Figure shows how the tribofilm thickness varies with rubbing time for the different ZDDP solutions in Indopol-H8 at the same concentration (800 ppm P), temperature (80 °C) and load (1000 N) condition. At the start of the tests, the tribofilm formation rate is approximately linear in all cases, which is indicative of zero-order kinetics. The tribofilm formation rates are much higher for all the secondary ZDDPs than the primary ones, which is consistent with previous tribometer experiments with commercial ZDDPs under mixed/boundary 8 and full-film EHL conditions. The three secondary ZDDPs and one of the primary ZDDPs (oct-1-yl) level out at a maximum tribofilm thickness of between 50-150 nm, while the other primary ZDDPs continue to increase linearly over the duration of the experiments. A similar range of maximum tribofilm thicknesses have been observed in previous experiments. This levelling-off behavior has been attributed to the reduction in contact stress for thicker ZDDP tribofilms due to their lower stiffness compared to the underlying steel substrate. 10 only the filled symbols are used for the fitting.