Fracture Toughness Measurement for B₄C Composites with Hierarchical Microstructures: Comparison of Micro-indentation and Four-Point Bending Methods

Because of the advantage of simple preparation of small-sized samples, micro-indentation has been widely adapted to evaluate the fracture toughness of materials including brittle ceramic composites. However, fracture toughness results measured by micro-indentation often differ from those measured by standardized methods such as four-point bending. Such differences can be attributed to loading condition differences (contact load for micro-indentation), fracture mechanism differences, and the associated usage of empirically obtained formulas for fracture toughness evaluation. These differences are expected to amplify when testing hierarchically microstructured ceramic composites, as multiple and complex fracture mechanisms can be triggered at different size scales and loads. Here, we compared the fracture toughness measured using micro-indentation (fracture under compression) and four-point bending (surface crack in flexure method) about three types of hierarchically microstructured boron carbide composites: one reinforced with sub-micron and micron-sized graphite platelets, one reinforced with titanium diboride particulates, and one reinforced with both. The fracture toughness value differences were correlated with fracture and deformation mechanisms observed after the micro-indentation and four-point bending tests. This study provides insights into the origin of the toughness value differences, and also insights into fair interpretation of micro-indentation data when the four-point bending test option is not available for brittle hierarchical composites.