Microstructure and mechanical properties of CaO/MgO-doped Si sub 3 N sub 4 sintered by hot isostatic pressing

Using glass-encapsulated hot isostatic pressing, additive-free Si{sub 3}N{sub 4} and Si{sub 3}N{sub 4} containing a 50/50 mol% mixture of CaO and MgO up to 6 wt% were sintered from 1,550{degree} to 1,750{degree}C at 160 MPa. Samples containing oxide additions achieved full density, whereas additive-free samples did not reach full density. In fully densified samples, the Vickers hardness decreased with increasing oxide content and sintering temperature. At 1,000{degree}C the oxide content, and, to a small extent, the sintering temperature, determine hardness. Fracture toughness increases with increasing oxide content and increasing sintering temperature. However, transformation of {alpha}-Si{sub 3}N{sub 4} to {beta}-Si{sub 3}N{sub 4} is enhanced by the temperature, but the oxide content has no significant effect.