Si_3N_4の焼結に及ぼすHfO_2添加の影響

概要

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The effct of HfO_2 addition on sintering of Si_3N_4-Y_2O_3-AIN system was investigated. Although the densification was improved by the addition of HfO_2, the behavior was divided into two types according to the selected firing temperature: below 1800℃, the maximum density was achieved with 1.5 mass% HfO_2 addition; on the other hand, above 1800℃ density continuously increased with increasing HfO_2 addition. This densification behavior could be explained according to phase diagrams of the HfO_2-Y_2O_3・SiO_2(eutectic temperature: 1800℃) and HfO_2-Y_2O_3・2SiO_2(eutectic temperature: 1600℃). As a tentative mechanism for densification, it was considered that, in the initial stage ofsintering, the formation of some liquid phases with low melting point promoted densification. Then precipitation of c-HfO_2 including Y,Si elements and of HfN including O occurred. This mechanism was supported by X-ray diffraction(XRD), transmission electron microscopy(TEM), and energy dispersive spectrometry(EDS) analysis. The phase transformation from α-Si_3N_4 to β-SiAION was found to be enhanced by HfO_2 addition. Consequently, sintered bodies obtained from the Si_3N_4-Y_2O_3-AIN-HfO_2 mixtures were consisted of not only elongated β-SiAION grains, and grain-boundary phases composed of Hf, Si, AI, Y and O, but also isolated spherical grains of both c-HfO_2 and HfN(200-400 nm in diameter).
2001-04-01