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.