Properties And Structures Of KD-S SiC_f/SiC Composites Fabricated By PIP Process

The fiber/matrix interface in SiCf/SiC composites plays an important role in the properties of composites.The interface design and preparation have become the key research direction in the preparation of SiCf/SiC composites.At present,the interface of SiCf/SiC composites includes pyrolytic carbon?PyC?,boron nitride?BN?and silicon carbide?SiC?.Compared with the strong bonding of SiC interface,BN interface and PyC interface can induce crack deflection,prolong crack propagation path and improve fracture toughness of SiCf/SiC composites because of its lamellar structure.Compared with the lower oxidation initiation temperature of the PyC interface,the BN interface is more suitable for use under oxidizing conditions because of its good oxidation resistance,which has attracted wide attention from scholars at home and abroad.In this paper,the third-generation SiC fiber?KD-S?with near-stoichiometric ratio was used as the reinforcement,and the single precursor borazine was prepared for chemical vapor deposition?CVD?.The effects of different interface coating thickness and different pyrolysis temperature on the structures and properties of KD-S SiCf/SiC composites were investigated systematically.The mechanical properties and structural evolution of KD-S SiCf/SiC composites under high temperature treatment were investigated.The research in this paper is mainly divided into three parts.The first part is to study the effect of interface coating thickness and heat treatment temperature on the structures and properties of KD-S SiC fiber.The results show that the existence of BN interface causes internal stress between interface and fiber,which reduces the tensile strength of SiC fiber.With the increase of thickness of BN coating,the effect of internal stress on KD-S SiC is stronger and the tensile strength of the KD-S SiC fiber decreases.The tensile strength of fiber remained unchanged at room temperature and heat treatment at 1200°C for 1 h.With the further increase of heat treatment temperature,the crystallinity of BN interface increased and BN grains also grow,resulting in a decrease in tensile strength.In the second part,the effects of PIP process preparation conditions on the structure and properties of KD-S SiCf/SiC composites were investigated.The results show that the density,porosity,flexural strength,fracture toughness and thermal conductivity of KD-S SiCf/SiC composites decrease with the increase of BN coating thickness.At the same time,it is found that the KD-S SiCf/SiC composites with different BN interface exhibit pseudo-plastic fracture mode at the time of fracture,which indicates that the crack in the matrix has deflected in the BN coating,which avoids the tip stress of the crack.The fracture toughness of KD-S SiCf/SiC composites was improved.The effects of different pyrolysis temperatures on the structure and properties of KD-S SiCf/SiC composites were investigated.The results show that the density of composites increases with the increase of PIP pyrolysis temperature.At the same time,bending strength and fracture toughness show a downward trend.The thermal conductivity and thermal diffusivity of KD-S SiCf/SiC composites increase with the increase of temperature.The crystallinity of composites under high temperature pyrolysis is high,which is helpful for heat transfer.The third part studied the effects of different high temperature oxidation environments on the structures and properties of SiCf/SiC composites.The results show that the flexural strength and fracture toughness of KD-S SiCf/SiC composites prepared with BN interfacial coatings are improved compared with room temperature at dynamic mechanical tests at 900°C,1100°C and 1300°C.The bending strength and fracture toughness of the PyC layer KD-S SiCf/SiC composites are lower than those at room temperature.In the micro-morphology of the fracture of KD-S SiCf/SiC composite with BN interfacial coating,the phenomenon of crack termination on the fiber surface can be found.Elemental analysis of the fiber interface matrix shows that oxygen is mainly concentrated in the matrix-interface and fiber-interface,and with the oxidation temperature increasing,the oxidation of the BN interface increased,the BN interface has been completely oxidized at 1300°C.The effect of oxidation time in air and water vapor environment on the mechanical properties of KD-S SiCf/SiC composites at 1300°C was also studied.With the treatment time prolonged in air?1h to 10h?,the flexural strength of KD-S SiCf/SiC composites first decreased and then remained stable,and the fracture toughness continues to decrease.With the prolongation of treatment time?1h to 10h?in water vapor,the bending strength of KD-S SiC_f/SiC composites continued to decrease,and the fracture toughness continued to decrease.At the same time,it is found that the density of the composite is slightly increased by oxidation,which is related to the formation of glassy SiO₂ and B₂O₃ during oxidation.