Fabrication And Fracture Mechanics Of Short Carbon Fiber Toughened Silicon Carbide Matrix Composites

Silicon carbon matrix composites have become candidate materials for space optical mirror because of low cast and stable performance following the optical materials such as Berullium, Glass, Aluminum alloy and Glass-ceramics and so on. However, the problems of inherent brittle of SiC ceramics have become the key restricted factor for wider application under inextreme circumstances. The manufacture technology of SiC matrix composites toughened short-fiber is an effective solution to deal with this problem. Short carbon-fiber with stable performance was introduced into SiC matrix composites, so the C_f/SiC composites with low cost and high fractrure toughness were fibricated by means of hot pressed technology, these results can provide technology support for SiC matrix composites applied for space mirror under extreme environment.Wet process was used to fabricate premixing material of C_f/SiC composites. Application of ball milling was benefit to refine SiC powder and disperse carbon fiber in SiC matrix material, and best processing parameter of ball milling was determined. The processing parameters include that the speed of ball milling was 180rpm,the time of ball milling was 8h and the proportion of ball and material was 4:1. The effect of carbon fiber content on the dispersity of carbon fiber in the matrix material was researched. When carbon fiber content was higher than 30%, the space bridging effect of carbon fiber occurred, but when carbon fiber content was lower than 30%, the carbon fiber dispersed uniformly.The relation between additives proportion and mechanical properties of hot-pressed C_f/SiC composites in the Al₂O₃/Y₂O₃,Al₂O₃/La₂O₃,AlN/Y₂O₃ and La₂O₃/Y₂O₃ system was studied. The result showed that C_f/SiC composites with high relative density could be obtained at 1900℃and 1h in the four additives system. When the sintered additives content was same, carbon fiber was damaged slightly in the C_f/SiC composites with Al₂O₃/Y₂O₃ and Al₂O₃/La₂O₃ system, but carbon fiber was damaged intensively in AlN/Y₂O₃ and La₂O₃/Y₂O₃ system. At the same time, the toughened mechanics of was researched, and the main toughened mechanics included particle bridging, crack deflection, fiber pull-out and micro-crack toughen, but the toughening effects of fiber pull-out were different in the four additive systems.The thesis researched effect of heat-treated technology on the microstructure and mechanical properties of C_f/SiC composites in Al₂O₃/La₂O₃ and La₂O₃/Y₂O₃ system. When heat treated temperature was higher than 1850℃and hold time was longer than 2h, SiC grain coarsed obviously and carbon fiber was damaged intensively in C_f/SiC composites with Al₂O₃/Y₂O₃ additive system. The fiber pull-out, crack deflection and particle bridging was the main toughened mechanics of heat treated C_f/SiC composites in three additives system. The in-situ formed SiC nanowire and a lot of dislocation structure were tested in Al₂O₃/La₂O₃ additives after C_f/SiC composites was heat treated at 1750℃for 1h, which resulted in the improvement of mechanical properties, and flexure intensity and fracture toughness was 395MPa and 7.42MPa·m1/2, respectively. The toughened mechanics including fiber pull-out, microcrack and nanowire toughened was the main reason for the improvement of mechanical properties.In-situ SiC nanowires were formed after C_f/SiC composites in Al₂O₃-La₂O₃ system were heat treated. The effect of heat treated temperature and hold time on the growth of SiC nanowires were analysized. The best process parameters to fabricate SiC nanowires in-situ toughened C_f/SiC composites were determined, and its parameters included that heat-treated temperature was 1750℃and hold time was 1h. The growth mechanism of SiC nanowire included "Vapour-Liquid-Solid" mechanism and"Oxide-Assisted Growth"mechanism,and different shaped SiC nanowires including straight line shape and branching structure could be fabricated by adjusting process parameters. The branched nanowire structure formed space net structure in the C_f/SiC composites, which was benefit to improve mechanical properties of the C_f/SiC composites. The formed SiC nanowires were distributed in the grain boundary of SiC grain, which strengthened SiC grain boundary and improved the fracture toughness.