Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glueckstrasse 11, 91054 Erlangen, Germany.
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glueckstrasse 11, 91054 Erlangen, Germany.
Dent Mater. 2020 Aug;36(8):1086-1095. doi: 10.1016/j.dental.2020.05.002. Epub 2020 Jun 3.
Due to past failures of orthopedic 3Y-TZP femoral implants linked to accelerated tetragonal-to-monoclinic phase transformation (t → m), the susceptibility to 'low-temperature degradation' or 'ageing' of 3Y-TZP has been advertised as detrimental to its long-term structural stability. However, no systematic mechanistic experiments on the fatigue resistance of aged 3Y-TZP under cyclic loading can support such statement. In this study, we aim to clarify this issue.
Here we evaluate the subcritical crack growth behavior of 3Y-TZP under cyclic loading after 0-50 h of accelerated ageing in an autoclave at 134 °C. The same 3Y-TZP sintered at two different temperatures (1450 °C or 1650 °C) allows for the comparison of materials containing grains with different susceptibilities to transformation. The volume fraction of surface transformed grains was measured using Raman spectroscopy, and the depth of the transformed surface layer from trenches milled with a Focus-Ion Beam. The fracture toughness before and after ageing was determined using the Chevron-notch Beam method. The quasi-static flexural strength was measured in dry conditions and the cyclic lifetime in water at 10 Hz and R-ratio = 0.3 in 4-point-bending at different applied stresses. The fatigue parameter n was derived from 3 different methods, namely SN curves, crack velocity plots and Weibull distributions.
The progression of transformation showed linear kinetics with higher rates for the 3Y-TZP sintered at 1650 °C. Accelerated transformation induced severe crack formation within the transformed layer with parallel orientation to the surface plane, which supposedly behaved as the critical crack size population governing fracture. The stress intensity factor within the transformed layer was increased due to compressive stresses. Consequently, the fatigue parameter n increased consistently from 5 to 50 h of ageing, regardless of the derivation method, suggesting an increased resistance against crack growth during cyclic loading in bending.
Our results do not support the long suggested negative clinical implications of LTD regarding mechanical performance, to the contrary, LTD seems to increase the resistance against subcritical crack growth in a humid environment in bending.
由于过去骨科 3Y-TZP 股骨植入物的失败与加速四方相到单斜相转变（t → m）有关，因此 3Y-TZP 的“低温降解”或“老化”易感性被认为对其长期结构稳定性有害。然而，没有关于循环载荷下老化 3Y-TZP 抗疲劳性的系统力学实验可以支持这种说法。在这项研究中，我们旨在澄清这个问题。
在这里，我们评估了在 134°C 高压釜中加速老化 0-50 小时后，循环载荷下 3Y-TZP 的亚临界裂纹扩展行为。在两种不同温度（1450°C 或 1650°C）下烧结的相同 3Y-TZP 允许比较具有不同转变敏感性的材料晶粒。使用拉曼光谱测量表面转变晶粒的体积分数，并使用聚焦离子束铣削的沟槽测量表面转变层的深度。使用 Chevron-notch Beam 方法测定老化前后的断裂韧性。在干燥条件下测量准静态弯曲强度，并在 10Hz 和 R-比= 0.3 的水中在不同应用应力下进行 4 点弯曲的循环寿命。疲劳参数 n 由 3 种不同的方法得出，即 SN 曲线、裂纹速度图和威布尔分布。
转变的进展表现出线性动力学，在 1650°C 烧结的 3Y-TZP 中速率更高。加速转变在与表面平面平行的转变层内引起严重的裂纹形成，这可能是控制断裂的临界裂纹尺寸群体。由于压缩应力，转变层内的应力强度因子增加。因此，疲劳参数 n 从老化 5 小时到 50 小时持续增加，无论采用哪种推导方法，这表明在弯曲循环载荷下，抗裂纹扩展的能力增加。
我们的结果不支持 LTD 对机械性能的长期负面影响，相反，LTD 似乎在弯曲时增加了潮湿环境下亚临界裂纹扩展的阻力。
