Hot isostatic pressing of silicon nitride based ceramics.

Several techniques have been developed for the encapsulation, and subsequent hot-isostatic pressing (HIPing), of silicon nitride (Si₃N₄) based ceramics. Green-state and densified billets of silicon nitride were vacuum encapsulated in either glass tube or powder. Glass powder encapsulation allows complex shape ceramic pieces to be HIPed. A selection of silicon nitride compositions were HIPed to near-theoretical density (> 97% T.D.) after encapsulation in either Pyrex glass tube or powder. The silicon nitride compositions studied included single yttria (Y₂O₃) additive materials that cannot be densified by conventional pressureless sintering, hence the requirement for pressurised sintering. Similar ceramic compositions were also densified using the commercial ASEA HIP process for comparison. The bulk ceramic microstructure was generally similar to pressureless sintered type materials, with a complete <IMG WIDTH=7 HEIGHT=7 ALIGN=BOTTOM SRC="/maths/alpha.gif">- to <IMG WIDTH=9 HEIGHT=24 ALIGN=MIDDLE SRC="/maths/beta.gif">-Si₃N₄ transformation, although a finer microstructure and lower matrix phase volume were apparent. The ceramic/encapsulant interaction during HIP was generally assessed using a boron nitride (BN) interlayer. When a thick layer (> 50 <IMG WIDTH=8 HEIGHT=14 ALIGN=MIDDLE SRC="/maths/mu.gif">m thickness) was retained after HIP negligible interaction was apparent. A thin silicon oxynitride (Si_2N_2O) surface layer was observed to form with thinner interlayers. Penetration of the molten encapsulant glass through the porous BN layer occurs during HIP, leading to an increase in the Si

4+ and O

2- concentration at the ceramic surface and the subsequent re-precipitation of Si_2N_2O in preference to <IMG WIDTH=9 HEIGHT=24 ALIGN=MIDDLE SRC="/maths/beta.gif">-Si₃N₄. Direct penetration of the encapsulant glass into the porous ceramic occurs in the absence of a BN barrier layer and a similar encapsulant dependent compositional modification was observed. Sub-surface ceramic contamination by boron was apparent in isolated samples HIPed in a Pyrex-type glass at ASEA. Interaction between the encapsulant and ceramic did not significantly affect the post-HIP surface oxidation rate, when compared with the bulk material. (D92473)