For the tribofilm growth measurements, we performed ball-on-disk tribometer experiments using the ETM (schematic shown in Figure ). We used the same ratio of the sliding velocity to the entrainment velocity, known as the slide-to-roll ratio (SRR = 2 %) as in our previous tribofilm growth study. Since Indopol H-8 has a higher viscosity than the traction fluid DM2H used in our previous study, a sufficiently thick fluid film could be generated with a lower entrainment velocity (U = 0.75 m s -1 ), and the sliding velocity was reduced accordingly (us = 0.015 m s -1 ) to maintain the same SRR. This approach reduced frictional heating compared to the conditions used in our previous study. The amount of frictional heating during the tribofilm growth experiments was estimated using the Archard equation, employing the recently measured thermal conductivity of through-hardened AISI 52100 steel. The rise in surface temperature in the contact due to frictional heating is expected to increase slightly with load from 2.6 °C at 600 N to 4.8 °C at 1500 N. Thus, the significant increase in tribofilm formation rate at higher loads observed below cannot be explained by frictional heating, which confirms mechanochemical acceleration.