Previous studies have shown that the tribofilm growth rate of antiwear additives is dependent on the shear stress as opposed to the normal stress. It is therefore important to understand the relationship between the normal stress and shear stress in our systems. The steady-state shear stress was block averaged over the final 0.5 ns of the NEMD simulations. Figure shows the relationship between applied normal stress and the measured shear stress for the three ferrous surfaces. At any temperature and normal stress, the shear stress is lower for the systems with TCP molecules sheared between iron oxide surfaces than iron and iron carbide surfaces. For all three surfaces, the shear stress increases linearly with normal stress with a finite intercept, as predicted by the extended Amontons-Coulomb law: τ = µσ + τ 0 , where τ is the shear stress, µ is the friction coefficient, τ 0 is the Derjaguin offset, and σ is the normal stress. The friction coefficient and Derjaguin offset for the different systems and temperatures are shown in the ESI (Tables and). The friction coefficient, given by the gradient of the linear fits in Figure , is similar for all three surfaces. The Derjaguin offset, given by the intercept of the linear fit, is similar for iron and iron carbide surfaces, but is around 50 % lower for iron oxide. This is likely due to the higher surface energy and adhesion of TCP molecules on iron and iron carbide compared to iron oxide. Previous DFT calculations suggested that the surface energy of α-Fe(110) (2.3 J m -2 ) and Fe 3 C(010) (2.2 J m -2 ) is much larger than Fe 3 O 4 (001) (1.0 J m -2 ). For the iron and iron carbide systems, the friction coefficient decreases with increasing temperature (Table ); in contrast, the Derjaguin offset increases with increasing temperature (Table ). As the temperature increases, the amount of TCP de-composition will also increase, which causes the temperaturedependence of the friction coefficient and Derjaguin offset. For the iron oxide surfaces, the temperature has a negligible effect on the friction coefficient and Derjaguin offset, which is probably due to the lower reactivity of TCP on this surface.