Tribochemical mechanisms of Si 3N 4 with additives

Abstract

The effects of several extreme pressure additives on the wear behaviour of silicon nitride (Si 3N 4) were investigated using a reciprocating wear tester; the tribochemical reaction mechanisms of Si 3N 4 with these additives were studied by means of scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The wear tests show that the additives have different antiwear functions compared with n-tetradecane in the order tridodecyl borate ≈ tributyl phosphate (TBP)< chlorinated paraffin < sulphurized olefin ≈ zinc dialkyldithio-phosphate (ZDDP). The analytical results of profiles and compositions of worn surfaces indicate that, under various lubricated conditions, Si 3N 4 oxidizes and forms crystalline SiO 2. ZDDP, sulphurized olefin and chlorinated paraffin are effective in antiwear action, because of the formation of tribochemical reaction products obtained from the reaction with SiO 2, while TBP and tridodecyl borate have little antiwear function because of their physical adsorption on the rubbing surfaces.