SI2N2OZRO2 COMPOSITES PREPARED BY HOT ISOSTATIC PRESSING

Fully dense Si2N2O-ZrO2 composites have been prepared by glass-encapsu lated hot isostatic pressing at 1600 and 1700 degrees C by two differe nt routes. Firstly, from reacting powder mixtures of equimolar amounts of Si3N4 plus SiO2 with 0, 5 and 20 wt% ZrO2 or ZrO2 (3 mol% Y2O3), a nd secondly by an in-situ reaction from equimolar mixtures of Si3N4 an d ZrSiO4 to form a Si2N2O composite with 36 wt% ZrO2. The effects of s mall additions of Al2O3 or Y2O3 in the two different preparation route s were also studied. It was found that the reaction of Si3N4 with SiO2 was strongly influenced by the addition of sintering aids and tempera ture, whereas the reaction with ZrSiO4 was mainly temperature controll ed proceeded rapidly at 1700 degrees C. Addition of zirconia or stabil ized zirconia did not appear to have any positive effect on the toughn ess, and the materials prepared were in general brittle with a fractur e toughness of similar to 3 MPa m(1/2). The observed hardness was typi cally 12-14 GPa for the composites. The composites which consisted of more or less unreacted starting materials had a very fine-grained micr ostructure. In the fully reacted samples which had the highest fractur e toughness the zirconia grains had increased in size compared to the more brittle samples.