Substituent Control of Tribofilm Growth under Mechanochemical Conditions

Mechanochemistry is known to play a key role in the function of some lubricant additives, such as the tribofilm growth of zinc dialkyldithiophosphate (ZDDP). This raises the intriguing possibility of tailoring the mechanochemical reactivity of additives by modifying their alkyl substituents. Here, we study the tribofilm formation rate of ZDDPs containing several different alkyl groups on steel surfaces from a high-friction base oil. We use macroscale tribometer experiments under full-film elastohydrodynamic lubrication conditions to ensure that the temperature and stress are carefully controlled. We show how the chain length and the presence of branches or bulky cycloaliphatic groups can lead to large differences in the temperature− and stress−dependencies of the tribofilm formation rate, which can be explained through changes to the packing density, steric hindrance, and stress transmission efficiency. Our rate data are successfully fitted using the Bell model; a simple modification of the Arrhenius equation that is commonly employed to model the kinetics of mechanochemical processes. Using this model, we observe large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs. Our findings show how structure−performance relationships can be identified for lubricant additives, which can be used to optimise their molecular structure.