Si₃N₄ ceramics formed by HIP using different oxide additions — relation between microstructure and properties

Abstract

Si₃N₄-based ceramic materials formed by glass encapsulation and hot isostatic pressing (HIP) using different additions of Al₂O₃, Y₂O₃ and ZrO₂ have been characterized by analytical electron microscopy and X-ray diffractometry. The microstructures have been related to formation process and to room temperature hardness and fracture toughness of the ceramics. A high volume fraction of retained α-Si₃N₄ after processing at 1550 °C gave the Si₃N₄ ceramics high hardness. The equi-axed grain morphology of the Si₃N₄ matrices in these materials, which contained only small amounts of residual glass, resulted in comparatively low fracture toughness values. Processing at 1750 °C reduced the amount of retained α-Si₃N₄ substantially. When Y₂O₃ was added, the microstructure contained a comparatively large volume fraction of residual glass, and the Si₃N₄ was present mainly as high aspect ratio β-Si₃N₄ grains. This type of microstructure gave an Si₃N₄ ceramic material with high fracture toughness combined with a lower hardness. Additions of ZrO₂ and/or Al₂O₃ resulted also at 1750 °C in an extremely small volume fraction of residual glass, and a major part of the Si₃N₄ was present as equi-axed grains. These ceramics exhibited medium hardness and toughness values, however, larger additions of ZrO₂ appeared to slightly increase toughness.