Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Abstract

Si3 N4 test bars containing additions of BN, B4C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room‐temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si3 N4 sintered with Y2O3, Y2O3 and Al2O3, or ZrO2. The 1370°C strengths were less than those obtained for additions of Y2O3 or ZrO2 but greater than those obtained from a combination of Y2O3 and Al2O3. Scanning electron microscope fractography indicated that, as with other types of Si3N4, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si3N4 having an amorphous grainboundary phase. The primary advantage of non‐oxide additions appears to be in facilitating specimen removal from the Ta cladding.