MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF SI3N4 DENSIFIED WITH ZRO2 AND AL2O3

Ceramic material based on Si3N4 has been formed using 5 wt% Zr2O stabilized by 3 mol% Y2O3 and 6 wt% Al2O3 as sintering aids. Densification was carried out by hot-pressing at temperatures in range 1700 - 1775 degrees C, and the resulting microstructure were related to the mechanical properties of as-machined and oxidized at 1200 degrees C samples. Densification at higher temperatures increased the room-temperature strength of the material. the highest value of fracture toughness, 5.2 MPa.root m, was obtained for samples hot-pressed at 1775 degrees C. The alpha to beta-transformation during sintering influenced the hardness of material, which value decreased for non-oxidized samples from 26.2 to 20.0 GPa. The oxidation treatment at 1200 degrees C of silicon nitride samples with very small quantities of ZrO2 increased only the mechanical strength, had a very small influence on the fracture toughness and decreased the hardness of the material.