Interfacial Modulation Of Single-phase Fiber C_f/C Composites And Its Effect On The Structure And Properties

C/C composites have low density,high specific strength,high specific modulus,low coefficient of thermal expansion,ablation resistance,friction resistance and unique high temperature mechanical properties,which are widely used in aerospace,military and other fields.However,due to the high interfacial bond strength between the fiber and the pyrolytic carbon matrix,which leads to brittle fracture upon material failure,the performance of the fiber cannot be fully exploited.the excellent performance of C/C composites mainly depends on three parts:carbon fiber,matrix carbon and interface.In this paper,SiO₂ sol and Al₂O₃ sol were prepared by the sol-gel method to obtain carbon fiber preforms coated with SiO₂ and Al₂O₃.Using a homemade TG-CVI deposition equipment,the preforms were densified,and the bonding strength of the interface was adjusted by high temperature heat treatment to improve the graphitization of pyrolytic carbon.The effects of the composite interface and the microstructure and mechanical properties of the pyrolytic carbon matrix were investigated by means of PLM,VGT,SEM,XRD and other analytical methods and density tests and tensile properties tests,and the main work and results are as follows:（1）The pyrolytic carbon structure of the uncoated modified C/C composites is mainly dominated by the SL structure,and the higher the density of the samples,the higher the content of pyrolytic carbon in the RL structure,which is caused by the transformation of the deposition mechanism at the late stage of deposition.After three hours of high temperature treatment at 1700°C,1800°C and 1900°C,the results showed that the graphitization increased as the heat treatment temperature increased,while the longitudinal cracks and annular cracks of the C/C composites increased,leading to a decrease in tensile properties,but the fracture model shifted from brittle fracture of the C_f/C-0 samples to pseudoplasticity of the C_f/C-1,C_f/C-2 and C_f/C-3 samples fracture,and the tensile elongation of the material increased from4%for C_f/C-0 to 11%for C_f/C-4.（2）The SiO₂-coated preforms were densified and heat treated by the same process.The prepared pyrolytic carbon structure is dominated by SL structure and the content of pyrolytic carbon in RL structure increases with the increase of density.However,the introduction of the interface reduced the bond strength of the interface,and the strength of the samples coated with SiO₂ in tension decreased before and after the heat treatment,with the tensile strength of C_f（SiO2）/C-0 decreasing by about 32%compared to C_f/C-0.The high-temperature heat treatment caused the SiO₂ coating to react with carbon fibers or pyrolyzed carbon matrix,and the generated Si C caused damage to the fibers and carbon matrix,but the molten SiO₂ could repair some of the cracks.High temperature heat treatment makes C_f（SiO2）/C-1,C_f（SiO2）/C-2 and C_f（SiO2）/C-3 tensile strength decrease compared with C_f（SiO2）/C-0,but elongation at break increases and toughness increases.（3）The Al₂O₃-coated preforms were densified and heat treated by the same process.the introduction of Al₂O₃ coating changed the active sites on the carbon fiber surface and affected the deposition mechanism,changing the pyrolytic carbon matrix from SL structure to RL structure compared to the uncoated C/C composites.the tensile strength of C_f（Al2O3）/C-0decreased by only 15%compared to C_f/C-0 at lower density.Al₂O₃ coating and high-temperature heat treatment decreased the interfacial bonding strength of the C/C composites,and the graphene lamellae of the RL structure increased the degree of graphene after high-temperature heat treatment,which changed the fracture mechanism.The high-temperature heat treatment caused chemical reactions near the interfaces of samples C_f（Al2O3）/C-1,C_f（Al2O3）/C-2,and C_f（Al2O3）/C-3,which changed the bond strength at the interface and improved the graphitization of C/C composites.