Necessary conditions for low friction between laser-irradiated aluminum alloy and bearing steel in engine oil

Abstract

Aluminum alloy SA162 was laser-irradiated with different scan pitches and laser energy densities to clarify the necessary conditions for low friction between laser-irradiated aluminum alloy and bearing steel in fully formulated engine oil. Friction tests demonstrated the three friction modes: no friction reduction (mode-I, µ:0.09–0.13), friction reduction after a high-friction period (mode-II, µ:0.06–0.09), and immediate and further friction reduction (mode-III, µ:0.04–0.06). Furthermore, two energy thresholds for the friction-mode transition (E'c1, E'c2) were found. For mode-II, intermediate-average-energy laser irradiation (E'c1< E < E'c2) promoted tribochemical reactions of zinc dithiophosphate (ZnDTP), suppressing aluminum adhesion and achieving friction reduction. For mode-III, however, high-average-energy-density laser irradiation (E > E'c2) reduced aluminum adhesion, resulting in immediate friction reduction. Additionally, the increased ZnDTP reactions helped minimize friction coefficient in molybdenum-dithiocarbamate-containing oil.