Refractory grain boundary phases

The hardness at elevated temperatures depends additionally on the softening of the glassy grain-boundary phase. Materials with Mg0/Al203 as sintering additives show a faster degradation of the hardness than materials with more refractory grainboundary phases (Fig. 20). 

This substitution of Al-0 for Si-N bonds results in an improved oxidation resistance of the material, compared to the original N-melilite phases (Chee et al. 1994). Such a kind of composition appears as an intermediate phase during sintering of a-sialon ceramics and also forms in SiAlON materials as a grain boundary phase with good refractory properties (Cheng and Thompson 1994a,b). 

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