Low-temperature degradation of yttria-stabilized zirconia treated with pulsed laser and annealing techniques

Low-temperature degradation (LTD) is a progressive degradation following tetragonal-to-monoclinic transformation in a water environment, and it represents a problem for the long-term integrity of zirconia implants. However, there is little documentation on the low-temperature degradation characteristics of zirconia with surface treatment. We investigated the LTD properties of such systems to assess the influence of surface treatment using nanosecond (thermal) and picosecond (less thermal) pulsed lasers, as well as an additional annealing process. The nanosecond and picosecond pulsed lasers generated periodic structures on the surfaces of tetragonal zirconia polycrystals stabilized with 3 mol% yttria (Y-TZP). Cross-sectional observations using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) revealed the characteristics of the microstructure and the heat effects. Both laser treatments resulted in heat-damaged layers. Moreover, the nanosecond laser induced grain growth on the outermost surface. However, annealing after nanosecond laser treatment resulted in a porous structure below the surface. X-ray diffraction analysis detected monoclinic crystals on the surfaces treated with the nanosecond and picosecond pulsed lasers after the LTD acceleration test. Cross-sectional observations revealed the propagation of the monoclinic-transformed layer. We concluded that the thermal shock from the lasers deteriorates the LTD resistance of the materials, and the use of additional annealing hinders LTD propagation. Stress relaxation and the microstructures produced by annealing may contribute to maintaining toward LTD resistance.