In-situ nanomechanical responses of zirconia-containing lithium silicate glass-ceramics

The high hardness and low fracture toughness of zirconia-containing lithium silicate glass ceramics (ZLS) leads to dominant brittle fracture behaviour, which prompts extensive surface and subsurface cracking during machining, reducing long-term stability of dental restorations. This study evaluated the in-situ nanoindentation behaviour of this material class aiming to describe the real-time deformation response and nanomechanical mechanisms of cracking in ZLS according to their crystallized state. The in-situ nanoindentation tests on polished pre-crystallized and crystallized ZLS (Vita Suprinity PC, Vita Zahnfabrik, Germany) were performed inside a scanning electron microscopy (SEM) at 500 mN peak load and 2 mN/s loading rate. SEM images were taken at different stages of loading-unloading cycles to correlate morphological responses to force-displacement events in real-time. Pop-in events in force displacement curves were matched to edge chipping in pre-crystallized state and to radial cracking in crystallized ZLS. Although both pre-crystallized and crystallized ZLS materials exhibited brittle fracture behavior under indentation, their microstructural responses revealed distinct fracture mechanisms. In the pre-crystallized ZLS, the brittle fracture was accompanied by edge chipping, rupture and radial cracks. In contrast, crystallized ZLS had shear-band, pile up, and radial cracks. Pre-crystallized ZLS had significantly lower hardness, Young's modulus, resistance to machining-induced cracking, and maximum shear stress compared with the crystallized state. Further, the larger indentation imprint volume observed in the pre-crystallized ZLS suggests higher material removal efficiency during machining. This study highlights the microstructure-property dependence of ZLS materials, providing valuable insights into their micromechanics upon abrasive machining.