Department of Biomaterials and Oral Biology, School of Dentistry, University of São Paulo, São Paulo, Brazil.
School of Mechanical and Manufacturing Engineering, UNSW, Sydney, NSW, Australia.
J Mech Behav Biomed Mater. 2019 Aug;96:261-268. doi: 10.1016/j.jmbbm.2019.04.049. Epub 2019 Apr 26.
Compare residual stress distribution of bilayered structures with a mismatch between the coefficient of thermal expansion (CTE) of framework and veneering ceramic. A positive mismatch, which is recommended for metal-ceramic dental crowns, was hypothesized to contribute to a greater chipping frequency in veneered Y-TZP structures. In addition, the multidirectional nature of residual stresses in bars and crowns is presented to explore some apparent contradictions among different studies.
Planar bar and crown-shaped bilayered specimens with 0.7 mm framework thickness and 1.5 mm porcelain veneer thickness were investigated using finite element elastic analysis. Eight CTE mismatch conditions were simulated, representing two framework materials (zirconia and metal) and six veneering porcelains (distinguished by CTE values). Besides metal-ceramic and zirconia-ceramic combinations indicated by the manufacturer, models presenting similar mismatch values (1 ppm/°C) with different framework materials (metal or zirconia) and zirconia-based models with metal-compatible porcelain veneers were also tested. A slow cooling protocol from 600 °C to room temperature was simulated. The distributions of residual maximum and minimum principal stresses, as well as stress components parallel to the long axis of the specimens, were analysed.
Planar and crown specimens generated different residual stress distributions. When manufacturer recommended combinations were analysed, residual stresses obtained for zirconia models were significantly higher than those for metal-based models. When zirconia frameworks were combined with metal-compatible porcelains, the residual stress values were even higher. Residual stresses were not different between metal-based and zirconia-based models if the CTE mismatch was similar.
Some conclusions obtained with planar specimens cannot be extrapolated to clinical situations because specimen shape strongly influences residual stress patterns. Since positive mismatch generates compressive hoop stresses and tensile radial stresses and since zirconia-based crowns tend to be more vulnerable to chipping, a tensile stress-free state generated with a zero CTE mismatch could be advantageous.
比较热膨胀系数（CTE）失配的双层结构的残余应力分布。假设金属陶瓷牙冠推荐的正失配会导致贴面氧化锆结构更频繁出现崩瓷。此外，还呈现了棒材和牙冠中残余应力的多向性质，以探索不同研究之间的一些明显矛盾。
使用有限元弹性分析研究了具有 0.7mm 基底厚度和 1.5mm 瓷贴面厚度的平面棒材和牙冠形状的双层试样。模拟了 8 种 CTE 失配情况，代表了两种基底材料（氧化锆和金属）和六种瓷贴面（通过 CTE 值区分）。除了制造商推荐的金属陶瓷和氧化锆陶瓷组合外，还测试了具有类似失配值（1ppm/°C）但不同基底材料（金属或氧化锆）的模型，以及具有金属兼容瓷贴面的氧化锆基模型。模拟了从 600°C 缓慢冷却到室温的过程。分析了残余最大和最小主应力以及与试件长轴平行的应力分量的分布。
平面和牙冠试件产生了不同的残余应力分布。分析制造商推荐的组合时，氧化锆模型的残余应力明显高于金属基模型。当氧化锆基底与金属兼容的瓷贴面结合时，残余应力值甚至更高。如果 CTE 失配相似，金属基和氧化锆基模型的残余应力没有差异。
一些从平面试件得出的结论不能外推到临床情况，因为试件形状强烈影响残余应力模式。由于正失配会产生压环向应力和拉径向应力，并且氧化锆基牙冠更容易崩瓷，因此零 CTE 失配产生的无拉伸应力状态可能是有利的。
