Balancing Color and Hardness in ZnO–Al₂O₃–SiO₂ Glass-Ceramics by Tailoring the ZrO₂/TiO₂ Nucleating Agents Ratio

Transparent and hard ZnO–Al2O3–SiO2 (ZAS) glass-ceramics (GCs) are desirable for various applications. However, TiO2, an effective nucleating agent for this system, can introduce an unwanted yellow coloration. This study explores the effect of partially replacing TiO2 with ZrO2 on the crystallization pathways, color, transparency, and hardness of ZAS glasses and corresponding GCs, by evaluating compositions containing a constant content (4.5 mol %) of the referred nucleating agents. Optical transmittance showed that all glasses and GCs prepared were transparent in the visible range. However, they presented a yellowish hue that faded as the ZrO2/TiO2 ratio increased, as indicated by the CIELab analyses. One possible source of such coloration is the presence of iron ions in different oxidation states such as Fe2+ and Fe3+, this last detected by Electron Paramagnetic Resonance measurements in all the glasses. The samples’ color is also likely affected by the Ti4+–O2– charge transfer mechanism. Concerning the mechanical performance, all GCs present similar hardness, which is a consequence of the slight variation in the crystallization kinetics with the varying ZrO2/TiO2 ratio. Therefore, the partial replacement of TiO2 with ZrO2 enables the preparation of glasses and GCs with a lighter coloration, without impairing their hardness.