Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Zahnklinik 1-Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Erlangen, Germany.
J Dent Res. 2023 Sep;102(10):1106-1113. doi: 10.1177/00220345231180565. Epub 2023 Jul 14.
Chemical and mechanical fatigue degradation in ceramic materials is generally inconspicuous yet ubiquitous, to the effect that clinical fractures still consist of the main cause of failure in all-ceramic restorations. Implications of this span wide, from a reduced survival prognosis for the affected teeth, including more frequent and increasingly invasive procedural interventions, to the financial burden borne by individuals and health care systems. To suffice as an effective corrective, restoration lifetimes need only to be extended so to outlive the patient. That opens a box of problems from a materials science standpoint, entailing inherent deficiencies of brittle materials to resist mechanical and environmental challenges. Efforts in developing more damage-tolerant and fatigue-resistant restoratives go thus hand in hand with understanding intrinsic mechanisms of crack growth behavior under conditions that simulate the oral environment. Here we developed experiments using size-relevant sharp precracked specimens with controlled size and geometry (truncated semielliptical crack in the surface-crack-in-biaxial-flexure method) to establish a relationship between crack size and strength. The tangent method was used to construct envelopes for the quasi-static (R-curves), which served as template for deriving residual cyclic R-curve analogs. By means of experimentally obtained stress-cycle curves, lifetime and fatigue parameters were employed within a mechanistic framework to reveal constitutive toughening mechanisms during subcritical growth under cyclic loading in a wet environment. Using 3 modern dental lithium disilicate glass-ceramics, we demonstrate the extent of R-curve degradation up to a threshold of 10 million cycles (~30 y in service) and draw parallels between the scope of fatigue degradation and the size of the microstructural units responsible for toughening mechanisms in glass-ceramic materials. Our results indicate that larger microstructural elements endow glass-ceramics with a higher reaching quasi-static R-curve at the onset but degrading more rapidly to comparable levels of lithium disilicates having submicrometric and nanometric crystal phases.
陶瓷材料的化学和机械疲劳退化通常不明显但无处不在，以至于临床骨折仍然是所有陶瓷修复体失效的主要原因。这一影响范围很广，从受影响牙齿的生存预后降低，包括更频繁和越来越侵入性的程序干预，到个人和医疗保健系统承担的经济负担。为了作为有效的矫正，修复体的寿命只需延长到超过患者的寿命。这从材料科学的角度来看，带来了一系列问题，即脆性材料在抵抗机械和环境挑战方面存在固有缺陷。因此，开发更耐损伤和耐疲劳的修复体的努力与理解在模拟口腔环境条件下的裂纹扩展行为的内在机制齐头并进。在这里，我们使用具有控制尺寸和几何形状的尺寸相关尖锐预制裂纹样品（表面裂纹双轴弯曲法中的截断半椭圆形裂纹）进行了实验，以建立裂纹尺寸与强度之间的关系。切线法用于构建准静态（R 曲线）的包络线，作为衍生剩余循环 R 曲线模拟的模板。通过实验获得的应力-循环曲线，在机械框架内使用寿命和疲劳参数来揭示在湿环境下循环加载下亚临界增长期间的本构增韧机制。使用 3 种现代牙科锂硅玻璃陶瓷，我们展示了 R 曲线退化的程度，直到 1000 万次循环的阈值（服务寿命约 30 年），并在疲劳退化的程度和负责玻璃陶瓷材料增韧机制的微观结构单元的大小之间建立了联系。我们的结果表明，较大的微观结构元素使玻璃陶瓷在开始时具有更高的准静态 R 曲线，但在达到类似水平时，降解速度更快，具有亚微米和纳米级晶体相的锂硅酸盐。
