Thermal effects on mild wear transitions in dry sliding of an aluminum alloy

Abstract

Wear mechanism maps previously developed for A356 Al in dry sliding against SAE 52100 steel [1]show the following. (1) A mild to severe wear transition, characterised by an abrupt increase in wear rates arising at a critical bulk surface temperature of approximately 125°C. (2) A gradual transition within the mild wear region between predominantly `mechanical mixing/oxidation' type wear and a `delamination' wear regime where metallic flake-like debris are produced by highly localised shear of the bulk metal at the sliding surface. The influence of A356 Al slider specimen asperity contact heating on the mild wear transition is studied using semi-empirical calculations of flash temperatures at different sliding velocities. This paper examines whether a critical temperature criterion can be applied to explain this transition. The calculations use experimentally determined load and sliding speed criteria to achieve the critical transition temperature for severe wear in A356 Al and assume a direct relationship between asperity contact size and debris particle diameters. The analysis shows that the onset of metallic flake debris formation coincides with a sharp increase in average asperity flash temperatures which exceed approximately 125°C, the critical bulk surface temperature ( T crit) at which the mild to severe wear transition occurred. The implications of a critical flash temperature criterion and tribosystem heat flow characteristics on current models of wear debris formation are discussed with reference to the thermo-mechanical behaviour of Al alloys.