Running-in for Low Friction between Silicon-Containing Aluminum Alloy and Bearing Steel in Engine Oil

Abstract

The friction properties of an aluminum-silicon alloy (Al-Si) can be improved by increasing its silicon content. However, this can deteriorate the material properties necessary for engine components. Therefore, an alternative approach to reduce friction is required. This study investigated the running-in behavior, the reduction of friction from the initial to the stable phase of sliding, between Al-Si with varying silicon contents and bearing steel in fully formulated engine oil. The friction of Al-Si and the silicon-free aluminum alloy (Al) was classified into three modes: no friction reduction (mode I), friction reduction after a high-friction period (mode II), and immediate friction reduction (mode III). Al exhibited only mode I, low-silicon-content Al-Si mainly displayed mode II, and high-silicon-content Al-Si mainly exhibited mode III. Energy-dispersive X-ray spectroscopy revealed that, for mode II, zinc dithiophosphate (ZnDTP) reacted with the aluminum matrix, suppressing aluminum adhesion and forming molybdenum-dithiocarbamate-derived tribofilm. Nanoindentation and micro-Vickers tests demonstrated that the initial high hardness of the Al-Si matrix did not increase the ZnDTP reactions. However, the friction-induced hardened layer on the matrix promoted the ZnDTP reactions, suggesting that forming the hardened layer is a key factor in the low-friction interface of Al-Si without increasing silicon content.