Influence of grinding-induced thermal softening on the material removal mechanism of lithium metasilicate glass-ceramics

Lithium metasilicate glass-ceramics (LMGC) are widely used in restorative dentistry; however, their hardness and brittleness make machining challenging, with material removal occurring predominantly through brittle fracture that degrades surface quality. This study investigates whether grinding-induced thermal softening can modify the removal mechanism and promote ductile-mode grinding in LMGC. Ductile-dominated removal was observed at wheel depths of cut (DOC) of 1 and 10 μm, while higher DOCs (20 - 40 μm) produced a mixed ductile–brittle regime, contradicting amended Bifano-Dow-Scattergood (aBDS) model predictions of brittle removal under all tested conditions. This discrepancy is attributed to frictional heating induced thermal softening, which promotes ductile removal and thus raises the ductile-to-brittle transition threshold, as supported by high-temperature nanoindentation showing a 94% hardness reduction at 700 °C. Ductile-dominated regions exhibit much lower surface roughness S a of 0.27 μm than brittle regions of 0.74-1.12 μm, demonstrating much improved surface quality. Increasing the workpiece temperature prior to grinding also promoted the transition from brittle to ductile removal and produced more uniform surface finish. At higher DOCs, ductile and brittle removal coexisted due to a transient, highly non-uniform thermal field where fracture and plastic deformation competed locally. These findings reveal that grinding-induced heat plays a critical role in LMGC material removal and thermal softening can be established as a practical pathway to improve grinding performance and surface integrity.