Effect of glazing application side and mechanical cycling on the biaxial flexural strength and Weibull characteristics of a Y-TZP ceramic

Abstract Glaze application on monolithic zirconia (Y-TZP) can be a practical approach to improve the mechanical properties of this material. Objective Our study evaluated the effect of glazing side and mechanical cycling on the biaxial flexure strength (BFS) of a Y-TZP. Methodology Eighty sintered Y-TZP discs (Ø:12 mm; thickness: 1.2 mm - ISO 6872) were produced and randomly assigned into eight groups (n=10), according to the factors “glazing side” (control – no glazing; GT – glaze on tensile side; GC – glaze on compression side; GTC – glaze on both sides) and “mechanical aging” (non-aged and aged, A – mechanical cycling: 1.2×106, 84 N, 3 Hz, under water at 37°C). Specimens were subjected to BFS test (1 mm/min; 1,000 Kgf load cell) and fractured surfaces were analyzed by stereomicroscopy and SEM. Hsueh’s rigorous solutions were used to estimate the stress at failure of glazed specimens. Two-way ANOVA, Tukey’s test (5%), and Weibull analysis were performed. Results The “glazing side”, “mechanical aging” and the interaction of the factors were significant (p<0.05). Groups GC (1157.9±146.9 MPa), GT (1156.1±195.3 MPa), GTC (986.0±187.4 MPa) and GTC-A (1131.9±128.9 MPa) presented higher BFS than control groups (Tukey, 5%). Hsueh’s rigorous solutions showed that the maximum tensile stress was presented in the bottom of zirconia layer, at the zirconia/glaze interface. Weibull characteristic strength (σo) of the GC was higher than all groups (p<0.05), except to GT, GTC-A and GTC, which were similar among them. The fractography showed initiation of failures from zirconia the tensile side regardless of the side of glaze application and fatigue. Conclusion Glazing zirconia applied on both tensile and compression sides improves the flexural strength of Y-TZP, regardless the mechanical aging.

**摘要**：对单片氧化锆（monolithic zirconia, Y-TZP）施釉是改善该材料力学性能的可行方案。
**研究目的**：本研究旨在评估施釉侧与机械循环加载对Y-TZP双轴弯曲强度（biaxial flexure strength, BFS）的影响。
**研究方法**：制备80片烧结态Y-TZP圆盘（直径：12 mm；厚度：1.2 mm，符合ISO 6872标准），依据“施釉侧”与“机械老化”两个因素随机分为8组（每组n=10）。其中“施釉侧”因素包括：对照组（无施釉）、GT组（受拉侧施釉）、GC组（受压侧施釉）、GTC组（双侧施釉）；“机械老化”因素包括：未老化组与老化组（A组：机械循环加载1.2×10^6次，载荷84 N，加载频率3 Hz，37℃水环境下）。所有试样均开展双轴弯曲强度测试（加载速率1 mm/min，载荷传感器量程1000 kgf），并通过体视显微镜与扫描电子显微镜（scanning electron microscopy, SEM）对断口表面进行表征分析。采用薛氏严格解析解法估算施釉试样的断裂应力；开展双向方差分析（Two-way ANOVA）、显著性水平为5%的图基检验以及威布尔分析（Weibull analysis）。
**研究结果**：“施釉侧”“机械老化”以及二者的交互作用均具有统计学显著性（p<0.05）。GC组（1157.9±146.9 MPa）、GT组（1156.1±195.3 MPa）、GTC组（986.0±187.4 MPa）与GTC-A组（1131.9±128.9 MPa）的双轴弯曲强度均高于对照组（图基检验，5%显著性水平）。薛氏严格解析结果显示，最大拉应力出现在氧化锆层底部、氧化锆与釉层的界面处。GC组的威布尔特征强度（σo）高于其余所有组别（p<0.05），仅与GT、GTC-A及GTC组无显著差异，此四组间结果相近。断口形貌分析表明，无论施釉侧与疲劳状态如何，断裂均起始于氧化锆的受拉侧。
**研究结论**：无论是否经过机械老化处理，在受拉侧与受压侧均施釉的Y-TZP均可提升其弯曲强度。