A dual-analysis approach in tribochemistry: application to ZDDP/calcium borate additive interactions

Tribochemical interactions between zinc dithiophosphate (ZDDP) and micellar calcium borate (CB) under boundary lubrication were investigated by coupli ng, in the same location of the wear track, both analytical TEM analysis of collected wear fragments from the tribofilm and XPS surface analysis of th e tribofilm directly underneath. This is the so-called dual-analysis approa ch, which improves the interpretation of tribochemical reactions. The eleme ntal composition inside the wear scars was analysed by micro-spot XPS. By d epth profiling, the film thickness could also be determined. In particular, the efficiency of the additive combination could be proven by quantificati on of iron oxide. The nature of wear particles was investigated in the TEM by using EELS and EDX simultaneously, with the result that phosphorus, boro n and sulphur contributions have been carefully distinguished. The techniqu e is very powerful for determining the composition of the material through quantification of both EELS and EDX spectra on the same specimen. The main result, when ZDDP and CB additives are used together, is the formation of a calcium and zinc borophosphate glass tribofilm. The overall data confirm t he general friction-induced glass model as being a unifying concept that ex plains the mechanisms of antiwear additives under boundary lubrication. Mor eover, the analytical results strongly suggest the role of viscous flow of the magma state glass tribofilm above its glass transition temperature to b e a main contribution to the antiwear mechanism under mild wear conditions.