Influence of TiO 2 and Cr 2 O 3 on crystallinity and properties of Ca,Li aluminosilicate glass

Abstract

The influence of TiO₂ and Cr₂O₃ on the crystallisation behaviour of a glass system based on CaO–Li₂O–Al₂O₃–SiO₂ has been studied. Local limestone and clays were used as raw materials in the batch composition. Various thermal, microscopic and diffractometric techniques were used to investigate the amorphous and crystalline phases present. Heat treatment led to the formation of β-eucryptite, wollastonite, anorthite and fayalite. Both parent and Cr₂O₃ containing glasses showed enhanced crystallisation of β-eucryptite and wollastonite, whereas anorthite formation was catalysed in TiO₂ containing glasses, in addition to the latter phases mentioned. Crystallisation of all glasses at 1100°C reduced wollastonite formation to trace levels; however, fayalite appeared in Cr₂O₃ containing samples. Glass–ceramics with a homogeneous bulk microstructure were developed, mainly characterised by accumulated fine rounded to subrounded crystals of 2·5–10 μm diameter of β-eucryptite phase. Intergranular rhombic-like and fibrous crystals were also developedand may be wollastonite and anorthite respectively, in TiO₂ containing samples. Depending on the content of β-eucryptite and other phases in the glass–ceramic samples, the coefficient of thermal expansion ranged from −8·75 × 10⁻⁷°C⁻¹ (25–300°C) to 54·16 × 10⁻⁷°C⁻¹ (25–680°C). The Vickers microhardness of glass–ceramic samples ranged between 676 and 593 kg mm⁻² and was high for anorthite containing samples.