TEM study of the interface between HIPed silicon nitride and encapsulationborosilicate glass

A transmission electron microscope study has been made of a silicon nitride component with 6 w/o yttrium oxide as a sintering aid hot isostatically pr essed (HIP) with an encapsulation glass of borosilicate. The TEM study conc entrated on the interface region between ceramic and glass. Two different t ypes of hexagonal boron nitride were formed near the interface. One, with a textured structure, seemed to nucleate heterogeneously on the surfaces of silicon oxynitride grains. The (001) planes of the crystals extended outwar ds, giving a thickness of approximately 0.5 microns. The other type formed as hexagonally shaped grains separate from the first type and appeared to h ave grown as several segments in different directions around a nucleus. In each segment BN layers are parallel to each other and perpendicular to thei r common [001](B)M direction. This second type of BN crystal was also detec ted a little further from the surface within the silicon nitride. The volum e fraction of additive glassy phase tended to be lower in this surface regi on than in the bulk. Possible mechanisms of prevention of encapsulation gla ss penetration into the porous ceramic component during HIP were discussed. (C) 2000 Kluwer Academic Publishers.