Role of tribofilms in wear protection

Engine valve train wear protection has been observed despite the absence of classical tribofilm formation. This was found with a lubricant containing zinc dialkyldithiophosphate (ZDDP), despite conventional understanding that ZDDP reduces wear by forming a thick, protective, inorganic phosphate tribofilm. We suggest that an antiwear additive can function by forming non-adherent triboproduct via a mechanically-stimulated chemical reaction. This triboproduct provides antiwear performance while entrained in the contact and is eventually expelled. In a reciprocating sliding tribometer it is observed as a pile of debris on the side of the contact. •ZDDP can protect from engine wear without forming classical tribofilm.•ZDDP mechanochemically converted to non-adherent triboproduct.•Entrained triboproduct lubricates contact.•Wear correlates with quantity of triboproduct expelled to wear scar boundary.•Data suggest that antiwear performance related to mechanochemical productivity.