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 an d the borate additive. The sulphonate moiety reacts to form sulfite and sul fate, 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 intera ction between the borate additive and ZDDP Calcium phosphate was detected i n the tribofilm as a result of this interaction. Although the additive cont ains boron, the film is quite similar to a film generated from ZDDP and cal cium sulphonate. The performance of the additives was determined by the Wea r Scar Width measurements. The data suggest that the antagonism between ZDD P and sulphonate is similar to the ZDDP-borate mixture. The antiwear functi on of ZDDP is almost completely retarded by sulphonate or borate-sulphonate additive. (C) 2001 Elsevier Science B.V All rights reserved.