Proposal Abstract: The demand for lightweight, high-strength, and cost-effective transparent materials is growing rapidly, particularly for use in military vehicles, marine vessels, electronics, and sensor systems. Recent progress in the synthesis of transparent nanocrystalline ceramics has positioned them as a superior alternative to traditional glass, primarily due to their outstanding mechanical properties and high temperature stability (no glass transition).

In this study, nanocrystalline magnesium aluminate spinel (NC-MAS) with grain sizes from 3.7 to 80 nm was synthesized using environmentally controlled, pressure-assisted sintering. Nanoindentation measurements revealed a transition from the Hall–Petch to the inverse Hall–Petch regime, which was examined using two representative grain sizes—80 nm and 3.7 nm—chosen to avoid overlap in deformation mechanisms. Electron microscopy showed that the 80 nm material deforms through dislocation activity and grain-boundary decohesion, whereas the 3.7 nm sample exhibited no dislocations and instead deformed through grain-boundary sliding, decohesion, and shear banding. Atomistic simulations confirmed that grain-boundary-mediated plasticity dominates in the inverse Hall–Petch regime, with no dislocation activity or stress-induced grain growth even at large strains.

Because fracture behavior is critical for the performance of ceramic materials, the proposed research aims to determine how grain size and the associated deformation mechanisms influence crack-tip plasticity and fracture toughness in NC-MAS. The future work will integrate micro-cantilever bending, nanopillar compression, advanced electron microscopy, constitutive modeling, and atomistic simulations to uncover how dislocation plasticity and grain-boundary-mediated shear banding alter crack-tip processes and energy dissipation. The resulting framework will not only clarify how grain size governs fracture resistance in NC-MAS but will also extend to other brittle nanostructured materials, enabling predictive models for designing next-generation, damage-tolerant ceramic components.