Influence of shading on zirconia's phase transformation and flexural strength after artificial aging.

Abstract

The aim of this study was to determine the phase transformation and the influence of aging on the flexural strength of different colored zirconia. The effect of hydrothermal aging in an autoclave was compared with the effect of mechanical stress by simulating chewing. High-strength zirconia (3Y-TZP) was investigated in three different colors: uncolored, A3, and D3. Disc-shaped specimens (N=3) were analyzed by X-ray diffraction (XRD), and flexural strength was determined on bar-shaped specimens (N=15) in a 4-point bending test before and after performing two different aging protocols: aging in an autoclave (134°C, 70h) and aging in a chewing simulator (5kg load, 1.2 million cycles). During autoclave aging, the fraction of monoclinic phase on the surface was determined every 5h. Once this exceeded 25vol%, aging of the bar specimens was stopped. While in the unstained group the mean value of the proportion of monoclinic phase already exceeded 25vol% after 30h in the autoclave, this was the case in both stained groups only after 70h. No measurable phase transformation could be detected after chewing simulation. Only color A3 showed a statistically significant (p≤0.05) decrease in flexural strength after aging in the chewing simulator. The colored zirconia showed a higher resistance to phase transformation through hydrothermal aging. The metal oxides in the staining solutions are assumed to hinder the phase transformation in the zirconia. Therefore, the significant reduction in the stained zirconia after chewing simulation is particularly interesting.