Objective

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.

Methods

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.

Results

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.

Significance

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.

中文翻译：

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低温降解会增加3Y-TZP弯曲时的抗循环疲劳性。

目的

由于矫形的3Y-TZP股骨植入物过去的失败与加速的四方-单斜相转变（t → m）有关，因此广告宣传了3Y-TZP对“低温降解”或“老化”的敏感性，不利于其长期的结构稳定性。然而，没有关于循环载荷下老化的3Y-TZP疲劳强度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。

方法

在这里，我们评估了在134°C高压釜中加速老化0-50小时后，在循环载荷下3Y-TZP在亚临界裂纹扩展行为。在两个不同的温度（1450°C或1650°C）下烧结的同一3Y-TZP，可以比较包含具有不同转化敏感性的晶粒的材料。使用拉曼光谱法测量表面相变晶粒的体积分数，并用聚焦离子束铣削沟槽后的相变表面层深度。使用雪佛龙缺口梁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，并在不同的施加应力下于4 Hz弯曲下在10 Hz和R比率= 0.3的水中循环寿命。疲劳参数n它是从3种不同的方法得出的，即S N曲线，裂纹速度图和威布尔分布。

结果

对于在1650°C烧结的3Y-TZP，转变过程显示出线性动力学，并且速率更高。加速的相变导致相变层中平行于表面平面的严重裂纹形成，据认为这是控制断裂的临界裂纹尺寸总体。由于压缩应力，相变层内的应力强度因子增加。因此，疲劳参数n从老化5h到50h一直增加，无论采用什么方法，都表明在弯曲循环载荷下抗裂纹扩展的能力增加。

意义

我们的结果并不支持LTD在机械性能方面长期以来提出的负面临床影响，相反，LTD似乎增加了在潮湿环境中弯曲时对亚临界裂纹增长的抵抗力。