Machinability: Zirconia-reinforced lithium silicate glass ceramic versus lithium disilicate glass ceramic

Diamond grinding used in dental adjustment of high-strength zirconia-reinforced lithium silicate glass ceramic (ZLS) and lithium disilicate glass ceramic (LDGC) is challenging in restorative dentistry. This study aimed to compare the machinability of ZLS and LDGC in diamond grinding in terms of machining forces and energy, debris, surface and edge chipping damage. Grinding experiments in simulation of dental adjustment were conducted using a computer-assisted high-speed dental handpiece and coarse diamond burs. A piezoelectric force dynamometer and a high-speed data acquisition system were used for on-processing monitoring for assessment of grinding forces and energy. Grinding debris and grinding-induced surface and edge chipping damage were examined using scanning electron microscopy. The results show that grinding of ZLS required higher tangential and normal forces and energy than LDGC (p < 0.05). ZLS was ranked the most difficult to machine among dental glass ceramics based on a machinability index associated with the material mechanical properties. The higher machinability indices of ZLS and LDGC pose a challenge for clinicians to conduct high-efficient material removal for dental adjustment and repair. Both ZLS and LDGC debris were micro fractured particles but the former were smaller than the latter due to the finer microstructure of ZLS. Ground ZLS surfaces contained more irregular microchipping and microfracture in comparison with LDGC surfaces with intergranular fracture or grain dislodgement. Grinding-induced edge chipping damage remained a serious issue for both ZLS and LDGC, which depths ranged approximately 20-100 μm and significantly increased with the material removal rate (p < 0.01). As the zirconia-reinforcement in ZLS only slightly reduced edge chipping damage (p > 0.05), continued efforts are required to explore new reinforcement technologies for optimized LDGC.