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.

Glass-ceramics are inorganic, non-metallic materials obtained by controlled crystallization of glasses through different processing routes; they contain at least one functional crystalline phase together with a residual glass, and the crystallized fraction may range from trace levels to nearly complete crystallization. Transparent glass-ceramics (TGCs) constitute the optically transparent subset of this class and combine a controlled crystalline microstructure with a residual amorphous matrix. Their transparency distinguishes them from conventional opaque glass-ceramics and is achieved by minimizing light scattering through careful control of crystallite size, volume fraction, spatial distribution, and refractive-index mismatch between the crystalline and glassy phases. Unlike conventional sintered ceramics, TGCs retain many of the processing advantages of glass while incorporating crystalline phases that can enhance mechanical, thermal, optical, or functional properties. Depending on their composition and microstructure, TGCs may exhibit improved hardness, fracture toughness, thermal stability, chemical durability, luminescence, nonlinear optical response, or ion-exchange strengthening capability. These features make TGCs attractive for applications requiring both optical clarity and advanced performance, including protective cover glass, transparent armour, precision optical substrates, laser and photonic components, optical sensors, and multifunctional host materials for rare-earth ions and nanoparticles.