Fabrication, microstructures and properties of SiCf/SiC composites prepared with two kinds of SiC fibers as reinforcements

Three-dimensional (3D) SiC/SiCf composites were fabricated by a polymer impregnation and pyrolysis (PIP) method using 3D four directional braided preforms from two kinds of SiC fibers as the reinforcements and liquid polycarbosilane as the SiC precursor. Both kinds of SiC fibers were prepared from the same precursor and process but with different heat treatment temperatures. The one prepared at a higher temperature (No.1) had a higher crystallinity than the another one (No.2). A thin pyrolytic carbon (PyC) interlayer was coated on the preforms by a methane CVD method before impregnation. Their microstructures and mechanical properties were investigated by TEM, SEM, Raman spectroscopy and mechanical tests. Results showed that both had a polycrystalline structure with grain sizes of 8-15 nm. Fiber pullout was seen from the fracture cross-section of the composites after a bending test, indicating pseudo-ductile fracture behavior. The composite using No.1 SiC fibers had an average flexural strength, elastic modulus and fracture toughness of 955.0±42.8 MPa, 110.3±1.7 GPa and 28.5±2.8 MPa·m1/2, respectively, which are superior to those of the material formed using No.2 SiC fibers. This was ascribed to the high modulus and excellent thermal resistance of the stoichiometric No.1 SiC fibers. The PyC layer adhering to No.1 SiC fibers was ordered and smooth while that deposited on No.2 SiC fibers had a loose and granular microstructure, which was attributed to the different surface chemistries of the two types of SiC fibers.