Effect of fiber directionality on the static and dynamic mechanical properties of 3D SiCf/SiC composites

The static and dynamic mechanical properties of three-dimensional (3D) 4-directional and 3D 5-directional braided SiCf/SiC composites fabricated by polymer infiltration and pyrolysis (PIP) were investigated using static and dynamic bending tests, as well as microstructural characterization. X-ray diffraction revealed that polycarbosilane was converted into a matrix of crystalline β-SiC after PIP cycling. Test results indicated that the density, flexural strength, elastic modulus, fracture toughness, and storage modulus of 3D 5-directional SiCf/SiC composites were superior to those of 3D 4-directional braided SiCf/SiC composites; the former also showed a smaller internal friction than the latter. Results from Weibull statistical analysis indicated that the scale parameter σ0 (736.9 MPa) and Weibull modulus m (21.7) of the 3D 5-directional specimen were higher than those of 3D 4-directional braided SiCf/SiC composites (629.6 MPa, 14.7). Both 3D braided composites demonstrated good toughness and avoided catastrophic brittle fractures under loading because of the effective crack energy dissipating mechanisms of crack deflection, interface debonding, and fiber pull-out. The internal friction and storage modulus of the 3D braided composites were sensitive to temperature. The cross angle of fiber placement in the preform and the direction of the applied force, as well as the pre-crack propagation remarkably influenced the static mechanical properties and failure behavior of the 3D braided SiCf/SiC composites. The dynamic mechanical properties of the 3D braided composites, including internal friction and storage modulus, were also considerably affected by fiber directionality in their preforms.