Improving the thermo-physical behavior of Li₂O-Al₂O₃-P₂O₅-SiO₂ glass–ceramic system by triple-step heat treatment processes

The thermo-physical behavior of Li₂O-Al₂O₃-P₂O₅-SiO₂ glass–ceramic system at different crystallization temperatures (CT) and heat treatment regimes (HTR) was investigated experimentally. The Li₂O-Al₂O₃-P₂O₅-SiO₂ glasses were fabricated using melt-quenching method and quickly annealed at 450 °C to relieve the internal stresses. The glass–ceramics were subjected to controlled nucleation process at 520 °C following the DSC data and crystallized at 550 °C/1 h (one-step HTR), 650 °C/1 h (one-step HTR), 650 °C/1 h + 700 °C/2 h (two-step HTR), and 650 °C/1 h + 700 °C/2 h + 780 °C/2 h (three-step HTR). The hardness, flexural strength, compressive strength, coefficient of thermal expansions (CTEs), and bulk density of glass–ceramics were determined. The thermal analysis (DSC), chemical compositions, surface morphology, crystal formation and transformation, and photographs of indentations, and crack patterns were also examined. The SEM and X-ray diffraction patterns revealed four major crystalline phases: LiAlSi₂O₆, Li₂Si₂O₅, Li₂SiO₃, and Li₃PO₄ which grow in quantity and size at increasing crystallization temperatures and heat treatment regimes. These microstructural changes and crystal transformations resulted in high bonding force and microstructure interlocking, leading to improvements of thermo-physical properties of the glass–ceramics. Maximum hardness, compressive strength, flexural strength, CTEs, and density of 6.72 GPa, 976 MPa, 137.44 MPa, 15.42 × 10⁻⁶/ °C, and 2.66 g/cm³ were obtained after three-step heat treatment process.