The length of the polyphosphate chains in tribofilms is important to their antiwear performance. Figure shows the change in the number of polyphosphate chains (dimers, trimers, tetramers, and ≥ pentamers) with sliding time for the three studied systems. In all cases, the total number of polyphosphate fragments increases at short times before asymptoting towards a steady-state value of ∼10. For Fe 3 C(010) and Fe 3 O 4 (001), the majority of TCP molecules form dimers, but longer chains, up to pentamers, form in some cases. In general, longer polyphosphate chains are formed on the α-Fe(110) surfaces than Fe 3 C(010) and particularly Fe 3 O 4 (001) surfaces. For α-Fe(110), a similar number of dimers and trimers form, with some of the chains growing up to octamers. The relatively short polyphosphate chains formed in our NEMD simulations are consistent with the average chain length of approximately 2.5 monomers found in tribometer experiments with ZDDP. Much longer chains (up to 40 linked monomers) were formed in previous NEMD simulations with ReaxFF of phosphoric acid confined between sliding silica surfaces at very high temperature (1400 K). The longer chains formed in these previous simulations is probably due to the higher reactivity of phosphoric acid compared to TCP and the lack of iron atoms to catalyse the depolymerisation (P-O dissociation) reactions.