High Temperature Hardness of Si₃N₄ Ceramics Fabricated under High Pressure

Abstract

The temperature dependence of the Vickers microhardness (VMH) was measured for highly dense Si₃N₄ ceramics without and with oxide or nitride additives prepared by the high pressure hot-pressing or hot isostatic pressing. Rapid increase in room temperature VMH of the Si₃N₄ ceramics without additives with decreasing the amount of open pores was observed. The VMH (200g loads) of the fully dense Si₃N₄ ceramics without additives was 23GN/m² at room temperature and 14GN/m² at 1200°C, and this VMH at elevated temperature was higher than that of SiC single crystal. The VMH of Si₃N₄ ceramics obtained from α-Si₃N₄ was higher VMH not only at room temperature but also at elevated temperature than those obtained from β-Si₃N₄. The decrease in the VMH with increasing temperature for the Si₃N₄ ceramics with oxide additives was more rapid than that for the Si₃N₄ ceramics with nitride of the same element, and it was found that controlling the composition of the grain boundary phase was very important in the fabrication of Si₃N₄ ceramics. It is expected that rapid fall in the VMH of the Si₃N₄ ceramics with additives of elevated temperature was due to the ductile-brittle transition of these materials.