Role of friction modifiers on the tribological performance of hypereutectic Al-Si alloy lubricated in boundary conditions

Hypereutectic Al-Si alloys are increasingly being as used as lightweight materials for engine components. This study is focused on understanding the tribochemical performance of a cast iron/Al-Si alloy system lubricated in the boundary regime using lubricating oils containing Zinc Dialkyldithiophosphate (ZDDP) and different friction modifiers. The tribological performance of two organic and two inorganic Mo-containing friction modifiers, blended in a ZDDP-containing oil, is evaluated on a Plint TE 77 reciprocating tribometer in contact conditions comparable to the conditions in a piston ring/cylinder liner system of an internal combustion engine. Surface sensitive analytical techniques such as secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and Raman spectroscopy have been used to determine the chemical nature of the tribofilms formed on the Al-Si material. The additive effects on tribofilm formation as well as tribological performance are discussed. This study shows that the Mo-containing additives, when blended into a ZDDP-containing lubricant, give superior friction performance when compared to lubricants containing organic friction modifiers. The role of substrate material in tribofilm chemical and physical characteristics for all lubricants is discussed.