Fracture behaviour of SiC/SiC ceramic matrix composite

Abstract

The fracture behaviour of SiC/BN/SiC CMC have been assessed using different approaches. The toughening mechanisms responsible for the enhanced fracture toughness have been identified. Apparent KIC have been measured. KIC values of 3.05-3.27 and 4.13-5.44 MPa.m1/2 were measured in-situ for the in-plane and out-of-plane orientations, respectively. A second KIC of 26.6 ± 3.37 MPa.m1/2 was measured using the proportional limit. However, this method was found to overestimate the fracture toughness as the matrix cracking initiates significantly before the proportional limit. KIC represents the resistance to crack propagation at low crack extensions and does not fully describe the composite’s fracture toughness. An equivalent fracture toughness Keq that reflect to some extent the contribution of toughening mechanisms has been measured and found to be notch sensitive: it increases from 16.3 MPa.m1/2 to a plateau of 27.7 MPa.m1/2. Resistance-curves were measured using a J-integral approach. JR-curve initiates at 0.11-0.17 kJ.m-2 but never reached a steady-state plateau. Finally, a fracture energy approach was used to measure the fracture matrix energy of 72-186 J.m-2 which corresponds to the energy required for fracture initiation and work of fracture that correspond to the energy absorbed during the whole fracture process. The work of fracture was found to be strain rate sensitive: it decreases with increasing displacement rate. The notch and size-sensitivity of the composite was investigated. Strength were found to be unsensitive, but the opposite was observed concerning the fracture nature and the work of fracture. Finally, the effect of degradation environment on the microstructure and flexural behaviour was investigated. These environments correspond to a humidity exposure at 65°C and a steam exposure at 800°C. The flexural behaviour was found to be considerably degraded after the humidity exposure, but significantly lower degradation was observed after the steam exposure.