Caracterização estrutural e mecânica de vitrocerâmicas de dissilicato de bário

Abstract

Barium disilicate (BaO.2SiO2) glass (BS2) is one of the few stoichiometric glasses that nucleates homogeneously via thermal treatment. It is a scarcely assessed model system in microstructure-property studies. This systematic research addresses the mechanical strength and fracture toughness variation as a function of the crystallized volume fractions, crystal size, and the effect of residual stresses in BS2 glass-ceramics. These microstructural features were independently modified; the average crystal diameter varied from 5 to 100 μm and the crystallized volume fraction from 0 to 90% (because the spherulites are not 100% crystalline). The average internal residual stresses in the spherulites in this system are tensile and estimated to be in the 40-170 MPa range. Samples with mean spherulite size >30 μm showed spontaneously fractured crystals caused by the residual stresses. These microcracking affected the biaxial strength, but not the fracture toughness, (KIC). The KIC increased with increasing the spherulite size and crystallized volume fraction. X-ray diffraction analysis revealed that the spherulites contain residual glass, and this amount depends on spherulite size. Toughening mechanisms were more effective in glass-ceramics containing spherulites with a higher degree of internal crystallization. The variation of KIC with the crystallized volume fraction was similar for glass-ceramics with different crystal sizes. Combination of the current findings on BaO.2SiO2 (crystals under tensile residual stresses) with previous studies of Li2O.2SiO2 glass-ceramics (compressive residual stresses) indicates that crystallization of a tougher phase – not the type of residual stress – is the crucial parameter controlling fracture toughness and strength. These results can be useful to design novel strong and tough glass-ceramics.