Hot isostatic pressing of silicon nitride based ceramics

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Abstract

Several techniques have been developed for the encapsulation, and subsequent hotisostatic
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 shaped 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 (Y203) 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 a- to ß- 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 μm thickness) was retained after HIP
negligible interaction was apparent. A thin silicon oxynitride (Si2N2O) 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 Si4+ and
02- concentration at the ceramic surface and the subsequent re-precipitation of Si2N2O in
preference to ß- 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.

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