Microstructural and thermal stress effects on mechanical properties of boron carbide particle-reinforced silicon carbide

Ceramic composites consisting of a silicon carbide matrix and boron carbide particles of various sizes and weight percentages were produced, characterized, and mechanically tested to elucidate the effect of microstructure on the mechanical response. All samples were observed to have flexural strength values lower than either of the constituent phases; however, blends with finer B4C particles (<10 mu m) exhibited improved fracture toughness. Residual stresses from the mismatch in thermal expansion of the materials are found to be fundamental to these variations in mechanical response, resulting in either microcracking during processing or producing an initial stress state within the composite prior to testing. Microcracks were observed in samples with larger B4C particles (similar to 77 mu m), causing a significant reduction in mechanical properties, while the finer B4C particles were found to be microcrack free. Residual stress measurements via X-ray diffraction and Raman spectroscopy show increasing compressive stresses within the SiC matrix as the weight percentage of boron carbide increases, however both flexural strength and fracture toughness are found to decrease, indicating that the boron carbide particles that are in a tensile stress state are an important strength-limiting feature. This is corroborated by fracture analysis. The results of both characterization and mechanical testing compare well with those predicted by micromechanics models.