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

Abstract

Transparent and hard ZnO−Al 2 O 3 −SiO 2 (ZAS) glass-ceramics (GCs) are desirable for various applications. However, TiO 2 , an effective nucleating agent for this system, can introduce an unwanted yellow coloration. This study explores the effect of partially replacing TiO 2 with ZrO 2 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 ZrO 2 /TiO 2 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 Fe 2+ and Fe 3+ , this last detected by Electron Paramagnetic Resonance measurements in all the glasses. The samples' color is also likely affected by the Ti 4+ −O 2− 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 ZrO 2 /TiO 2 ratio. Therefore, the partial replacement of TiO 2 with ZrO 2 enables the preparation of glasses and GCs with a lighter coloration, without impairing their hardness.