Interaction of encapsulation glass and silicon nitride ceramic during HIPing

Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and boro silicate glass. Attention has been focused on what happens when the pressur e is first applied in the HIP-cycle, i.e. between about 1200 and 1500 degre es C. At this stage the pore system of the ceramic green body is still rath er unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass composition s, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Da rcy's law it was predicted that the encapsulation glass with the lowest vis cosity should penetrate about 1200 microns into the still open pore structu re at 1450 degrees C, but this was not observed experimentally. In the calc ulations no chemical reactions were assumed to take place. However, increas es in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen i ncreases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibit ed a phase composition gradient of silicon oxynitride, down to approximatel y 100-200 microns into the bulk. (C) 1999 Elsevier Science Ltd. All rights reserved.