Properties and microstructure of hot isostatically pressed Si₃N₄

The hot isostatic pressing (HIP) of Si₃N₄ was performed in the temperature range 1500-1800°C and within the pressure range 90-172 MPa. Both additive-free Si₃N₄ and Si₃N₄ with Y₂O₃ additions were studied. During the HIP process the transformation of α-Si₃N₄ to β-Si₃N₄ occurs and the amount of α→β transformation increases as the HIP temperature and HIP pressure increase. The α→β transition also depends on the Y₂O₃ content. The density of the Si₃N₄ consolidated by HIP increases as the HIP pressure, HIP temperature and Y₂O₃ content increase. The hardness and Young's modulus of the product increase gradually with increasing Y₂O₃. The effect of outgassing the starting material prior to HIP on the properties of the final product is discussed.