Monoclinic phase transformations of zirconia-based dental prostheses, induced by clinically practised surface manipulations

Full-ceramic zirconia crowns and bridges have become very popular with dentists and patients because of their excellent esthetics and mechanical properties. We studied phase transformations within the outermost surface layer of 3 mol.% yttria-stabilized zirconia (Y-TZP) samples of small, clinically relevant thicknesses, manipulated by polishing, grinding and fracture as might be encountered in everyday clinical practice. Stress-induced transformations of the tetragonal phase were studied in three dimensions in order to better understand the organization and extent of the monoclinically transformed phase. By means of laboratory- and synchrotron-based X-ray diffraction measurements, coupled with electron microscopy and multimodal tomography, it was possible for the first time to visualize and quantify the phase distributions non-destructively and in three dimensions. Highly variable degrees of local transformation result in ragged transformed zones of very inhomogeneous thickness. The overall thickness of the transformation layers strongly depends on the severity and rate of loading. Gentle diamond cutting resulted in surprisingly low transformation ratios of less than 0.1%. When Y-TZP constructions are manipulated before bonding, toughness of the outer layers is reduced and they may become brittle with important implications for the stability of the bond: dental practitioners thus need to be cautious when altering the surfaces of these materials after sintering.