Effect of the K2O/(CaO+ZnO) ratio on crystallization behavior, phase formation, and mechanical properties of transparent borosilicate glazes

Abstract

The crystallization behavior of transparent borosilicate glass–ceramic glazes is strongly influenced by their chemical composition, particularly the ratio of network modifiers. In this study, the effect of the K2O/(CaO+ZnO) ratio (S ratio) on the phase evolution, microstructure, optical properties, and mechanical performance of transparent borosilicate glazes was investigated. Glazes with different S ratios were prepared under identical processing conditions and characterized by differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), optical measurements, and Vickers microhardness testing. The results showed that decreasing the S ratio significantly affected the thermal stability and crystallization behavior of the glass network. XRD and SEM analyses revealed the formation of wollastonite, willemite, calcium silicate, and calcium aluminosilicate (anorthite) phases, accompanied by distinct changes in crystal morphology. These phase transformations directly influenced the optical appearance and mechanical properties of the glazes. The highest microhardness was obtained for the glaze containing calcium aluminosilicate as the dominant crystalline phase, whereas glazes with a higher residual amorphous content exhibited lower hardness values. Variations in the crystalline phase assemblage also affected the surface gloss and opacity of the fired glazes. Overall, the results demonstrate that the K2O/(CaO+ZnO) ratio is an effective compositional parameter for controlling the crystallization behavior and optimizing the functional properties of transparent borosilicate glass–ceramic glazes.