The contribution of a soft thin (metallic) film to a friction pair in the running-in process

Analytical and experimental studies have been conducted on silver-film-covered and uncoated metallic surfaces to investigate the role of the soft, thin, metallic film in the running-in process. The experimental results have shown that a low coefficient of friction of about 0.22 and especially low wear can be obtained at the inception of sliding when there is a thin film. The analysis has shown that for the coated case the improvements can be obtained owing to the increase of the true area of contact and thus the reduction of contact pressures. In the presence of such a thin film, it has been pointed out that the friction force arises due to shear of the film and substrate instead of solely shearing of the film. Based on the experimental and analytical results, a new physical model for the film-covered case was proposed where a plastic deformation for the thin film was assumed. To account for the role of the film in preventing the substrate/pairing materials from wear, the effect of the coefficients of friction on the maximum shear stresses and maximum tensile stresses in the subsurface was calculated in light of the von Mises yield criterion vJ₂.