Preparation And Study On Properties Of The Cf/ZrB₂-SiC Composites By Ceramic Slurry Injection/precursor Method

In recent years,the continuous carbon fiber toughened ultra-high temperature ceramic matrix composites?Cf/UHTCMCs?have become important candidate materials for hypersonic aircraft nose cone,wing leading edge and other extreme thermal components because the material combines the anti-oxidation ablation properties of ultra-high temperature ceramics and the strength and toughness of carbon fibers.However,the structural damage which carbon fibers are prone to in the composite preparation process and the inhomogeneous combination of continuous carbon fibers and ultra-high temperature ceramic phase restrict the engineering application of Cf/UHTCMCs.In this paper,the C_f/ZrB₂-SiC composites were fabricated by a combination process of slurry injection/vacuum impregnation/precursor impregnation and pyrolysis innovatively.The effects of the introduction of the pyrolytic carbon coating and the carbon fiber content on microstructure,mechanical properties,thermal shock resistance and oxidation resistance of the C_f/ZrB₂-SiC composites were studied in particular.The mechanism of the pyrolytic carbon coating in the C_f/ZrB₂-SiC composites was analyzed,and the intrinsic relationship among the fiber content,microstructure and properties was revealed.The synergy of“strengthening and toughening”and“oxidation resistance”of Cf/UHTCMCs was realized,which laid the theoretical foundation for the engineering application of ultra-high temperature ceramic materials.In terms of microstructure and mechanical properties,the introduction of the pyrolytic carbon coating on the carbon fiber made the mechanical properties of the C_f/ZrB₂-SiC composites greatly improved because it could promote the effectiveness of toughening mechanisms of fiber pull-out,fiber bridging,crack deflection,crack branching,crack termination and so on during the crack propagation process through weakening the strong interface bonding between the carbon fiber and the matrix and inhibit the generation of microcracks through coordinating the mismatch of the thermal expansion coefficient between the carbon fiber and the matrix despite the decrease of the density and the relative density.When the carbon fiber content was certain,the fracture toughness and the work of fracture of the 33.3 vol.%C_f-PyC/ZrB₂-SiC composite were5.20±0.46 MPa?m^1/2 and 1629 J/m²,which were about 3 and 42 times higher than those of the 33.3 vol.%C_f-AS/ZrB₂-SiC composite respectively.When the carbon fiber was coated with the pyrolytic carbon coating,with the increase of the carbon fiber content?from 8 vol.%to 33.3 vol.%?,the content of ZrB₂ phase,the density and the bending strength of the C_f-PyC/ZrB₂-SiC composites decreased gradually,whereas the fracture toughness and the work of fracture of the materials showed a trend of increasing first and then decreasing.The fracture toughness and the work of fracture of the 21.3 vol.%C_f-PyC/ZrB₂-SiC composite were the highest,which were 8.20±0.55 MPa?m^1/2 and 2045J/m²,respectively.In terms of thermal shock resistance and oxidation resistance,the pyrolytic carbon coating could serve as a sacrificial layer to protect carbon fibers and effectively inhibit carbon fiber damage in high-temperature air atmosphere.It could prevent further oxidation of the C_f/ZrB₂-SiC composites and maintain the toughening effect of carbon fibers.When the carbon fiber content was certain,the critical thermal shock temperature difference of the 33.3 vol.%C_f-PyC/ZrB₂-SiC composite was 776?C,which was 103?C higher than that of the 33.3 vol.%C_f-AS/ZrB₂-SiC composite.And after the static oxidation experiment,the fiber bundles of the latter samples were almost completely oxidized and the oxide layer was thicker than that of the former.When the carbon fiber was coated with the pyrolytic carbon coating,the critical thermal shock temperature difference of the C_f-PyC/ZrB₂-SiC composites had little change?789?C,792?C,776?C?with the increase of the carbon fiber content?from 8 vol.%to 33.3 vol.%?.However,the oxidation layer of the 21.3 vol.%C_f-PyC/ZrB₂-SiC composite was the thinnest?62?m?and its oxidation resistance was the best benefiting from the higher relative density.