Effects of heat pressing on microstructure and mechanical properties of lithium disilicate glass ceramics with different crystal morphology

Abstract

The purpose of this study is to investigate effects of heat pressing on microstructure and mechanical properties of lithium disilicate glass ceramics with different crystal morphology. Lithium disilicate glass-ceramics (LD) were melted to form precursory glass blocks, which were crystallized by two-step heat treatment. The second steps at 900 °C, 915 °C, and 930 °C (below, equal to and above the heat pressing temperature) were designed for specimens in group G1–G3 (n = 20, per group) respectively to form LD. Then specimens in group G1–G3 were heat-pressed at 915 °C to form Group G1p, G2p and G3p. Six groups of glass ceramics were characterized using X-ray diffraction (XRD) and subsequently evaluated via flexural strength testing, nanoindentation testing, and toughness measurements. The microstructures and fracture morphologies were analyzed using scanning electron microscopy (SEM). One-way analysis of variance (ANOVA) was used to analyze the data. After heat pressing, the crystal size of LD in G1p was larger than that in G1, while these was no significant changes from G2 to G2p, or from G3 to G3p. The crystals displayed varying degrees of oriented distribution along their long axis after heat pressing: mild in G1p, pronounced in G2p and G3p. G1p showed lower flexural strength than G1, whereas G2p exhibited higher flexural strength than G2 (p < 0.05). G1p, G2p, and G3p exhibited lower average hardness than G1, G2, and G3. The fracture toughness along the longitudinal (Gpl) direction was different from that along the transverse (Gpt) direction. As the heat treatment temperature increased, the crystal size in LD increased and crystal content declined. After heat pressing, the crystals in the 915 °C and 930 °C groups became oriented, the flexural strength increased, the fracture toughness in specific directions improved, while the hardness had no significant change.