Effect of crystalline phase assemblage on reliability of 3Y-TZP

Strengthening mechanisms of zirconia ceramics stabilized with 3 mol% yttria (3Y-TZP) are complex and dictated by the crystalline phase assemblage. Although their clinical performance for dental restorations has been excellent, there is evidence that framework fractures do occur and have been underreported. Meanwhile, the relationship between phase assemblage and reliability of 3Y-TZP is not properly understood.

The purpose of this in vitro study was to elucidate the relationship between crystalline phase assemblage and the reliability of 3Y-TZP and to calculate the associated probabilities of survival.

Disks of 3Y-TZP were prepared from cylindrical blanks and randomly assigned to 12 experimental groups (n=20 per group). Different crystalline phase assemblages were produced by either varying the sintering temperature from 1350 °C to 1600 °C and/or treating the surface by airborne-particle abrasion with 50-mm alumina particles at a pressure of 0.2 MPa for 1 minute with or without subsequent heat treatment. Crystalline phases were analyzed by standard and grazing incidence X-ray diffraction (GIXRD). The relationship between phase assemblage and reliability was determined by measuring the biaxial flexural strength (BFS) according to ISO standard 6872 and by using Weibull statistics to calculate the Weibull modulus (m), probability of survival, and maximum allowable stresses. XRD results were analyzed by ANOVA to detect statistically significant differences between groups. Adjustment for all pairwise group comparisons was made using the Tukey method (α=.05).

Standard incidence XRD confirmed the presence of a small amount of cubic phase after sintering at 1350 °C. A cubic-derived nontransformable tetragonal t'-phase was observed at sintering temperatures of 1450 °C and above, the amount of which increased linearly. GIXRD revealed that airborne-particle abraded groups sintered at 1350 °C and 1600 °C had the highest variability in monoclinic phase fraction as a function of depth. These groups were also associated with the lowest reliability. Groups as-sintered at 1350 °C and 1600 °C had the lowest modulus (m=8.1 [0.5] and 7.0 [0.8], respectively) and probability of survival (Ps) for a maximum allowable stress of 700 MPa, while treated groups sintered at 1450 °C and 1550 °C were associated with the highest modulus (from 15.0 [1.4] to 20.9 [1.4]) and Ps (≥0.9999). The lower strength and reliability of groups sintered at 1600 °C was consistent with the presence of a significant amount of nontransformable t'-phase. The pattern of BFS results indicated that ferro-elastic domain switching was a dominant strengthening mechanism in 3Y-TZP.

The present study first reported on the detrimental effect of the cubic-derived nontransformable t'-phase on the mechanical properties of 3Y-TZP. It was demonstrated that phase assemblage determined reliability and was directly linked to the probability of survival.