Results for the other primary and secondary ZDDPs are shown in Figure and Figure , respectively. Figure shows the temperature and shear stress dependence of the tribofilm growth rate for all the ZDDPs. The rates are calculated from the slopes of the linear fits in Figure , Figure , and Figure . For all the ZDDPs, the rate increases exponentially with both temperature and shear stress, which is indicative of a SATA process. Under all the conditions studied, tribofilm growth rate is much faster for the secondary ZDDPs than the primary ZDDPs. The tribofilm growth rate is negligible (< 0.1 nm min -1 ) for the dodec-1-yl ZDDP when T < 80 °C, irrespective of the load. This could be due to the formation of a close-packed SAM by the dodec-1-yl ZDDP at low temperature that become less ordered (or possibly partially desorbed) at higher temperature, allowing additional ZDDP molecules to adsorb, decompose, and form tribofilms.