Here, we investigate the mechanochemical tribofilm formation of ZDDPs containing several different aliphatic substituents on steel surfaces in a ball-on-disk tribometer under full-film EHL conditions. The methods used here are similar to those described by Zhang et al. , but employing different base oils and additives. In these experiments, the temperature and shear stress conditions can be carefully controlled, which is not the case in mixed/boundary lubrication conditions as present in most mechanochemistry apparatus, such as ball mills. This is because the stress is applied through confined base oil molecules rather than randomly contacting solid asperities. This approach also avoids tribofilm wear and other potential drivers of tribofilm growth, such as frictional heating, triboemission, and generation of catalytic surfaces. For measurable tribofilm formation to occur under these conditions, large stresses are required, which can be achieved by using either hard surfaces or very high applied loads. Moreover, base oils that provide high shear stress such as traction fluids with bulky cycloaliphatic groups or polymers with pendant methyl groups are required. High additive-surface affinity is also needed to form ZDDP tribofilms under these conditions. Unlike previous studies using lowerpurity commercial additives, the ZDDPs used here contain a wide range of well-defined alkyl groups to provide a more detailed understanding of the substituent effects on tribofilm growth under mechanochemical conditions. Using the Bell model, large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs studied become apparent. We anticipate that these results will be useful both for the development of high-performance antiwear additives for specific applications 2 and for improving the accuracy of macroscale tribology models that include the effects of tribofilm growth.