To validate the calculations of the adsorption free energy, friction was used as a proxy for surface coverage using the Jahanmir and Beltzer model. The obtained surface coverage can then be related to the adsorption energy. Friction measurements were performed using a highfrequency reciprocating rig (HFRR) from PCS Instruments (Acton, UK). The standard specimens were used, namely a 6 mm diameter ball (upper) loaded on a 10 mm diameter disk (lower). Both of the specimens were made from AISI E-52100 steel. A 2 N load was applied to the upper specimen, which followed a reciprocating movement of 10 Hz with a stroke length of 0.1 mm, giving a maximum velocity of 2 mm s -1 . The corresponding maximum Hertzian pressure was 0.8 GPa, whereas the mean was 0.5 GPa. High pressure and low speed were used to ensure that boundary lubrication conditions were maintained over most of the stroke.