Fracture mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal exposure
Fracture mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal exposure
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DOI：
10.1016/j.actamat.2015.10.019
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发表时间：
2016-01
期刊：
影响因子：
9.4
通讯作者：
B. Richards;Stephen Sehr;Foucault de Franqueville;M. Begley;H. Wadley
中科院分区：
文献类型：
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作者：
B. Richards;Stephen Sehr;Foucault de Franqueville;M. Begley;H. Wadley
采用最近优化的空气等离子喷涂工艺在α-SiC衬底上沉积了Yb_2SiO_5/Al_6SiO_(13)/Si三层环境阻挡涂层系统，采用低功率沉积工艺，以减少硅损耗，改善界面结合，降低缺陷浓度。在冷却过程中，由于其热膨胀系数超过了衬底的热膨胀系数，因此在单硅酸Yb层中产生了拉应力。这些应力驱动了垂直的泥浆裂缝，这些裂缝在Al6Si2O13(莫来石)层内或在其界面上经历了裂缝分支。在90%H2O + 10%O2环境中，在1316oC到110oC的温度之间进行随后的热循环，分支泥浆裂纹扩展到硅粘结层，并沿该层的中面横向扩展。分支裂纹的表面可接触到蒸汽环境，导致方石英表层的形成，由于在热循环过程中β ↔ α方石英的反复相变，该表层泥浆破裂。在长时间循环后，这些裂纹连接在一起，导致涂层部分剥落。用有限元分析方法对裂缝分支现象进行了分析，并从泥浆裂缝尖端控制扭结的裂缝驱动力来评价裂缝轨迹。目前的优化沉积工艺(在低沉积功率下进行)与以前的非优化工艺(在高功率下进行)的比较突出了降低裂纹驱动力和控制微结构缺陷的重要性。有限元模拟为量化涂层设计对各种开裂形式失效的敏感性提供了一种有效的手段。
A recently optimized air plasma spray process has been used to deposit a model tri-layer Yb2SiO5/Al6Si2O13/Si environmental barrier coating (EBC) system on α-SiC substrates using low power deposition parameters to reduce silicon losses, improve interface adherence and decrease defect concentrations. During cooling, tensile stresses developed in the ytterbium monosilicate layer since its coefficient of thermal expansion exceeded that of the substrate. These stresses drove vertical mud cracks that underwent crack branching either within the Al6Si2O13(mullite) layer or at one of its interfaces. Upon subsequent thermal cycling between temperatures of 1316 °C and 110 °C in a 90% H2O + 10% O2environment, the branched mud cracks propagated into the Si bond coat and grew laterally along the mid-plane of this layer. The faces of the branched cracks were accessible to the steam environment resulting in the formation of a cristobalite surface layer, which mud cracked due to repeated β ↔ α cristobalite phase transformations during thermal cycling. After extended cycling, these cracks linked to cause partial spallation of the coating. The crack branching phenomenon was analyzed using finite element analysis, and the crack trajectory was assessed in terms of the crack driving force controlling kinking from the tip of the mud cracks. A comparison between the present optimized deposition process (performed at low deposition power) with a previous study of a non-optimized process (performed at high power) highlights the importance of reducing the crack driving force and controlling microstructural defects. Finite element simulations provided an effective means to quantify the susceptibility of coating design to failure by the various cracking modalities.