SILICON SURFACE SEGREGATION AND OXIDATION EFFECTS IN SLIDING WEAR

Friction and wear experiments were conducted on couples consisting of Invar (containing 0.18% Si), Fe-3% Si steel, iron, and nickel pins sliding against a tool steel disc.;It has been established that silicon surface segregation and oxidation, associated with high flash temperatures and mechanical activation, led to the formation of a glassy film on the surface. At loads higher than the critical value as well as under air and nitrogen gas environments, no silicon segregation could be detected. The corresponding values of friction and wear rate were much higher. It is suggested that the beneficial layer can build up only when the rate of silicon diffusion to the surface is higher than that of material removal as wear.;It has been found that that the wear rates in air at room temperature are well related to the oxidation characteristics. Nonlubricating powdery oxides formed on the surface. However, in a mild vacuum environment at loads below a critical value, protective films were formed on the surface. Under these experimental conditions, the friction and wear values were very low. These films were identified as compacted oxide layers. A critical film thickness was observed. This thickness was thinner in the case of Invar (6 (mu)m) than for Fe-3%Si steel (22 (mu)m) presumably because of a larger difference in the thermal expansion coefficient between oxide and metal in the former compared to the latter. 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 due to higher flash temperatures.