In this paper, AIN was studied. In order to get high performance structural materials, preliminery investigation about processing conditions to produce the in-situ AIN composite was conducted. Macrostructure was observed and analysed by SEM and TEM mechanical components were determined by X-ray diffraction. The mechanical properties thermal expansion coefficient and weight gain after oxidation have been measured. Moreover, powders of AIN was synthesized by mean of reduction of Al_2O_3 with pure C powders. The results show that both flexural strength and fracture tougheness of the hot-pressed in-situ AIN composite were composite were greatly improved. It was found that fibrous structure found that fibrous structure formed in some specimens and consequently improved the mechanical behavior of the materials. Further study demonstrated that the fibrous in microstructure were AIN polytype (e.g. 27R, 21R ct al.). Epitaxial growth of fibrous AIN polytype was found by TEM observation. It can be concluded that Y_2O_3 for the in-situ AIN composite is not only as a densification aid but also as a medium for the growth of AIN polytype. In addition, SiO_2 was regarded as a fibre-growth promoter. According to the results mentioned above, crack deflection was regarded as the principal toughening mechanism to the in-situ AIN composite. The main fashion of fracture for specimens without fibrous microstructure was brittle.
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