Comparing the linear alkyls, the ZDDP with the longer chain (dodec-1-yl) forms tribofilms much more slowly than that with shorter chains (oct-1-yl). A reduction in friction with increasing chain length has also been noted for oct-1-yl and dodec-1-yl ZDDPs under boundary lubrication conditions. These observations suggest that ZDDP (or its dissociation products) form selfassembled monolayers (SAMs) on top of the growing ZDDP tribofilm that are responsible for both the boundary friction response and surface reactivity. SAM formation has also been linked to the antiwear performance of ZDDPs in previous studies where molecules with larger monolayer cohesive energies gave lower wear. It has been shown that for a range of systems and processes at solid-liquid interfaces that SAMs formed from surfactants with longer alkyl chains generally show lower chemical reactivity. This is due to the formation of more densely packed, ordered monolayers 60 that are more difficult for approaching molecules to penetrate to react with the surface. This effect is likely responsible for the slower tribofilm growth of dodec-1-yl ZDDP than octyl-1-yl ZDDP, since the more densely packed monolayer hinders adsorption, decomposition, and ultimately tribofilm formation on the steel surface. For the primary ZDDPs with C8 alkyl groups, the tribofilm growth rate increases in the order: 2-cyclohexylethyl < 2ethylhexyl < oct-1-yl. This suggests that the increased steric bulk has a larger effect on the rate than the lower packing density expected for the ethyl-branched and particularly cyclohexylcontaining ZDDPs compared to the linear one. Previous studies of other systems have also shown lower reactivity for surfaces protected by branched alkyl monolayers compared to linear ones. For the secondary alkyl ZDDPs at 80 °C and 1000 N, the reactivity increases in the order: oct-2-yl < 4-methylpent-2-yl < but-2-yl. The trend is similar to the primary ZDDPs in that the secondary ZDDPs with longer alkyl chains give slower tribofilm formation rates, which can again be attributed to the formation of more densely packed SAMs. The presence of methyl branches in the 4-methylpent-2-yl ZDDP seems to have a minimal effect on the tribofilm growth rate, since the rate is approximately halfway between oct-2-yl and but-2-yl, as would be expected for a linear hex-2-yl group. A similar order of tribofilm growth rate for ZDDPs with the same substituents has recently been reported under mixed/boundary conditions, where the tribofilm will be partially worn away as it grows. To obtain a more complete understanding of substituent effects on mechanochemical reactivity of ZDDP, we studied how the tribofilm formation rate varied with temperature (60-120 °C) and load (600-1500 N). Figure shows the tribofilm growth for a representative primary (oct-1-yl)