Effect of Aging Time and Thickness on Low-Temperature Degradation of Dental Zirconia

Abstract

Purpose: To evaluate the influence of specimen thickness and low-temperature degradation (LTD) on yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP). Materials and Methods: Thin discs of Y-TZP from four manufacturers were sintered according to each manufacturer's recommendations, cut into 23 mm (length) × 4 mm (width) × 0.8 mm/1.5 mm (thickness) specimens, artificially aged under standard autoclave sterilization conditions (34°C at 0.2 MPa for 10 and 20 hours), and finally ground and polished. Tetragonal to monoclinic transformation was confirmed by X-ray diffraction (XRD) analysis. Flexural strength was measured by 3-point bending tests and Vickers hardness measurements. Fracture surfaces were examined by scanning electron microscopy (SEM). Results: SEM investigation revealed that with increasing aging time, the surface defects and grain size increased, particularly in the 20-hour group. Compared with the 1.5 mm group, the 0.8 mm group showed more significant defects, irrespective of aging time. The flexural strengths of Y-TZP materials decreased with a decrease in the thickness. Moreover, LTD of Y-TZP can cause significant tetragonal to monoclinic transformation, which also results in a statistically significant decrease in the flexural strength. Conclusion: A thinner Y-TZP specimen was likely to present surface defects and microcracks after aging. In addition, the flexural strengths decreased with a decrease in the thickness, a notable fact for further studies.