Effect of hydrothermal degradation on three types of zirconias for dental application

Statement of problem: Concern has been expressed with regard to hydrothermal aging of yttria-stabilized tetragonal zirconia polycrystalline.

Purpose: The purpose of this study was to assess the accelerated aging characteristics of a new yttria-stabilized tetragonal zirconia polycrystalline material and 2 commercially available yttria-stabilized tetragonal zirconia polycrystalline materials by exposing specimens to hydrothermal treatments in steam at 134°C, 0.2 MPa, and at 180°C, 1.0 MPa.

Material and methods: Thin bars of zirconia: Prettau, Zirprime, and a new zirconia, ZirTough, n = 55 for each brand (22 × 3 × 0.2 mm) were cut and ground from blocks sintered according to the manufacturer's specifications. The control specimens for each group were evaluated in the nonaged condition, and their chemical composition was measured with energy dispersive spectroscopy. The experimental specimens were artificially aged under standard autoclave sterilization conditions, 134°C at 0.2 MPa (n = 5 per group at 5, 50, 100, 150, and 200 hours), and under standard industrial ceramic aging conditions, 180°C at 1.0 MPa (n = 5 per group at 8, 16, 24, and 48 hours). The tetragonal to monoclinic transformation was measured by using x-ray diffraction for all groups. Flexural strength was measured with a 4-point bend test (ASTM 1161-B) for all the groups, and the fracture surfaces were examined with scanning electron microscopy. The data were analyzed as a function of aging time. To test for an aging effect on the flexural strength and the monoclinic-tetragonal ratio, a 1-way ANOVA (with heteroscedasticity-consistent standard errors) was used to test for a general time effect. For the analyses of the monoclinic-tetragonal ratio, the same specimens were used at 0 hours and after aging, and the data were analyzed with an ANOVA for an incomplete block design. The relationship between flexural strength and monoclinic-tetragonal ratio was assessed with the Spearman rank correlation coefficient based on the average value at each aging.

Results: After 200 hours at 134°C and 0.2 MPa, the flexural strength decreased from a mean (standard deviation) of 1328 ± 89.9 MPa to all fractured during aging for Prettau (P < .001); 1041 ± 130 to 779 ± 137 MPa for Zirprime (P = .<.001) and 1436 ± 136 to 1243 ± 101 MPa for ZirTough (P = .017). After 200 hours at 134°C and 0.2 MPa, a portion of the tetragonal crystals transformed to the monoclinic phase in all specimens. The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 78.8% ± 2.0% for Prettau, 1.95% ± 0.48% to 74.8% ± 0.52% for Zirprime, and 12.4% ± 0.60% to 31.4% ± 4.4% for ZirTough (all P < .001). After 16 hours at 180°C and 1.0 MPa, all Prettau specimens had spontaneously fractured during aging. The Zirprime and ZirTough specimens were intact after 48 hours at 180°C and 1.0 MPa, and the mean (standard deviation) flexural strength had decreased from 1041 ± 130 MPa to 595 ± 88.4 MPa for Zirprime and 1436 ± 136 MPa to 1068 ± 76.8 MPa for ZirTough (all P < .001). The mean (standard deviation) monoclinic phase fraction increased from 3.08% ± 0.28% to 79.0% ± 0.13% for Prettau, from 1.95% ± 0.48% to 68.1% ± 4.4%, for Zirprime, and from 12.4% ± 0.60% to 39.5% ± 5.56% for ZirTough (all P<.001). The flexural strength decreased with an increase in the monoclinic phase for all the groups (Spearman rank correlation coefficients, -0.71 to -1.0). Scanning electron microscope micrographs revealed a transformed layer on the fracture surfaces. The decrease in flexural strength was related to the increase in monoclinic phase from long-term degradation.

Conclusion: Hydrothermal aging of zirconia caused a statistically significant decrease in flexural strength of thin bars of zirconia, which was the result of the transformation from tetragonal to monoclinic crystal structure. ZirTough exhibited the least decrease in strength and smallest amount of monoclinic phase after aging.