The effect of film thickness on initial friction of elastic-plastically rough surface with a soft thin metallic film

The effect of a deposited soft thin metallic film on friction properties of a hardened steel substrate has been investigated experimentally and theoretically. The dependency of the film thickness and contact load oil the static friction coefficiency is preserved. The experimental observations show that deformation of the film in contact was plastic, thereby confirming the assumption of the theoretical calculation. The effect of the film thickness on the contact area has been analyzed. A model for calculating the static friction coefficient of contacting rough surfaces in the presence of a soft thin film has been used. Results from the numerical calculations have been compared with the present static friction measurements performed on a pill oil plate reciprocating apparatus. The rise in friction that occurs with increasing thickness for very thin films is discussed ill detail. The calculated results, which predict the correct trend of the friction behavior from the present experiment, cover on extremely large range of F/A(n)E from 10(-12) to 10(-2), where three different dependencies of F/A(n)E on the static friction coefficient can be identified. An investigation into the discrepancy between the calculated and experimental values for the satic-friction coefficient mu suggests that an accurate prediction of the magnitude of mu depends to a great extent on the level of accuracy in measuring the value of the constant zeta, the effective hardness of the film.