High Indentation Resistance of Aluminum Borate Based Glass-Ceramics

Abstract

This study investigated the bulk crystallization of 54B2O3-19SiO2-17Al2O3-5BaO-5MgO (mol%) glass. Melting was carried out at 1500°C for 1 h using a bottom-load electric furnace. The glass melt was cast into a block and annealed at 500°C for 2 h. Isothermal heat treatment was carried out at 1100°C for 2, 4, 8, 16, 32 h to form bulk crystallized glass-ceramics using a heating and cooling rate of 5°C/min. Phases present in the glass-ceramic samples were studied by x-ray diffraction. Crystalline Al4B2O9 and Al18B4O33 were the main phases and the phase stability depended on the isothermal time. Microstructures were observed by a scanning electron microscope. The size of aluminum borate whiskers/rods tend to increase with longer isothermal holding period. The whisker/ rod-like crystals uniformly oriented throughout the microstructure in all heat treated samples. This led to interlocking microstructure and hence an increase in hardness and fracture toughness. Glass-ceramics synthesized at longer heat treatment times resulted in an increase in the surface hardness and shorter path length at the corner of the diamond pyramid-shaped indenter. Glass-ceramics synthesized in this study can be applied as high temperature resistant machinable materials because their microstructures can resist micro-cracking upon indentation.