Mechanical properties and machinability of lithium silicate glass-ceramics with varying MgO content

This study focused on the role of lithium metasilicate crystalline phase and microstructural evolution on altering the mechanical properties and machinability of two glass-ceramic system, (Li₂O-K₂O-Na₂O-CaO-MgO-ZrO₂-Al₂O₃-P₂O₅-SiO₂), previously developed by our research group. Our previous study showed that increasing MgO content introduced phase separation and thus controlling the crystallization and microstructure of the glass-ceramics. For this study we chose the glass ceramic system with 1.5 and 4.5 mol% MgO. The glass-ceramics were prepared by melt cast method followed by heat treatment at 650 °C to induce partial crystallization. The crystalline phase in the glass-ceramics was identified by x-ray diffraction and the quantification of crystalline and amorphous phase content was done using Rietveld refinement. The hardness and elastic moduli were measured by nanoindentation. Vickers hardness and fracture toughness were determined by micro indentation. Nano scratch test was performed and scratch morphology was characterized using AFM and FEG-SEM. In order to compare the machinability of the prepared glass-ceramic samples, milling experiments were carried out using CNC milling machine, and machining characteristics were compared by analysing machining force, surface roughness and microstructure of the machined surface. Presence of higher amount of lithium metasilicate (LS) crystals in higher magnesia containing glass-ceramic (G2) resulted in lower average force (Fy_rms) as compared to lower MgO containing glass-ceramic. However, due to a wide range of variation in crystal size (150 nm - 2 μm) in higher MgO containing glass-ceramic, the force fluctuation was more than lower MgO containing glass-ceramics.