Effects Of Al₂O₃ Interface Coating And CNTs Introduction On The Structure And Properties Of PyC Matrix

C/C composite materials are widely used in aerospace,military,biomedical and other fields benefit by its low density,high specific strength and specific modulus,friction resistance,ablation resistance and ultra-high temperature strength retention rate,etc.Traditional C/C composites have some serious problems such as multi-structures in one sample and low mechanical properties.From a macro perspective,the strength of C/C composites come from three aspects:the strength of the reinforcement,matrix strength,and the F/M interface bonding strength.Because the strength of the fiber is constant,therefore,this dissertation studies the microstructure and mechanical properties of the composites from the aspects of interface bonding strength and matrix strength.The high temperature heat treatment and the addition of Al₂O₃ coating improve the interface of the composite.On the other hand,the PyC matrix was prepared by using SiO₂ fiber felt as a preform,and use CNTs to induce the structure and improve the mechanical properties of the PyC matrix.In the experiment,The TG-CVI method was used,with liquefied gas as the carbon source precursor and nitrogen as the diluent gas.The analysis method of PLM,VGT,SEM,and three-point bending,fracture toughness,and tensile test methods were used to modify the composite interface and the influence of the microstructure and mechanical properties of the CNTs modified pyrolytic carbon matrix has been studied.The main work and results are as follows:?1?C/C composites were prepared with 3D needle-punched carbon fiber preforms.The effects of 1600?,4h high temperature heat treated performs on the microstructure and properties of C/C composites were studied.The results show that the structure of both C_f/C and C₁₆₀₀/C is SL.After heat treatment of the preform,the three-point bending strength has decreased from 76.89 MPa to 57.55 MPa,decreased 25%.But the fracture toughness increased from 0.938 MPa·m^1/2 to 2.141 MPa·m^1/2,increased 128%.Heat treatment removes the oxygen-containing functional groups on the fiber surface,which can significantly improve the fracture toughness of C/C composites.?2?After the Al₂O₃ coating was applied to the fiber surface of the preform after heat treatment,the structure of the C_1600-Al2O3/C matrix became RL,indicating that the Al₂O₃ coating can significantly affect the structure of the carbon matrix.By adding Al₂O₃ coating to improve the FMBS of the composite,its fracture toughness was increased from 0.938 MPa·m^1/2 of C_f/C to 3.061 MPa·m^1/2 of C_1600-Al2O3/C,increased226%.Through the interface modification and the induction of the matrix structure,C_1600-Al2O3/C obtained the best interface bonding strength between the fiber and matrix and the bonding strength between graphene sheets in the matrix.However,the flexural strength is mainly affected by the final density of the C/C composite.?3?Bare quartz fiber reinforced pyrocarbon composites characterize with SL PyC matrix and strong interface bonding.Both the composite and the matrix show brittle fracture behavior with low flexural and tensile strength.PVA modified quartz fiber reinforced pyrocarbon composites characterize with RL PyC matrix.The composites also show a strong interface bonding with slightly improved mechanical properties.PVA containing CNTs treated quartz fiber reinforced pyrocarbon composites show significantly improved flexural and tensile strength as well as a much weaker interface bonding compared with the former two.Randomly oriented CNTs reinforced PyC micro-composites account for the pseudo ISO structure and contribute to the improved mechanical properties of the PyC matrix.?4?Interface bonding and relative strength between reinforcing fiber and matrix greatly affects the mechanical behaviors of the quartz fiber reinforced pyrocarbon composites:Low volume fraction and low strength of reinforcement with strong interface bonding results in limited strengthening effect on flexural strength of a composite.By contrast,same volume fraction and strength of reinforcement with weak interface bonding leads to more effective strengthening effect on flexural strength of a composite.?5?Higher volume fraction and low strength of unidirectional fiber with weak interface bonding reinforcing a much stronger matrix results in limited strengthening effect on tensile strength of a composite,whereas same volume fraction and strength of reinforcement with strong interface bonding leads to more effective strengthening effect on tensile strength of a composite.