Figure shows the change in the number of intact TCP molecules with time for α-Fe(110), Fe 3 C(010), and Fe 3 O 4 (001) surfaces at constant normal stress (4 GPa) and varied temperature (400-900 K), and constant temperature (600 K) and varied normal stress (1-5 GPa). The reduction in the number of intact TCP molecules with time is shown for several other conditions in the ESI (Figure ). The TCP molecules are counted as dissociated, i.e., no longer intact, if any bond other than C-H is broken. During the NEMD simulations, there is an exponential decay in the number of intact TCP molecules with time, which is indicative of a first-order reaction. This observation is in agreement with previous NEMD simulations of trialkyl phosphate decomposition between α-Fe(110) surfaces. In mechanochemistry experiments with ZDDP, the rate of tribofilm growth followed either zero-order or fractionalorder kinetics. The observation of zero-order kinetics indicates that the rate-determining step for tribofilm growth involves surface-adsorbed molecules, which masks the underlying firstorder kinetics. In experiments, the surface-adsorbed additive molecules are continually replenished from the base oil solution and the bulk concentration does not significantly decrease. On the other hand, there is a finite number of additive molecules in the NEMD simulations, which means that the dissociation rate inevitably decreases as fewer molecules are available to react. The dashed lines in Figure show the simulation data and the solid lines are fits to an exponential function. The exponential decay curves are used to calculate the decomposition rate for each temperature and stress condition. The full tables of the TCP decomposition rates obtained from the 2D fits for all the conditions and surfaces can be found in the ESI (Tables S4, S5, and S6).