Influence of hydrothermal and mechanical conditions on the strength of zirconia

Low temperature degradation and mechanical and thermal cycling may decrease the strength of zirconia and jeopardize the long-term success of dental restorations made of this material. The objective of this study was to reveal the influence of different environmental and loading conditions on the strength of 3 mol.% yttria-stabilized polycrystalline tetragonal zirconia (3Y-TZP). A total of 144 disk specimens were produced from each of two 3Y-TZP materials, and subjected to one of the following conditions: (A) no further treatment (control); (B and C) 106 and 5 × 106 mechanical cycles, respectively, with an upper load limit of 100 N; (D) 104 thermal cycles between 5 and 55 °C; (E) 200 days storage in water at 36 °C; (F) a successive combination of conditions B, D and E; (G) storage in water at 80 °C for 64 days; (H) storage in an autoclave at 134 °C for 8 h. Monoclinic phase content was evaluated by X-ray diffraction (XRD) analysis. Specimen strength was determined in a biaxial bending test. The two ceramics exhibited average strengths of 995 and 1239 MPa, respectively. No statistically significant influence of any treatment on strength was demonstrated for either material. However, XRD measurements revealed a substantial increase in monoclinic phase content, from an initial 2% (control) to up to 10%, according to storage conditions. As a consequence of hydrothermal loading a tetragonal to monoclinic phase transformation took place at the surface of the 3Y-TZP materials investigated, but, like thermal and mechanical cycling, this did not lead to significant changes in bulk strength.