Lubricant Degradation and Wear III. Antioxidant Reactions and Wear Behavior of a Zinc Dialkyldithiophosphate in a Fully Formulated Lubricant

Earlier model systems studies have shown that the products of the antioxidant reactions of a zinc Dialkyldithiophosphate (ZDTP) with peroxy radicals are ineffective as antiwear agents. Now a similar study has been carried out with a fully formulated synthetic hydrocarbon/ester engine oil aged in fleet tests. The results are consistent with those reported earlier and indicate that the previous conclusion can be extended to a fully formulated lubricant in service. It was found that base stocks containing esters can exhibit increased wear upon addition of ZDTP. However, this can be suppressed by other oil additives. A study of the wear behavior of admixtures of a ZDTP and a hindered phenol reacted with peroxy radicals showed that the hindered phenol did not protect the ZDTP from degradation by antioxidant reactions, a result consistent with analysis of IR spectra presented in the accompanying paper (1).