Preparation And Properties Of C/ZrC（ZrB₂）-SiC Composites By Polymer Infiltration And Pyrolysis

Development of the hypersonic aircraft proposed new requirement for the thermal structure components. It is pressing to research and development ultra high temperature resistant material with excellent mechanical properties and ablation performance. Continuous fiber reinforced ultrahigh temperature ceramic matrix composites with excellent properties, such as thermal resistance, toughness and ablation resistance.such as thermal resistance, toughness and ablation resistance. In this paper, C/ZrC-SiC and C/ZrB₂-ZrC-SiC composites were prepared via precursor infiltration and pyrolysis（PIP）with the ZrC and ZrB₂ precursor Produced by Institute of Process Engineering. The influence of PIP-SiC and CVD-C interface coating on the properties of the composites was studied and discussed.Structure and pyrolysis mechanisms of ZrC and ZrB₂ precursor were investigated. Two kinds of precursor polymer at 1500 ℃ for 2 h, the purity of ceramic is high. The ceramic yield of ZrC and ZrB₂ precursor were 36.1% and 29.4%, respectively. Inorganic process of the ZrC precursor is compelet at 1200°C for 1h, its products are Zr O2 and C. For ZrB₂ precursor, to form ZrB₂ at 1300 ℃, its transformation process is implemented by the chemical reaction between Zr O2, B2O3 and C.The C/ZrC-SiC and C/ ZrB₂-ZrC-SiC composites were prepared with 2D Z-pin carbon fiber cloth and carbon felt. Vacuum impregnation were selected, whose densification efficiency was same to vacuum pressure impregnation. 3 cycles inorganic treatment at 1200°C and following carbo-thermal treatment at 1500°C were employed for preventing C fibers from injuring with long time high temperature treatment. The density of the C/ZrC-SiC composites was 2.3g/cm3, the void content was 33%, the flexural strength and modulus were 60 MPa and 17 GPa, respectively. Those of the C/ZrC-SiC-ZrB₂ were 1.90g/cm3, 20%, 56.9MPa and 22.7GPa, respectively.Analysis of composite microstructure showed that carbon fiber got a certain degree of damage in the process of material reparation, the strength of fiber decreased,and formed a strong interface bonding between fiber and matrix, the matrix was porous mesh structure with low load capacity.In order to avoid or reduce the fiber damage in the process of pyrolysis. SiC and C interface coating was fabricated on carbon fiber by PIP and CVD processes respectively.The influence of PIP-SiC coating thickness on the properties of C/ZrC-SiC composites was investigated. After the preparation of PIP-SiC the coating, composite density increased. When the content of PIP-SiC is 15.4vol%, its density Increases from 2.30g/cm3 to 2.50g/cm3, and its porosity decreases by 33.0% to 20.0%. When PIP-SiC content is 5.6vol%, its mechanical properties is best, with bending strength increasing from 58.1MPa to 89.0MPa, and the fracture toughness increasing from 3.6MPa·m1/2 to 4.5MPa·m1/2. After the preparation of the coating, bending strength retention at 1600℃ for 10 min oxidation can be up to 91%, slightly higher than the 87% of uncoated.The influence of CVD-C coating thickness on the properties of C/ZrC-SiC composites was investigated. When the deposition time is 20 h, the density of composite with fiber cloth and carbon felt as Strengthen the body is the largest, 2.50g/cm3, 2.90 g/cm3, respectively, which is higher than no coating·s,whose is 2.30 g/cm3,at the same time, its Bending strength and fracture toughness is lower than that of uncoated composites, is 52.9MPa·m1/2, 1.9MPa·m1/2, respectively. But the density decreased with thick coating. With the increase of coating thickness, the bending strength and fracture toughness of the two composites are improved. With carbon cloth, casting coating preparation time for 20 h, 40 h, the strength retention after oxidation can reach more than 94%, and the strength retention of the others is under 40%, or even structure is failure.