Professor, Iowa Institute for Oral Health Research, University of Iowa College of Dentistry and Department of Prosthodontics, Iowa City, Iowa.
Clinical Assistant Professor, Division of Prosthodontics, Department of General Dentistry, East Carolina University School of Dental Medicine, Greenville, NC.
J Prosthet Dent. 2021 Aug;126(2):238-247. doi: 10.1016/j.prosdent.2020.05.023. Epub 2020 Aug 15.
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.
稳定的氧化锆陶瓷（3Y-TZP）的强化机制较为复杂，且取决于晶体相组合。尽管其在牙科修复中的临床性能优异，但有证据表明框架确实会发生断裂，且断裂情况报告不足。同时，3Y-TZP 的相组合与可靠性之间的关系尚未得到充分理解。
本体外研究的目的是阐明 3Y-TZP 的晶体相组合与可靠性之间的关系，并计算相关的生存概率。
从圆柱形坯料制备 3Y-TZP 圆盘，并将其随机分配到 12 个实验组（每组 20 个）。通过将烧结温度从 1350°C 改变至 1600°C 以及/或用 50mm 氧化铝颗粒在 0.2MPa 的压力下进行 1 分钟的空气喷射颗粒磨损处理，同时或不进行随后的热处理，从而产生不同的晶体相组合。通过标准和掠入射 X 射线衍射（GIXRD）分析晶体相。根据 ISO 标准 6872 测量双轴弯曲强度（BFS），并使用威布尔统计计算威布尔模数（m）、生存概率和最大允许应力，从而确定相组合与可靠性之间的关系。通过方差分析（ANOVA）来检测组间是否存在统计学差异。使用 Tukey 方法（α=.05）对所有两两组间比较进行调整。
标准入射 XRD 证实，在 1350°C 烧结后存在少量立方相。在 1450°C 及以上的烧结温度下观察到立方衍生的不可转变的四方 t'-相，其含量呈线性增加。GIXRD 显示，在 1350°C 和 1600°C 下烧结的空气喷射颗粒磨损组，其单斜相分数的深度变化最大。这些组也与最低的可靠性相关。在 1350°C 和 1600°C 下烧结的组的模数（m=8.1[0.5]和 7.0[0.8]）和最大允许应力为 700MPa 时的生存概率（Ps）最低，而在 1450°C 和 1550°C 下烧结的处理组与最高的模数（15.0[1.4]至 20.9[1.4]）和 Ps（≥0.9999）相关。在 1600°C 下烧结的组的强度和可靠性较低，这与存在大量不可转变的 t'-相一致。BFS 结果表明，铁弹畴转变是 3Y-TZP 的主要强化机制。
本研究首次报道了立方衍生的不可转变的 t'-相对 3Y-TZP 机械性能的有害影响。结果表明，相组合决定了可靠性，并且与生存概率直接相关。
