Effects of Microstructural Engineering on the Mechanical Properties of Boron Carbide Ceramics

Anthony Jason Weaver

Boron carbide (B4C) is an ultra-hard, lightweight ceramic widely used in structural, nuclear, and defense applications, yet its performance is limited by low fracture toughness and a susceptibility to shear-induced amorphization. This thesis investigates how targeted microstructural engineering strategies—specifically stoichiometric modification, dopant incorporation, and secondary-phase reinforcement—affect the quasi-static mechanical response of six novel boron carbide variants: (1) no free carbon (NFC) which lacks all carbon impurities/inclusions, (2) NFC reinforced with TiB2, (3) boron-rich with a nominal stoichiometry of B5.5C, (4) boron carbide containing 1% residual carbon, (5) Si-doped, and (6) Si-dopedreinforced with TiB2. Comprehensive microstructural characterization was performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) to quantify grain morphology, phase distribution, and chemical homogeneity. Mechanical behavior was evaluated through nanoindentation, Vickers hardness testing, indentation fracture toughness measurements, and quasi-static uniaxial compression. These methods enabled direct comparison of elastic modulus, hardness, crack propagation behavior, and failure mechanisms across boron carbide variants. Findings indicate that microstructural modifications have no meaningful influence on overall mechanical performance. The average measured properties: young’s modulus, hardness (Vickers and nanoindentation), indentation fracture toughness, and ultimate compressive strength, are not heavily influenced by these microstructural modifications. In fact, the only influence these modifications seem to have on these quasi-static test methods is the stability of the measurements. The more drastic the modification the wider the standard deviation becomes while the averages remain similar. Overall, this work provides a systematic assessment of how compositional and microstructural design strategies can be used to tailor the mechanical behavior of boron carbide ceramics. These results contribute to ongoing efforts to develop next-generation lightweight high strength ceramics with improved damage tolerance.