Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

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A new kind of oxidation protection coating of Si-MoSi2 was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550°C. The overall oxidation behavior could be divided into three stages: (i) 500°C < T < 800°C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800°C < T < 1100°C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100°C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi2 layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550°C for 50 h in air and 1550°C ⇔ 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.

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