Interfacial fracture behavior and adhesive strength in tensile and shear loading of SiC-PyC-SiC composites by micro-scale specimens

Micromechanical tests were performed to understand the local fracture behavior and adhesive strength of a fiberreinforced ceramic matrix composite (silicon carbide fiber reinforced silicon carbide matrix with the pyrolytic interphase in between, SiCfiber/PyC/SiCmatrix) and to determine the mode-dependent interfacial fracture toughness. A combined approach of pico-second laser ablation and focused ion beam milling was used to fabricate notched and unnotched micro-cantilever and in-plane micro-shear specimens for mode I and II testing. Due to the complexity of the sandwich system, finite element simulations were implemented, which took into account the elastic heterogeneity, the influence of PyC thickness, Young's modulus and beam thickness. This allowed us to obtain mode-specific geometry functions and to increase the accuracy of the obtained stress intensity factor. In all cases, a failure at the SiCfiber/PyC interface was observed. While straight through notched specimens exhibit a systematic overestimation of fracture toughness due to the less accurate alignment of notch root and PyC layer, curved notched specimens show a very low interfacial fracture toughness of 0.24 & PLUSMN; 0.02 MPa & RADIC;m and 0.17 & PLUSMN; 0.06 MPa & RADIC;m in mode I and mode II, respectively. Combined with tests on unnotched specimens, a critical flaw size for the present microstructure of the PyC phase of ca. 21 nm was identified. The data underlines the important role of the PyC interlayer in damage resistant ceramic composites and the developed methodology, which can be used for systematic studies of their properties as a function of process, geometry, and boundary conditions.