Soft machining-induced surface and edge chipping damage in pre-crystalized lithium silicate glass ceramics

Soft machining is a key procedure in fabrication of high-strength lithium-based silicate glass ceramic (LS) restorations. This paper reports on the diamond machining-induced surface and edge chipping damage in two pre-crystalized LS materials: pre-crystallized lithium metasilicate/orthophosphate glass ceramic (Pre-LS, IPS e.max CAD) and pre-crystallized zirconia-containing lithium metasilicate glass ceramic (Pre-ZLS, Vita Suprinity). Indentation techniques were used to measure the material mechanical properties. Soft machining was conducted using a robotic controlled apparatus mimicking dental CAD/CAM machining processes at different removal rates and enabling in-process force measurement. Machined surface roughness was assessed using 3D confocal optical profilometry in terms of the average and maximum surface heights. Scanning electron microscopy was used to assess diamond tool and machined surface and edge morphology. Soft machining of both materials was dominated by brittle fracture mixed with localized ductile flow. However, the higher brittleness index of Pre-ZLS than Pre-LS yielded higher degrees of machining-induced conchoidal fractures in Pre-ZLS in comparison with irregular fractures in Pre-LS. Thus, much larger surface roughness and deeper edge chipping damage were produced in Pre-ZLS than Pre-LS. Machining forces for Pre-ZLS were significantly smaller than Pre-LS, due to the lower machinability index associated with a complex relation of the mechanical properties as well as less debris adhesion for Pre-ZLS than Pre-LS. Further, increased material removal rates resulted in significantly increased machining forces, maximum surface roughness and fracture, and edge chipping damage in both Pre-ZLS and Pre-LS materials. Therefore, optimization of soft machining processes needs to be practiced to achieve accepted surface and edge quality at balanced removal rates.