Microstructure and mechanical properties of Li2O-Al2O3-SiO2 glass-ceramics

Glass-ceramics are fine-grained polycrystalline materials formed when glasses of suitable composition are heat- treated and undergo controlled crystallisation. Their design consists in the choice of both chemical composition and firing cycle. Glass-ceramics can be used as bulk as well as, after being ground, as powders. Their application widens in many fields such as the electronic and biomedical one, but also as raw materials for both ceramic bodies and glazes. Moreover, they can be prepared through melting of raw materials and cast in order to form laminar sheets further used as facing tiles in buildings. In this work, microstructure and mechanical properties of two lithium-aluminosilicate-based glass-ceramics (LAS) have been characterised in order to investigate the relationships among process conditions, phase composition, microstructural characteristics and mechanical properties. In detail, microstructure was studied by scanning electron microscopy and phase composition by X-ray powder diffraction performed at both room and high temperature, followed by Rietveld refinements of data. Ultrasound speed, flexural stength, young's modulus, poisson's ratio, fracture toughness and Vickers hardness were measured. Moreover, the study of the weakening effect due to the industrial finishing process was carried out, comparing the results of fracture strength tests carried out on both industrial finished samples and samples finished according to the European standard EN 843-2. The most suitable firing cycle, determining crystalline composition and crystallite size, was designed by combining microstructural and phase composition analyses, achieving an insight also in glassy phase amount and composition. The results of mechanical characterisation have confirmed that such materials have outstanding performances for facing tiles application (modulus of rupture up to 200 MPa and fracture toughness around 2 MPa.m1/2). Mechanical properties resulted mostly influenced by process conditions, for example by thermal cycle and finishing procedure.