In contrast to monolithic ceramic components that can fail by the propagation of a single crack, ceramic matrix composites (CMC) are designed to deflect cracks at the interface between fiber and matrix to increase toughness and damage resistance. Hence, for optimally designing crack deflecting composites, of great importance it is to have a quantitative understanding of the interfacial properties.

In this study, novel micromechanical specimens were designed based on an earlier study that allow determining the interfacial toughness and strength of fiber reinforced composites in both tensile and in-plane shear loading. Finite element analysis was performed with particular focus on interface geometry and preparation of curved notches to characterize failure purely along curved fiber interfaces. Furthermore it allowed determining stress distribution, geometry factor and stiffness evolution as a function of interface thickness, elastic properties of the interface, and crack propagation. Specimens under mode 1 and mode 2 loading, both notched and unnotched, were prepared by focused ion beam. Micromechanical tests were performed to understand the local fracture toughness and adhesive strength of the SiC-C-SiC interface system. Experimental results were found to be highly repeatable and consistent. Interfacial toughness of the studied SiC-C-SiC system was found to be approximately 0.28 ± 0.03 MPam1/2 in mode 1 and 0.22 ± 0.07 MPam1/2 in mode 2. The crack travelled mostly along the interface for both testing configurations. Besides, the unnotched samples showed highly consistent results with an interfacial strength of 934 ± 137 MPa under tensile and 418 ± 36 MPa under shear load.

In this study, for the first time the interfacial fracture and strength properties of SiC-C-SiC composites were measured by micromechanical experiments on microshear samples in addition to microcantilevers. As expected, cracks nucleated and propagated preferentially at the interface of the SiC fiber and the C interface. The study shows the high potential of FIB-based micromechanical testing in combination with mechanical modelling for studying the interface properties in complex materials such as fiber reinforced ceramic matrix composites. It also stresses the significance of using proper geometry factors to correctly capture the interfacial toughness between elastically dissimilar materials.