Characterizing the transformation near indents and cracks in clinically used dental yttria-stabilized zirconium oxide constructs.

OBJECTIVES: Tetragonal zirconia polycrystal (TZP) materials are widely used for full ceramic partial dentures, even though their mechanical properties might change during service. A key property for the durability of the constructs is thought to be an inhibition of crack propagation by phase transformation toughening. Because dental prosthesis are ground and polished for adjustment purposes, it is important to understand the effects of mechanical surface treatments, on localized transformation and around the propagating cracks. METHODS: Sintered samples of commercially available 3 mol-% Yittria-doped TZP were ground and polished and the surface structure and phase composition were compared with those of re-transformed annealed samples. Microindentation was used to induce cracks and nanoindentation was performed to determine the local variety of hardness and indentation modulus, coupled with XRD and SEM investigations. RESULTS: Y-TZP polished surfaces exhibited 9% monoclinic phase content and have reduced hardness and indentation modulus amounting 16.3 GPa and 210.6 GPa, respectively. Y-TZP re-transformed annealed sample revealed 19.4 GPa and 242.3 GPa, respectively. A localized reduction of the stiffness around the crack tips on the annealed surface reveals an irregular arrangement of t-m-transformed grains. Electron micrographs show more damage on the transformed surface following microindentation than on indented annealed surfaces. SIGNIFICANCE: Y-TZP prostheses are adapted and roughened by clinicians prior to bonding to teeth. Annealing recovers properties and microstructure that is changed by the adaptation of the outer layer. This might be important to ensure long-term toughening functionality of the dentures and optimal comfort for the patients.

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.