The effects of crossover pores on high temperature bending behavior of 3-dimensional braided C/SiC

Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) are potential for application as high temperature materials and components in severe environments. The three-point-bending strength of 3-dimensional braided C/SiC composites (3D C/SiC) was tested from room temperature up to 2050°C in vacuum. Below 1000°C, the flexural strength of 3D C/SiC composites increased with temperature and from 1000°C to 1500°C decreased with temperature. Over 1900°C, the composite specimens retained high strength and deformed over the nominal maximum deflection in the test machine without fracture. The microstructure of 3D C/SiC after bending test at 2050°C were systematically characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). No glass phase was found in the composites. The crossover pores were investigated and its configuration change with time was observed on tensile creep specimen. Under loads, the angle between fiber bundles changed from buckling and torsion because of the large crossover pores and weak matrix restraint at these sites. When the structure of the test specimen finally determined by machining made the particular arrangement of crossover pores in a vertical face, the crossover pores would evolve synergistically, which explained the abnormal bending properties of 3D C/SiC above 1700°C.