S. Jahanmir, Y. Ozmen, and L. K. Ives, “Water Lubrication of Silicon Nitride in Sliding,” Tribology Letters, 17 (2004) 409-417.
Several studies have shown that the coefficient of friction of self-mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run-in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e., hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. The purpose of our study is to investigate the influence of load, speed, and surface roughness on the duration of the run-in period. The results confirmed that a low coefficient of friction is obtained following a run-in period when a wear scar of sufficient size is developed to reduce the contact stress. The run-in period, during which the coefficient of friction is fairly high, was shorter for smoother surfaces and at higher loads and speeds. The low friction behavior was found to be unstable and occasional high friction spikes were observed. The surfaces of the wear tracks and wear scars contained a series of striations parallel to the sliding direction and exhibiting plastic deformation, delamination and fracture. The striations that appeared to be associated with the high friction spikes, could form as a result surface film breakdown. Although these results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low friction behavior may be also related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen-terminated oxide films.