X-ray absorption study of antiwear films generated from ZDDP and borate micelles

The possible partial replacement of zinc dialkyldithiophosphate (ZDDP) by a multifunctional additive (borate–sulphonate micellar compound) was studied. X-ray absorption spectroscopy at the phosphorus, sulphur and boron edges was used to determine the chemistry of the tribochemical films, both at the surface and in the bulk. XANES data suggest an interaction between ZDDP and the borate additive. The sulphonate moiety reacts to form sulfite and sulfate, which allows the release of the borate core. However, this process is limited and a substantial proportion of unreacted micelles could be found in the film. The sulphonate moiety seems to play a major role in the interaction between the borate additive and ZDDP. Calcium phosphate was detected in the tribofilm as a result of this interaction. Although the additive contains boron, the film is quite similar to a film generated from ZDDP and calcium sulphonate. The performance of the additives was determined by the Wear Scar Width measurements. The data suggest that the antagonism between ZDDP and sulphonate is similar to the ZDDP–borate mixture. The antiwear function of ZDDP is almost completely retarded by sulphonate or borate–sulphonate additive.