The Evolution Of Matrix Crack In Three Dimensional Carbon/Silicon Carbide Composites

C/SiC composites is a kind of promising structural materials which can be used at 1600C for a long time, at 2200C for a limited time and at 2800C for a short time. In order to design the composites and understand the properties evolution of the composites during their service, a systemic and in-depth study on the microstructural evolution in the composites is very important.Against the background of the microstructural evolution of three dimensional braiding C/SiC composites, used such analytical methods as LM, SEM and TEM, it is studied that the matrix cracks propagate in the composites with different interphase thickness under three-point-bend testing, as well as under the creep testing. The main conclusions are as follows:1. There are three main kinds of matrix cracks within the as-received composites: cracks induced by mismatch of thermal expansion coefficients, cracks between bundles and cracks induced by matrix's delamination. They locate within the bundles, between the bundles and on the surface of the bundles respectively.2. The mechanical properties and fracture mode of 3D C/SiC depend on the matrix cracks' propagation in interphase. It was found that the cracks can penetrate the interphase and the fiber rapidly when the interphase is thin, which causes the low flexural strength and fracture toughness, as well as the composites' brittle fracture. While the interphase is thick, multiple toughening mechanisms such as the crack deflection and matrix cracking can be activated, which cause the composites' flexural strength and fracture toughness is high and the composites' non-brittle fracture.3. The two stages of primary creep and steady-state creep of 3D C/SiC correspond to the gradual saturation of matrix crack and the opening of these cracks respectively. The transverse crack is the main contributor to the macroscopic creep strain of 3D C/SiC, and its opening is controlled by the creep temperature.4. The matrix cracks' deflection can occur along the M/I interface, within the interphase and along the I/F interface, which can be predicted by modeling the matrix cracks' defection.