Effect of Low Thermal Degradation on Flexural Strenght of Zirconia

Objectives: The purpose of this study was to investigate the aging characteristics of three yttria-stabilized tetragonal zirconia polycrystalline (Prettau, Supra Zr, CopraPretty) and effect of low thermal degradation on flexural strenght.
Methods: Total of 120 discs (d=12.0 mm, h=1.2±0.2 mm as per ISO 6872) were obtained from three presintered zirconia-based materials. Specimens sintered according to the manufacturers specifications. Control specimens (n=10) for each brand left in distilled water at 37°C for 5 h. Experimental specimens were artificially aged at standard autoclave sterilization conditions, 134^○C at 0.2 MPa for 5h in Low Thermal Degradation (LTD) group; fatigued for 10,000 thermal cycles between baths hold at 5°C and 55°C in Thermal Cyclus (TC) group and left in 4% asetic asid at 80°C for 168 h in Asetic Asid (AA) group.

Tetragonal to monoclinic transformation was measured by using X-ray diffraction for all groups. Flexural strength was measured in biaxial flexural strenght test for all groups.

Datas were analyzed using one-way ANOVA and Kruskall Wallis follewed by Tukey HSD and Mann Whitmey U test (α=.05).
Results: Corresponding to the XRD results; Prettau showed the highest; CopraPretty showed the lowest relative amount of the monoclinic phase content due to aging. The lowest biaxial flexural strength (FS) mean was recorded in CopraPretty. No significant difference was found between Prettau and Supra Zr (p>0.05).
Conclusions: Low temperature degradation produced the positive and negative effects on the mechanical properties of Y-TZP ceramics depending on the experimental conditions applied. Long term in vivo studies are insisted to evaluate whether the results presented are transferable to the clinical situation.