Boron Nitride Nanotubes Toughen Silica Ceramics

Ceramics are renowned for their exceptional mechanical stiffness, strength, chemical inertness, and thermal stability. However, their intrinsic brittleness significantly limits their engineering applications. Enhancing the bulk mechanical properties of ceramics with fillers presents a viable solution. Boron nitride nanotubes (BNNTs) are promising reinforcing fillers for ceramics due to their extraordinary structural and mechanical properties, as well as their thermal stability. While improvements in bulk mechanical properties have been reported, the underlying reinforcing mechanism remains unclear. This ambiguity is partly due to the difficulty in directly and quantitatively evaluating the interfacial load transfer, which is critical for mechanical reinforcement, from bulk mechanical measurements and microscopic imaging. In this study, we investigate how BNNTs reinforce silica nanocomposites by quantifying their bulk and interfacial mechanical properties using in situ Raman micromechanical characterization techniques and microstructural analysis. Additionally, we employ the digital image correlation (DIC) technique in conjunction with finite element method (FEM) simulations to directly quantify the local deformation of the matrix and correlate the data with Raman analysis. Silica is chosen for its technological relevance to devices and composites and its widespread industrial usage. The incorporation of even small amounts of BNNTs can result in substantial microstructural changes in the ceramic matrix (such as crystallinity, grain size, porosity, etc.), thereby influencing the overall bulk mechanical properties of the ceramic nanocomposite. Our studies reveal that incorporating small amounts of BNNTs (0.1 to 0.5 wt.%) significantly increases the flexural strength (~51% to ~153%) and fracture toughness (~44% to ~167%) of silica. The effective interfacial shear stress in the bulk BNNT-silica nanocomposite follows a shear-lag model, reaching a maximum value of ~92 MPa. Microstructural analysis shows that incorporating BNNTs in silica prominently influences its crystallization, increasing porosity and decreasing crystal size and lattice strain. These collective microstructural changes substantially contribute to the bulk mechanical properties of the BNNT-silica nanocomposite. Our findings provide valuable insights into the reinforcement mechanism of BNNTs in ceramics and contribute to the optimal design of light, strong, tough, and durable ceramic materials.