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.