Adsorption film thickness ha along the central line at the outlet was obtained at different sliding speeds, and results are illustrated in Fig. 12. The thickness of the adsorption film initially increased with the running time. However, no further increase and accompanied by a slight decrease in film thickness was observed beyond approximately 60 min. A similar phenomenon was observed by Vrbka et al. [18] that the adsorption film thickness in a ball-on-disc contact initially increased and subsequently decreased.