Preparation And Evaluation Of Antioxidant SiC Coating For C/C Composite

As an advanced high-temperature thermal structure composite material,C/C composite has the characteristics of high specific strength,low density,and excellent the rmal shock resistance.It can still perform excellent mechanical properties under non-oxygen and above2200°C service conditions.It has been successfully applied to engine thermal structure components,missile cones,etc.The C/C composites undergo oxidation under the conditions of oxygen at 400°C,and the oxidation rate increases significantly at 500°C,resulting in catastrophic failure of the C/C composite.Therefore,solving the high temperature and easy oxidation problem of C/C composites is the prerequisite for ensuring its application in high temperature aerobic environment.Coating technology is an effective means to improve the oxidation resistance of materials.SiC is a high-temperature ceramic with high strength and excellent oxidation resistance.It has a small difference in thermal expansion coefficient and good compatibility with C/C composites,which can improve the service temperature of C/C composites under high temperature and aerobic environment.CVD and PC are common process technologies for preparing SiC coatings,enabling the preparation of low temperat ure SiC coatings.In this thesis,C/C composites were used as matrix materials,and SiC coatings were prepared by CVD and PC processes,respectively.The effects of different deposition times of CVD-SiC coatings and different Al2O3 additives of PC-SiC coatings on the microstructure and oxidation resistance were investigated.The oxidative failure modes and oxidation failure mechanism of SiC coatings prepared by different methods were investigated.The study found that the SiC coating prepared by low temperature CVD method is amorphous,the coating has spherical particles,compact and uniform,and has a linear interface with the C/C matrix.The bonding strength is low and the damage to the C/C matrix is small.As the deposition time increases,the thickness of the CVD-SiC coating increases and tends to be integrated,and the oxidation resistance is enhanced.When the deposition time is 60 minutes,the oxidation loss rate of the coating at 1200°C for 0.5 hours is 25.49%;The PC-SiC coating has high crystallinity,and the coating consists of irregular polygonal particles.The bonding interface with the C/C matrix has a mosaic structure,and the bonding strength is high,and the matrix fibers have different degrees of Si corrosion.As the content o f Al₂O₃ additive increases,the PC-SiC coating particles are refined,the thickness is increased,and the density and oxidation resistance are improved.When the Al₂O₃ additive content is 5%,the oxidation weight loss rate after oxidation for 1 hour is 10.51%.The thermal expansion coefficient of the SiC coating and the C/C matrix are different and cracks occur.Oxygen diffuses through the crack into the interface between the CVD-SiC coating and the matrix composite,where the oxidation reaction of the C matrix occurs,thereby causing the substrate to lose the coating.The support of the layer causes the coating to peel off and the coating to fail.For the PC-SiC coating,the oxidation reaction takes place at the bonding interface between the SiC coating and the C fiber.After the C fiber is oxidized,leaving a hole inside the SiC coating can accelerate the oxidation of the substrate as a rapid diffusion channel of oxygen,thereby causing oxidation failure of the material.