Local chemo-mechanical insights into the efficacy of ZDDP additives from in situ single asperity growth and mechanical testing

This work combines nanoscale tribological wear methods, used to prepare zinc dialkyldithiophosphate (ZDDP) tribofilms under conditions of controlled loading, with site specific nanopillar compression testing to derive a relationship between the ZDDP growth conditions and resultant mechanical properties. The key finding is that ZDDP films grown at higher loads exhibit lower yield strengths, which correlate with differences in tribofilm chemistry. We hypothesize that the load sensitivity of tribofilm properties may underlie the efficacy of ZDDP in suppressing wear by functioning as a hard coating at low loads while exhibiting enhanced lubricity at high loads.