performance at 100 °C was found to be superior to that at 25 °C. At elevated temperatures, island-like tribofilm formation on silicon particles and the composition of oil-residue layer were the characteristic differences compared to 25 °C. The island like-tribofilm consisted of zinc sulphide and short chain zinc pyrophosphate, and the ORL was formed by mixing of tribofilm with aluminum matrix. The long term durability of ORL investigated for different Al-Si alloys at 100 °C elucidated the effectiveness of it in maintaining UMW. Findings from the numerical study were in good agreement with experimental observations. The parametric study showed that higher yield strength and strain hardening exponent of the matrix, and presence of tribofilm on silicon were beneficial in prolonging UMW and delayed the transition of UMW-I to UMW-II stage. The experimental findings suggested that the eutectic alloy can be used as a cost effective replacement of the hypereutectic alloys.