On the critical thickness of protective films at sliding interfaces

Friction and wear experiments were conducted on couples consisting of Invar and Fe-3%Si steel pins sliding against a tool steel disk in a mild vacuum (0.1 mmHg) at room temperature. At loads below a critical value, protective films, identified as compacted oxides, were observed on the sliding surfaces. The resulting friction and wear values were very low. A critical film thickness was observed that was thinner for Invar (6 μm) than for Fe-3%Si steel (22 μm), presumably because of a larger difference between the thermal expansion coefficients of oxide and metal for Invar than for Fe-3%Si steel. This critical thickness was found to be independent of sliding speed or applied load. However, at higher loads, the critical thickness was reached at lower sliding times, probably as a result of a higher flash temperature.