Preparation And Oxidation Behavior Of C/C-SiC Composites Coated With High-temperature Protective Coatings

Liquid silicon infiltration?LSI?or gaseous silicon infiltration?GSI?as a simple,economical and time saving fabrication method for C/C-SiC composites has been widely applied in the fabrication of CMC composites.The C/C-SiC composites fabricated by LSI or GSI have C/C-SiC composites have characteristics of high specific strength,high thermal conductivity,low thermal expansion coefficient and high thermal shock resistance,which are derived from their unique compositions and structures.However,the C/C-SiC composites can be oxidized by oxygen above 773K.Much effort has been made to improve their high temperature oxidation resistance.One most important attempt to contribute to the performance of C/C-SiC composites in high temperature environment is applying a high-temperature protective surface coating on the composites substrate.Based on GSI and slurry coating process,a new approach to fabricating surface coating on C/C-SiC substrate is proposed to improve the high temperature oxidation resistance of C/C-SiC composites.The C/C performs were prepared after the densification of carbon felts through CVD and PIP process.The pre-coatings containing ZrO₂,B₄C and C particles,were prepared on the C/C performs by slurry coating process.During siliconization,ZrB₂ phase was obtained by reducing ZrO₂ using B₄C and C,and Si reacted with residual C to form SiC simultaneously.In addition,Si passed through the pre-coating and reacted with C matrix to form SiC substrate.The pores in the pre-coating were filled with the residual Si and strongly bonded with the C/C-SiC substrate.The carbon felt C/C-SiC composites with a Si-rich Si-ZrB₂-SiC coating were obtained after gaseous silicon infiltration process.The influence of temperature,atmosphere and proportion of the raw materials on the species of the resultants was investigated.The ZrO₂ and B₄C powder were mixed thoroughly in a 7:6 molar ratio and heated to 1500?for 30min,thus the ultrafine ZrB₂particles were obtained.The C and O element content in the synthetic ZrB₂ particles was below 1.5wt.%.The influence of the thickness of pre-coating on the density,open porosity and flexural strength of the final coated C/C-SiC composites was discussed.Measure standard of the thickness of the pre-coating is the mass addition per unit area?mg/mm²?for the C/C samples.It was found that about 30⁵5?m increased in the thickness of the Si-ZrB₂-SiC coating for every increased 0.1mg/mm² of the pre-coating.The density of the coated C/C-SiC composites increased with the thickness of the outer coating in some scope,as the outer coating is dense and Si-rich,also contains a certain amount of ZrB₂,which has a higher density.However,when the thickness of the coating continued to increase,there was a sharp decline in the density of the coated composites.The effect of the thickness of the coating on the open porosity and flexural strength of the coated C/C-SiC composites was very small or non-existent,when the thickness of the pre-coating was below 0.4mg/mm².The distribution and morphology of cracks within the coating,and the effect of the species of carbon felts on them were investigated.Two different kinds of carbon felts C/C-SiC composites were fabricated by siliconization of C/C performs using needled carbon cloth composite fabric and needled short fibers whole carbon felt as the reinforcement respectively.Cracks within the coating on the needled carbon cloth composite fabric C/C-SiC substrate run preferentially perpendicular to the fiber orientation of the C/C-SiC composites,as result of the high shrinkage impediment along the fiber direction during cooling.The needled short fibers whole carbon felt C/C-SiC composites containing more short fibers have a higher coefficient of thermal expansion,which can lead to a crack-free surface.Oxidation behavior of the coated C/C-SiC composites at 800?and 1400?respectively in static air was discussed.It was found that the mass loss rate of the coated needled carbon cloth composite fabric C/C-SiC composites was much faster than the coated needled short fibers whole carbon felt C/C-SiC composites respectively.After oxidation for 20h in the air at 800?,the weight loss of the coated needled short fibers whole carbon felt C/C-SiC samples was 0.013wt.%.The coated needled short fibers whole carbon felt C/C-SiC samples were still in weight gaining state after oxidation for80h in the air at 1400?.Double-layer Si-ZrB₂-SiC coatings were prepared on the needled carbon cloth composite fabric C/C-SiC samples by double slurry paintings followed by siliconization each time.The double-layer Si-ZrB₂-SiC coatings can provide a better protection on the needled carbon cloth composite fabric C/C-SiC than the single-layer coatings.The needled carbon cloth composite fabric C/C-SiC samples with double-layer Si-ZrB₂-SiC coatings were still in weight gaining state after oxidation for 18h in the air at 1400?.