Abstract
Laser synthesized, ultrafine silicon nitride was densified via hot isostatic pressing (HIP'ing) without any oxide-sintering-aid additions. HIP'ing was performed on exposed samples made from powder that had been exposed to the atmosphere, thereby picking up an oxide surface layer; and unexposed samples made from powders processed entirely under glove-box conditions, i.e. with minimal oxygen contamination. High-resolution transmission electron microscopy (TEM) studies indicate that the exposed Si3N4 powder samples, HIP'ed at temperatures in excess of the melting point of SiO2, densified via a solution-reprecipitation mechanism, with an intergranular glassy phase of high-purity SiO2. In contrast, samples of unexposed Si3N4 powder had to be HIP'ed to 2050°C to achieve a density of ∼ 70%ρTh (where ρTh is the theoretical density of silicon nitride). In this state, the sample consisted of equiaxed β-Si3N4 grains, with localized high-density regions. These regions had clean grain boundaries.
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Lu, P., Danforth, S.C. & Symons, W.T. Microstructure and grain-boundary characterization of HIP'ed high-purity silicon nitride. JOURNAL OF MATERIALS SCIENCE 28, 4217–4222 (1993). https://doi.org/10.1007/BF00351257
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DOI: https://doi.org/10.1007/BF00351257