Preparation, properties and formation mechanism of transparent anorthite-based glass-ceramic glaze with high hardness

The transparent anorthite-based glass-ceramic glaze was prepared at 1200 degrees C by mineral raw materials. The effect of ZnO content on the structures and properties of the anorthite glass-ceramic glaze was studied by XRD, SEM-EDS, HR-TEM, high temperature microscope, micro-Vickers hardness tester and comprehensive thermal analyzer. Furthermore, the formation mechanism of anorthite was investigated systematically. The results reveal that the introduction of ZnO results in the decrease of melting temperature, elimination of the diopside, reduction of the thermal expansion coefficient, and improvement of the mechanical properties of the glazes markedly. The major crystalline phases of glazes change from the coexistence of anorthite and diopside to single anorthite with the increase of ZnO content from 0 wt% to 6.45 wt%, and the relative anorthite content increases from 26.87 wt% to 49.83 wt%. Both the size and number of anorthite grains with a distribution of the card structure gradually increases as increasing the ZnO content. The increase of anorthite improves the scattering ability of visible light and then reduces the transparency of the glaze. The Vickers hardness and flexural strength of glazes improve from 5.26 GPa and 45.95 MPa to 7.20 GPa and 72.79 MPa, respectively. The thermal expansion coefficient decreases from 6.328 x 10-6/degrees C to 5.781 x 10-6/degrees C (600 degrees C). Anorthite crystal generates from the reaction of genhlenite, SiO2 and Al2O3 at 1100 degrees C, and the gehlenite forms through the reaction of metakaolin and CaO. The study shows that the plate-like anorthite crystallizes when the content of ZnO is 3.33 wt%, the Vickers hardness of glass-ceramic glaze is 6.56 GPa with a great transparency.