Oxidation Damage Anaiysis Of C/SiC Composites On The Microscpic Scale

Continuous carbon fiber toughened silicon carbide composites(C/SiC)have been successfully used in aerospace,national defense and military fields due to their excellent oxidation resistance at high temperature,lightweight,high strength and high stiffness.However C/SiC composites as a result of the limitation of the preparation process and the influence of the thermal expansion mismatch between the component materials,material can produce a variety of internal pore and micro cracks matrix,the defects in the high temperature oxidation environment such as oxidizing atmosphere in diffusion channels,lead to C/SiC in the actual service of high temperature oxidation environment cannot fully exert ideal antioxidant effect.The mechanism of oxidation damage of C/SiC at high temperature has attracted extensive attention and research by scholars at home and abroad.At present,the theoretical modeling of oxidation combined with the high temperature oxidation experiment is an effective means to explore the oxidation mechanism of C/SiC.In this paper,a mathematical model describing the oxidation of C/SiC at high temperature is established based on the gas phase diffusion and gas solid reaction kinetics theory.In this model,the material is approximately regarded as a porous material,and the diffusion and absorption of oxygen are described in the diffusion equation containing the absorption term.The oxidation damage state of the material is defined by the conversion rate of the solid phase in the gas-solid reaction kinetics,and the relationship between the oxidation rate constant and the temperature is established by the Arrhenius formula.In addition,the reaction activation energy of carbon fiber was calculated as 96 k J /mol based on thermal analysis kinetics method.The activation energy of SiC reaction is 111 k J /mol,and the most probable oxidation mechanism mode is phase interface reaction.In order to verify the rationality of the oxidation model,the governing equations of the model were numerically solved based on the COMSOL mathematical module,and the damage morphology simulation of uniform oxidation and local oxidation of carbon fiber and the parabolic growth law of the SiC oxide layer were simulated and verified.In addition,a multi-scale analysis method was used to establish the micro fiber bundle cell model and the micro fiber braided cell model,respectively.The periodic boundary conditions were applied to investigate the effect of carbon fiber oxidation notch and matrix microcrack on the equivalent engineering elastic constant of C/SiC.The results show that the longitudinal equivalent elastic properties of C/SiC micro filamentous cells are more sensitive to the oxidation notch and matrix crack of carbon fiber.For the C/SiC plain braided meso-scale cell,the oxidation notch at the local crack of the wire bundles mainly affects the in-plane elastic properties of the cell.High-temperature oxidation test and tensile mechanical properties test of plain braided precursor C/SiC composites(PIP-C/SiC)were carried out for plain braided pioneer impregnation cracking process.With the help of observation of scanning electron microscope and elemental energy spectrum analysis,the differences between morphology evolution of C/SiC at high temperature and tensile fracture characteristics before and after oxidation were mainly studied.The results show that the oxidation of uncoated C/SiC in high temperature oxidation environment mainly starts at the interlacing of wire bundles and mainly depends on the weight loss of carbon fibers.The protective effect of passive oxidation of silicon carbide depends on the rate,thickness and fluidity of oxidation layer formation.High temperature oxidation results in a decrease in ultimate strength and stiffness of the material,and the tensile fracture shows typical ductile fracture characteristics.