SI3N4-ZRO2 COMPOSITES WITH SMALL AL2O3 AND Y2O3 ADDITIONS PREPARED BY HIP

Si3N4-ZrO2 composites have been prepared by hot isostatic pressing at 1550 and 1750-degrees-C, using both unstabilized ZrO2 and ZrO2 stabilized with 3 mol % Y2O3. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt %, with respect to the silicon nitride, together with 0-4 wt % Al2O3 and 0-6 wt % Y203. Composites prepared at 1550-degrees-C contained substantial amounts of unreacted alpha-Si3N4, and full density was achieved only when greater-than-or-equal-to 1 wt% Al2O3 or greater-than-or-equal-to 4 wt % Y2O3 had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO2 starting powder was stabilized by Y2O3, fully dense Si3N4-ZrO2 composites could be prepared at 1 750 C even without other oxide additives. Densification at 1750-degrees-C resulted in the highest fracture toughness values. Several groups of materials densified at 1750-degrees-C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm-2 and 4.5 MPa m1/2, respectively. Examples of such materials were either Si3N4 formed with an addition of 2-6 wt% Y2O3 or Si3N4-ZrO2 composites with a simultaneous addition of 2-6 wt % Y2O3 and 2-4 wt % Al2O3.