Effects of MgO Content on Properties of Hot-Pressed Silicon Nitride

Abstract

Silicon nitride containing various amounts of MgO were hot-pressed at 1800°C and 300kgf/cm2 for 60min, and their physical and tribological properties were in vestigated. Nearly full densities were obtained for sintered bodies containing 1-15wt% MgO. The fracture toughness was low and its dispersion was much greater for MgO addition less than 5wt%, as compared with MgO addition more than 5wt%. The Vickers hardness decreased up to 5wt% MgO content, and was almost constant with further addition of MgO. The tribological properties were measured, using a pin-on-disk friction and wear tester. The coefficient of friction was almost constant regardless of MgO addition. On the other hand, the specific wear rate scattered much for MgO addition less than 5wt%, and decreased with MgO content over 5wt%. The negative correlation was recognized between fracture toughness and specific wear rate for specimens less than 5wt% MgO addition, and it is supposed that the wear was caused by brittle fractures at contact points. Above 5wt% MgO addition, on the other hand, the wear is mainly controlled by the amount or some properties of glassy phase, which is formed as a result of excessive MgO addition.