Balancing Color\nand Hardness in ZnO–Al<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub> Glass-Ceramics by Tailoring\nthe ZrO<sub>2</sub>/TiO<sub>2</sub> Nucleating Agents Ratio

Transparent and hard\nZnO–Al<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub> (ZAS)\nglass-ceramics (GCs) are desirable for various applications.\nHowever, TiO<sub>2</sub>, an effective nucleating agent for this system,\ncan introduce an unwanted yellow coloration. This study explores the\neffect of partially replacing TiO<sub>2</sub> with ZrO<sub>2</sub> on the crystallization pathways, color, transparency, and hardness\nof ZAS glasses and corresponding GCs, by evaluating compositions containing\na constant content (4.5 mol %) of the referred nucleating agents.\nOptical transmittance showed that all glasses and GCs prepared were\ntransparent in the visible range. However, they presented a yellowish\nhue that faded as the ZrO<sub>2</sub>/TiO<sub>2</sub> ratio increased,\nas indicated by the CIELab analyses. One possible source of such coloration\nis the presence of iron ions in different oxidation states such as\nFe<sup>2+</sup> and Fe<sup>3+</sup>, this last detected by Electron\nParamagnetic Resonance measurements in all the glasses. The samples’\ncolor is also likely affected by the Ti<sup>4+</sup>–O<sup>2–</sup> charge transfer mechanism. Concerning the mechanical\nperformance, all GCs present similar hardness, which is a consequence\nof the slight variation in the crystallization kinetics with the varying\nZrO<sub>2</sub>/TiO<sub>2</sub> ratio. Therefore, the partial replacement\nof TiO<sub>2</sub> with ZrO<sub>2</sub> enables the preparation of\nglasses and GCs with a lighter coloration, without impairing their\nhardness.