Preparation And Properties Of C_f/ZrC-SiC Ultra High Temperature Ceramic Composite

With the urgent demand of ultra-high temperature thermal structure material for themilitary and aerospace fields, the proposal to prepare Cf/ZrC-SiC ultra-high temperatureceramic composites by reactive melt infiltration (RMI) process was presented toimprove the properties of Cf/C-SiC prepared by precursor infiltration and pyrolysis (PIP)through converting C into ZrC ultra-high temperature ceramic.In this paper, preparationand properties of Cf/ZrC-SiC composites by RMI method was investigated.According to the principle of RMI process, Zr-Si alloy was chosen as the reactiveinfiltrant (The melting point of Zr-Si alloy was1570°C, which was lower than the pureZr (1850°C), and the certificate of quality showed that the content of Zr was97.1wt.%).The presence of element Si could improve the wettability between Zr and C, and themain product SiC was one component of the target products. Zr-Si alloy could reactwith C at1600℃accordings to thermodynamics calculations and experiments.The structure of Cf/C-SiC substrate was designed from the fabric structure, carbonmatrix and pore structure on the base of theoretical model. Three dimensional and fourdirectional braided carbon fibers were chosen as reinforced phase, which hadoutstanding mechanical properties and fitting micro-structure for infiltration. TheCf/C-SiC substrate was prepared by PIP technology using polycarbosilane (PCS) andphenolic, respectively.The microstructure of Cf/ZrC-SiC composite prepared by RMI technology wasinvestigated. With the increase of infiltration temperature, the density of the resultingcomposites and the content of residual alloy would reduce. When infiltrationtemperature was reached to1700℃, the density of the composites increased with theincrease of time.The permeability was demonstrated to be a function with infiltration temperatureand infiltration through the analysis of the RMI process. With the increase of infiltrationtemperature, the developement of structure was complex. On the one hand, C atomscould diffuse much faster through ZrC layer to react with alloy at higher temperature,and formed much more ZrC phase, which could reduce the residual alloy. On the otherhand, the velocity of Zr-C reaction would be quickly, the ZrC phase leaded to volumeexpansion. The volume expansion would create much more “close pore” in thecomposite, which resulting in the fall of density of the composite.Preparation of Cf/ZrC-SiC composites by RMI method was investigated. The openporosity of Cf/C-SiC was controlled to be about21.26%by PIP process, then thepreforms were infiltrated with Zr-Si at1700℃for6h. The resulting showed that theCf/ZrC-SiC composites had a bending strength of198.65MPa and a bending modulusof66.13GPa. Thermophysical properties of Cf/ZrC-SiC composites in Z direction were researched.At room temterature, the coefficient of thermal expansion (CTE) is5.2×10-6℃-1, and theCTE at high temperature is higher than that at low temperature. The specific heatcapacities (Cp) is0.612J.g-1·K-1, the thermal diffusivity (α) is3.295mm2·s-1and thethermal conducivicy (λ)5.93W·m-1·K-1. From room temperature to1200℃, α decreasedgradually, while Cpand λ increased gradually.The oxidation behavior of Cf/ZrC-SiC composites was researched. The resultsshowed that the masses of the composites were increased after oxidation at1200℃, andthe oxidation product SiO2and ZrO2distributing at the area where1mm below thesurface of the composite, which could prevent a severe reaction between O2and thecomposite.The Cf/ZrC-SiC composites had outstanding high temperature ablation resistancethan the Cf/SiC composites prepared by PIP. After ablating by oxyacetylene flame for30s, the mass ablation rate of the Cf/ZrC-SiC composites was0.013g·s-1and the linearablation rate was0.022mm·s-1.