Prepareation And Properties Of Modified Carbon Fiber Reinforced Silicon Carbide Composite

Carbon fiber reinforced silicon carbide(C_f/SiC)composite is a new type of structural functional material using carbon fiber as reinforcement while SiC ceramic as matrix.It has excellent properties such as low density,high strength,high modulus,high thermal conductivity,good corrosion,wear,oxidation and ablation resistance,thus has widely been used in high-tech fields such as aerospace and military.However,traditional preparation process has the disadvantages of cumbersome,long cycle and uneconomical.The as-received C_f/SiC products usually prone to brittle fracture in high temperature causing catastrophic damage.Their service life and stability in harsh environment still need to be improved.In order to solve this problem,two processes of compression molding-melt silicon infiltration and hot pressing method were designed to prepare the composite materials.Their advantages and disadvantages were carefully analyzed and compared.The graphene and ZrO₂ coating were deposited on carbon fiber surfaces,and Ti element was introduced into the SiC matrix to modify the reinforcement and the matrix respectively.rGO-C_f/SiC,ZrO₂-C_f/SiC and C_f/Ti₃SiC₂-SiC composite were successfully prepared.Compared with the traditional C_f/SiC composite,the mechanical properties and high temperature oxidation resistance have been significantly improved,which is contributed to the development and application of C_f/SiC composite.The pretreated carbon fiber was chopped into a small length of 4-6mm,then uniformly dispersed with ceramic powders in an aqueous solution of sodium carbonxymethylcellulose by wet mixing.C_f/SiC composites with different fiber content were prepared by combining the compression sintering and reactive melt infiltration process.It was found that,after pretreatment process,the sizing agent was significantly removed,and obvious grooves were generated on fiber surface with large numbers of oxygen-containing functional groups attached on it.All short carbon fiber bundles were dispersed into monofilaments which uniformly distribute in the C_f/SiC composite at appropriate dispersant concentration and temperature.The excessive thermal stress promoted the transformation of SiC crystal from???,resulting in the coexistence of?-SiC and?-SiC.In addition,C_f/SiC composite with 30vol.%fiber content shows the higheset flexural strength,fracture toughness and shear strength,and the weight loss rate reach 10.1%after oxidation at 1600?for 5h.The modified Hummer's method was used to prepare the graphene oxide(GO)solution using natural flake graphite as raw material.Then,the GO nanosheets were deposited on carbon fiber surface by electrophoretic depositon process.After the reduction,the rGO-C_f/SiC composite was prepared by mixing the modified fibers with the ceramic powders.It was found that the deposition of GO nanosheets on fiber surface was mainly affected by the depositon voltage,GO concentration and deposition time.After modification,the specific surface area and affinity of the carbon fiber were significantly improved,thus enhancing the interface between the fiber and the matrix,the mechanical properites of the rGO-C_f/SiC were improved as well.Compared with the untreated C_f/SiC composite,the flexural strength,fracture toughness and shear strength increased by more than 20%.After oxidation for 5h at 1600?,the weight loss rate was merely 9.6%and maintained about 48%of the flexural strength.Zirconium hydroxide sol-gel solution was prepared under acidic conditions using zirconia powder as the raw material.A uniform ZrO₂ coating was applied on carbon fiber surface by cyclic dip-coating process.After annealing,ZrO₂-C_f/SiC composite was prepared by mixing the ZrO₂ coated carbon fiber with ceramic powders.It was found that the stability of the zirconium hydroxide sol solution is influenced by the concentration of Zr⁴⁺at a specific temperature and p H.As the cycle number increases,the thickness of the ZrO₂ coating on carbon fiber increases continuously.After 5 cycles,the carbon fiber surface is covered completely.The chemical composition of this coating is detected to be m-ZrO₂ by XRD analysis,which gradually transformed into t-ZrO₂ with more stable thermodynamic properties during sintering.Compared with the rGO-C_f/SiC composite,the improvement of mechanical properites is not obvious,but the high-temperature oxidation resistance is significantly improved.The weight loss rate dropped to 7.8%after oxidizing for 5h at 1600?.Ti C,Siand C powders were added into the pretreated carbon fiber dispersant solution according to the ratio of 3:(3+x):x.The ternary Ti₃SiC₂ phase was synthesized under high temperature and C_f/Ti₃SiC₂-SiC composite was prepared via hot pressing process.It was found that the content of Ti₃SiC₂ in the composite is the hightest when the value of x is 0.8,which is uniformly distributed in the SiC matrix and shows a typical lamellar morphology.The as-prepared C_f/Ti₃SiC₂-SiC composite shows a dense structure and low porosity without any obvious defects appering on its surface.Compared with the untreated C_f/SiC,the mechanical properites and oxidation resistance have been greatly improved.After high-temperature treatments,the weight loss rate reduced to 5.6%and the ablation rate is merely 0.498mg/s.The retention of flexural strength and shear strength reached 72.1%and 52.7%,respectively.