FLEXURAL PERFORMANCE OF 3D-C/SiC COMPOSITES AT HIGH TEMPERATURE AND INFLUENCE OF POST-HEAT-TREATMENT ON FLEXURAL PERFORMANCE

The preform fabricated with T300 carbon fibers weaved into 3-dimensional and 4-directional structure with 22° weaving angle, is deposited pyrocarbon and SiC matrix at 950℃～1*!000℃ with CVI(chemical vapor infiltration) method. Fiber volume fraction is 40%～45%, the thickness of the pyrocarbon is about 0.2*!μm, 3D-C/SiC density is 2.01*!g/cm 3 and porosity is 17%. Finally the thickness 50*!μm of SiC coating is formed on the surface of specimen. Three points flexural experiments are carried out at YKM-2200 super-high-temperature flexural machine with 10 -3 Pa vacuum and 0.5*!mm/min cross head moving speed. The results indicate that flexural strength appeare two peak values, at 900℃ and 1*!700℃ respectively. Maximum value of flexural modulus is found at 1*!100℃. The flexural strains nearly keep constant when temperature lower than 1*!500℃. Above 1*!500℃, the obviously irreversible flexural deformation is obtained. After keeping one hour at 1*!800℃ in vacuum, the flexural modulus decreased obviously at 1*!100℃, whereas flexural deformation increased. Laser Raman spectrum analysis indicates that 3D-C/SiC exists residual compressional stress in carbon fiber, and that residual tensile stress after 1*!300℃ treatment, after 2*!050℃ treatment residual stress disappear. At high temperature the tendency for 3D-C/SiC to destroy interface bond is ascribed to different thermal expansion coefficient between matrix and carbon fiber. It would lead that carbon fiber easily slide, at the same time keep carbon fiber strength because matrix crack unlikely entering into fiber. Therefore flexural deformation increases, flexural modulus decreases and the flexural strength slightly increases when elevating temperature.