∆V * is sometimes interpreted as the difference in molecular volume between the transition state and the initial equilibrium state of the reactant. A positive ∆V * indicates that the volume of the transition state is larger (bond breaking), whereas a negative ∆V * indicates the volume of the transition state is smaller (bond formation). We use shear stress, τ, in the equation rather than normal stress, σ , due to experimental evidence that tribofilm growth is controlled by shear stress rather than normal stress. While τ and σ are proportional (Figure ), using σ results in smaller activation volumes by a factor of µ. For firstorder reactions, as is the case here, the reaction rate, r = k[C], where k is the rate constant and [C] is the concentration of reactant. The Bell model accurately describes the rate of TCP decomposition with temperature and shear stress on the three surfaces, as shown in the ESI (Figure ).