The activation volume, ∆V * , is calculated from the 2D fits of the rate versus shear stress data to Equation 1 shown in the insets of Figure . With this fitting method, ∆V * is found to increase monotonically with temperature from 4.9 ± 1.7 Å 3 at 400 K to 17.5 ± 8.4 Å 3 at 900 K for TCP between the α-Fe(110) surfaces. For the Fe 3 C(010) surfaces, ∆V * also increases with temperature from 6.6 ± 3.7 Å 3 400 K to 15.3 ± 2.4 Å 3 at 900 K. For the Fe 3 O 4 (100) surfaces, ∆V * increases from 8.3 ± 4.5 Å 3 at 400 K to 12.8 ± 6.6 Å 3 at 900 K. Thus, the calculated ∆V * values are consistent with the dimensions of single bonds. The molecular volume of TCP is approximately 500 Å 3 , meaning that the activation volumes for TCP on the three surfaces are between 1 and 4 % of the molecular volume. This percentage range is is consistent with previous NEMD simulations using trialkyl phosphates (4 %), while experiments using ZDDPs with different alkyl substituent showed larger percentages (7-17 %). Based on the 2D fits to Equation 1 shown in Figure , the activation volume is highly temperature-dependant, but there is no clear trend between the three surfaces due to the relatively large uncertainties. Previous experimental studies of mechanochemical tribofilm growth have treated the activation volume as a temperature-independent reaction constant; however, these studies considered a much smaller temperature range than the present study. In our recent NEMD study of trialkyl phophate mechanochemical decomposition, we also observed that the the activation volume increased with increasing temperature. This was partially explained through a reduction in the elastic modulus of the α-Fe(110) slabs. A recent DFT study suggested that the contribution of the elastic deformation of the surrounding bulk to the activation volume of mechanochemical reactions can be significant and, in some cases, dominant. The elastic modulus of all three surfaces decreases with increasing temperature (Table ). Since the activation volume is inversely proportional to the contact stiffness, it is expected that higher temperatures will lead to lower activation energies, as observed in Figure and Table . However, the increase in the activation volume (up to 350 %) is much larger than the decrease in the stiffness of the surfaces (up to 10 %) between 400-900 K. Thus, other factors, such as a decrease in stiffness of the TCP molecules and degradation products, must also contribute to the increased activation volume at higher temperature.