Ultra-Strong Transparent ZnAl₂O₄ Glass-Ceramics via Controlled Crystallization and Ion Exchange

Enhancing the mechanical strength of transparent glass-ceramics (TGCs) without compromising their optical performance remains a key challenge for advanced optical and photonic materials. Among aluminosilicate systems, ZnO-MgO-Al₂O₃-SiO₂ (ZMAS) glasses are particularly attractive due to their ability to form ZnAl₂O₄-based nanostructures; however, their ion-exchange (IE) strengthening has not been systematically explored due to the absence of single-charged cations in their composition. In this study, a sodium-modified ZMAS glass was developed to enable efficient chemical strengthening while preserving glass-forming ability and optical clarity. Controlled two-stage heat treatment produced TGCs containing 5 mol% Na₂O, composed solely of ZnAl₂O₄ (gahnite) nanocrystals with an average size of 4-5 nm. The obtained TGCs showed a Vickers hardness of ~8.5 GPa, increasing to ~10-10.5 GPa after ion exchange in molten KNO₃ at 450 °C, without changes in phase composition or optical transmittance. Compared with literature data on alkali-containing TGCs, the developed material demonstrates a higher hardness level while maintaining full transparency. The results reveal a practical route toward chemically strengthened ZnAl₂O₄-based glass-ceramics combining optical clarity, high hardness, and damage tolerance for optical, photonic, and protective applications.