Damage Mechanism Of Carbon Fibers During Preparation Of Ceramic Matrix Composites By Precursor Infiltration Pyrolysis

Focusing on serious damage of carbon fibers in ceramic matrix composites (CMCs) prepared by precursor infiltration pyrolysis (PIP), and with emphasis on carbon fiber reinforced silicon carbide composites (Cf/SiC) derived from polycarbosilane (PCS), the influences of raw materials (including carbon fibers and precursors, etc.) and preparing conditions on carbon fiber damage during the preparation of Cf/SiC composites were investigated systemically. Pyrolysis process, fiber-matrix interface and carbon fiber damage process were observed by SEM, TEM, IR, XRD, XPS and TQ etc. The characterization method of carbon fiber damage degree was established and the models for carbon fiber damage mechanism were also established. The carbon fiber coatings and unidirectional Cf/SiC composites with coated carbon fibers were prepared and studied.The multifilament tension strength test was firstly applied to characterizing carbon fibers damage degree in CMCs during PIP, which as an intractable problem was resolved successfully. The test results show that the measured strength is similar to marked strength, and with a deviation of 12-17%. The differences of carbon fiber damage caused by different preparing conditions are showed clearly. A good test method is applied for studying carbon fiber damage in PIP process.The mechanism of carbon fiber damage during preparation of CMCs by PIP was clarified firstly. The results show that the damage of carbon fibers includes chemical damage, heat damage and physical damage of thermal stress. The carbon fiber damage mainly occurs in the first cycle of PIP, in which carbon fiber strength lose is greater than the total strength lose in following cycles, and the main form of damage is physical damage of thermal stress. The chemical damage is not the main factor of strength lose. It was first proved that the combined oxygen does not damage carbon fibers obviously, which corrects the long-term misunderstanding about the influence of oxygen on carbon fiber damage.The mechanism of physical damage of carbon fibers was clarified firstly. The precursor which attached on carbon fibers changes to inorganic ceramic under high temperature with large volume shrinkage which appearing at cracks with fiber traction and the cracks are mainly vertical to fiber axis. Because of mismatched coefficients of thermal expansion, carbon fibers suffer multiple thermal stresses and are damaged. The physical damage modelsof carbon fibers are established, which are matrix shrink stress damage model, thermal tension stress damage model, thermal bend stress damage model and thermal shear stress damage model. The influence factors are crack size, matrix hardness, carbon fiber surface defect and carbon fiber modulus. Similar damage models have not been reported before.The influence of different precursors and carbon fibers on carbon fiber damage during preparation of CMCs by PIP was systemically studied firstly. The results show that PSO pyrolysis matrix has low hardness and good extensibility which can relax stress concentration in matrix and interface, and damages carbon fibers slightly. However, PSZ derived matrix damages carbon fibers largely in physical and chemical ways. The carbon fibers with low strength are broken easily according to thermal tension and bend stress damage models, and the carbon fibers with high modulus are broken easily according to thermal shear stress damage model in spite of less damage caused by heat. The carbon fibers with surface glue of epoxy resin are damaged easily in physical and chemical ways because of high oxygen extent and rigidity of epoxy resin. So, the carbon fiber with high strength, low modulus and flexible surface glue is suitable for the preparation Cf/SiC composites with high properties.The Coatings of SiC and Si-O-C were prepared by precursor pyrolysis and studied, and the unidirectional Cf/SiC composites with coated carbon fibers were also prepared and studied. The results show that coatings with suitable thickness can reduce damage degree of carbon fibers as b...