The tribological behavior of monolithic Si3N4 and solid lubricants dispersed Si3N4-based composites has been assessed under a high load and low speeds in an aqueous environment. The results showed that the friction coefficient of the Si3N4 was not significantly reduced when compared to dry sliding and this was attributed to the failure to maintain a lubricating layer between the solid-solid surfaces. In the case of the composites, the initial high friction coefficient was reduced shortly after the beginning of the wear test and maintained a low value (approximately 0.03) throughout. This was attributed to the solid lubricating effect of the composite resulting in a lower stress at the contact asperities preventing the removal of the lubricating layer. The solid lubricant content did not affect the value of the friction coefficient under hydrodynamic type lubrication, but the running in distance decreased with the solid lubricant dispersion. The type and amount of solid lubricant affected the distance required to reach a low friction regime, but all types of solid lubricant showed decreasing friction with increasing amount. In Stribeck analysis, the addition of the solid lubricant resulted in a highly graphitic transfer layer on the Si3N4 material, and shifted the transition points from hydrodynamic to mixed and from mixed to boundary lubrication regimes to more severe conditions. It also reduced the friction coefficient in the boundary lubrication regime.

Keywords: Tribology, Friction, Solid lubricant, Silicon nitride, Composites.