Strain rate–dependent indentation responses and microstructural evolution mechanisms of aluminum oxynitride ceramics

Aluminum oxynitride (AlON) ceramics, valued for their superior mechanical properties and optical transparency, are promising candidates for advanced armor protection. However, their mechanical response under varying loading rates remains insufficiently understood, constraining reliability-based design and performance optimization for impact conditions. In this study, quasi-static and dynamic indentation tests were conducted to elucidate the strain rate–dependent response of AlON ceramics. The results reveal a pronounced strain-rate hardening effect, with dynamic Vickers hardness markedly exceeding quasi-static values. Correspondingly, crack morphology transitions from simple radial cracks to complex crack networks with secondary branching. Microstructural analysis further shows that dynamic loading activates multiple plastic-related deformation processes within the indentation region, including high-density dislocation structures, slip-band interactions, dislocation-free zones, and deformation twins. These features help explain both the enhanced hardness and the evolution of crack patterns and demonstrate that AlON ceramics exhibit a brittle–plastic coexisting failure mode at high strain rates. This work provides new insight into the strain rate–dependent indentation response and microstructural evolution of AlON ceramics, offering valuable guidance for the design of high-performance transparent armor materials.