Microstructural characterization of a microwave-sintered silicon nitride based ceramic
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The microstructure of a microwave-densified silicon nitride based ceramic has been assessed in the as-sintered, post-sinter hot-isostatically pressed (HIPed) and annealed conditions. The grain size of the as-sintered material, which is a low substitution yS'-Sialon, was significantly finer than observed in conventionally processed materials of similar composition. The as-sintered ceramic exhibits a reverse porosity gradient (with the highest porosity level at the surface) due to heat dissipation to the cooler surroundings during microwave processing. This also results in a higher /3' aspect ratio close to the surface arising from an increased glass viscosity (due to heat loss) and compositional change in this region during sintering. HIPing results in removal of all porosity from the sample core; however, a reduced porosity surface layer is retained. Significant /3'-Sialon grain growth is also apparent after HIPing. A fine f3' grain structure was retained after annealing, with partial devitrification of the glassy grain boundary phase to /3-Y 2 Si 2 O 7 .
I. INTRODUCTION Several processing routes exist for the fabrication of high density silicon nitride (Si3N4) based ceramics, with the most commonly applied being uniaxial hot-pressing and pressureless-sintering. Hot-pressing is limited to the fabrication of relatively simple shapes, although a high degree of compositional control is possible. Pressureless-sintering of Si 3 N 4 allows the production of complex-shaped components; however, a high volume of sintering additives is necessary for densification. Glassencapsulated hot-isostatic pressing (HIPing) offers the benefits of both these routes, at the expense of significantly higher processing costs. Recently, a further alternative has been reported that utilizes microwave energy to heat and sinter Si3N4-based ceramics.1"5 Microwave heating has previously been used to sinter various ceramics, including alumina and related composites,6"8 boron carbide,9 zirconia,10 zinc oxide,11 andhydroxyapatite. 12 Tiegs et al.1 demonstrated that enhancement of densification could be obtained for microwave-sintered Si 3 N 4 , with A12O3 and Y 2 O 3 additives, when compared to conventional heating. It was noted that higher microwave-sintered densities could be obtained by either increasing the additive content, sintering at a higher frequency microwave power (i.e., 28 GHz as opposed to 2.45 GHz), or adding secondary particulate phases that couple well during microwave heating (i.e., SiC or TiN).1 a)
Present address: Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6068. J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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Further development of microwave sintering techniques for the densification of Si3N4-based ceramics has been performed by Alcan International.3"5-13 This work has concentrated upon the use of a powder bed around the Si 3 N 4 articles which contains components that act as a susceptor during microwave processing,13 al
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