where A is the pre-exponential factor, Ea is the activation energy, σ is the applied stress, ΔV * is the activation volume, and kB is the Boltzmann constant. Although both the normal stress and the shear stress have been used in this equation for ZDDP tribofilm formation, recent experiments have demonstrated that the latter controls the reaction rate in macroscale experiments. Tribometer experiments at different scales have suggested that ZDDP tribofilm formation follows either fractional-order or zero-order kinetics, indicating that ZDDP adsorbs onto steel surfaces before it dissociates and forms a tribofilm.