Substituent Control of Tribofilm Growth under Mechanochemical Conditions

Lubricant additives that reduce wear by forming protective tribofilms on sliding surfaces are crucial to maintaining the efficient and reliable operation of many engineering systems. The most important of these additives, zinc dialkyldithiophosphate (ZDDP), has been in use for almost a century; however, several aspects of the physicochemical mechanisms through which it reduces wear remain unclear. While changes to the molecular structure of ZDDP are known to affect tribofilm formation and antiwear performance, the underlying mechanisms are not well understood. Here, we show using macroscale tribometer experiments under well-defined temperature and stress conditions, how the ZDDP tribofilm formation rate on steel from a high-friction base oil can be controlled by tailoring the additive’s alkyl substituents. Our results suggest that the chain-length, branching and presence of cycloaliphatic groups can affect the packing density, steric hindrance, and stress transmission, leading to large differences in the temperature- and stress-dependencies of the tribofilm formation rate. These changes can be successfully explained using the Bell model; a simple modification of the Arrhenius equation commonly used to model the kinetics of mechanochemical processes. Using this model, large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs studied become apparent. We expect these results to be useful both for the development of high-performance lubricant additives for specific applications and for improving macroscale tribology models that consider the effects of growing tribofilms.