Residual stress distribution along interfaces in thermal barrier coating system under thermal cycles

The residual interfacial stress plays an important role in crack initiating and propagating along the interface, which could result in delamination failure of the thermal barrier coatings (TBCs). In this study, the finite element model of air plasma spraying(APS) TBCs was established to assess the level and distribution of residual stress along top coat(TC)/thermally grown oxide (TGO) and bond coat (BC)/TGO interfaces under thermal cycles. Instead of using vertical stress S₂₂ in global coordinate system, the normal and tangential components in the local system along the interfaces, transformed from stress components S₁₁, S₂₂, and S₁₂ in the global one, were used to evaluate the way the cracks initiate and propagate along the interfaces. Firstly, the effect of the number of thermal cycles on residual stress was investigated. It was found that, for the TBCs model without TGO growth and crack, the impact of the number of thermal cycles on the stress is very insignificant and could be ignored. So the present study only chose to focus on the first thermal cycle. Then the influence of the TGO thickness and the interface amplitude on the normal and tangential residual stresses for both homogeneous and inhomogeneous temperature fields was explored. The results show that the TGO thickness, interface amplitude and temperature field affect the residual stress level and distribution, leading to different fracture mechanisms along TC/TGO and TGO/BC interfaces. Finally, the difference between the vertical stress in the global coordinate system and the normal stress in the local coordinate system was studied. Compared with vertical stress S₂₂, the stress components normal and tangential to the TC/TGO and TGO/BC interfaces are more appropriate to describing the stress distribution along the interfaces and predicting the propensity of crack initiating and propagating along the interfaces.     

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.     

Polymer/clay nanocomposites as VOC barrier materials and coatings

The transport properties of select volatile organic compounds were measured in polyurethane/clay nanocomposite barrier membranes as a function of clay content. The nanocomposites were fabricated by two different processing methods involving stirring and sonication of the clay particles. The concentration of Cloisite^® 30B in the nanocomposite was varied from 0 to 50 wt%. Characterization of membrane transport properties was achieved via a gravimetric sorption method. Material-phase diffusivity coefficients (D) decreased with increasing Cloisite^® concentration, while changes in the material/VOC partition coefficients (K) depended on the molecular interactions of the VOCs with the membrane material.     

Lithium ion diffusion in Li4+xTi5O12: From ab initio studies

Lithium ion dynamics in Li_4+xTi₅O₁₂ spinel are investigated from first principles calculations. The diffusion pathways are optimized and the energy barriers of lithium migration under four types of dilute defect extremes: Li_4+δTi₅O₁₂, Li_4−δTi₅O₁₂, Li_7+δTi₅O₁₂ and Li_7−δTi₅O₁₂ (δ ? 1) are calculated with the nudged elastic band method. Results show that lithium diffusion in the charged state (energy barriers are 1.0 and 0.7 eV for interstitial Li and Li vacancy diffusion, respectively) is much slower than in the discharged state (energy barriers are 0.13 and 0.35 eV for interstitial Li and Li vacancy diffusion, respectively). The diffusion coefficients are evaluated based on lattice gas model and hopping mechanism. The obtained results are compared with available experimental data within a two-phase co-existence framework.     

Effect of substrate temperature on the microstructure and properties of thick plasma-sprayed YSZ TBCs

Thick (∼1.2 mm) thermal barrier coatings (TBCs) consisting of YSZ were deposited by plasma spraying. Spraying parameters were varied in a controlled manner to produce different microstructures. The effect of substrate temperature on the microstructural features and subsequently on the Young's modulus was investigated. In addition, the residual stresses in the coatings were estimated using a numerical model and they were related to the microstructural features observed. Results showed that crack segmentation density, residual stresses in the coatings and thus coating properties are strongly affected not only by the average substrate temperature during spraying but also the variations between the minimum and maximum substrate temperature.     

Influence of residual solvent on the gas transport properties of dense Hyflon® AD 60X gas separation membranes

The solvent retention by dense Hyflon AD 60X membranes were studied. The effect of the residual solvent on the transport properties of the membranes was evaluated. The permeability, diffusion coefficient and solubility of six permanent gases in membranes prepared under different conditions were determined and compared with the results of simulation studies.     

基体条件对Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力的影响

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力分布进行了数值仿真。结果表明:基体厚度不同时,涂层界面Sm2Zr2O7/YSZ及界面YSZ/NiCoCrAlY对应应力及应力梯度基本不变,表明应力及应力梯度与基体厚度无关;但基体材质热膨胀系数对涂层系统界面的径向、轴向及剪切应力梯度有决定性的影响,且各应力梯度随金属基体的热膨胀系数差异增加而增大,表明基体材质是影响涂层界面径向残余热应力及应力梯度的根本原因。采用多层陶瓷结构并合理选择各层材质的热膨胀系数将更加有利于降低涂层应力梯度,进而改善涂层性能,延长涂层寿命。     

Effects of post-deposition annealing ambient on band alignment of RF magnetron-sputtered Y2O3 film on gallium nitride

The effects of different post-deposition annealing ambients (oxygen, argon, forming gas (95% N₂ + 5% H₂), and nitrogen) on radio frequency magnetron-sputtered yttrium oxide (Y₂O₃) films on n-type gallium nitride (GaN) substrate were studied in this work. X-ray photoelectron spectroscopy was utilized to extract the bandgap of Y₂O₃ and interfacial layer as well as establishing the energy band alignment of Y₂O₃/interfacial layer/GaN structure. Three different structures of energy band alignment were obtained, and the change of band alignment influenced leakage current density-electrical breakdown field characteristics of the samples subjected to different post-deposition annealing ambients. Of these investigated samples, ability of the sample annealed in O₂ ambient to withstand the highest electric breakdown field (approximately 6.6 MV/cm) at 10⁻⁶ A/cm² was related to the largest conduction band offset of interfacial layer/GaN (3.77 eV) and barrier height (3.72 eV).     

Electroless copper on refractory and noble metal substrates with an ultra-thin plasma-assisted atomic layer deposited palladium layer

Electroless Cu was investigated on refractory metal, W and TaN_X, and Ir noble metal substrates with a plasma-assisted atomic layer deposited palladium layer for the potential back-end-of-the-line (BEOL) metallization of advanced integrated devices. The sodium and potassium-free Cu electroless bath consisted of: ethylenediamine tetraacetic acid (EDTA) as a chelating agent, glyoxylic acid as a reducing agent, and additional chemicals such as polyethylene glycol, 2,2′-dipyridine and RE-610 as surfactant, stabilizer and wetting agent respectively. The growth and chemical characterization of the Cu films was carried out with a field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). Group VIII metals such as Pt, Pd, etc., are stable in the electroless bath and catalytic towards the oxidation of glyoxylic acid and therefore work well for the electroless deposition of Cu. From RBS analysis, the amount of carbon and oxygen in Cu films were less than 1-3%. The Cu films were electroless deposited at 45-50 °C on patterned tantalum nitride with plasma-assisted atomic layer deposited (PA-ALD) Pd as a catalytic layer. Electroless Cu trench fill was successful with ultrasonic vibration, RE-610, and lowering the temperature to 45-50 °C on TaN_X with the PA-ALD Pd catalytic layer.     

带腐蚀缺陷采油管线剩余强度和剩余寿命的评价

为了评价某采油厂采油管道实际工况,基于ANSYS有限元法对管道腐蚀缺陷进行模拟,研究了不同缺陷类型下,缺陷长度、宽度、深度因素对管道失效的影响,分析了不同腐蚀类型缺陷对管道最大工作压力的影响。利用检测数据,对管道最大腐蚀速率进行了计算,并对管道剩余寿命进行了预测。结果表明:缺陷长度和深度是管道失效的直接影响因素,等效应力和工作压力成正比关系,管线的剩余寿命为10.2 a。此结果对管道的安全运行及维护具有重要的参考价值。     

Self-tightening of spider silk fibers induced by moisture

Spider dragline silk has a unique combination of desirable mechanical properties—low density, high tensile strength and large elongation until breaking—that makes it attractive from an engineering perspective [Nature 410 (2001) 541]. Nevertheless, this outstanding performance is threatened by the way mechanical properties are affected by a wet environment, particularly if the stress of these fibers can relax when exposed to moisture. Tests on spider dragline silk (Argiope trifasciata) performed by the authors have shown that when the fiber is clamped and exposed to a wet enough environment non-vanishing supercontraction forces develop. When the moisture is removed the residual stresses increase, and this effect has proven long lasting, as the fiber remains stressed for hours. In addition, the tensile properties of the fiber remain unaffected by the residual stresses build up after removing the moisture or after a wetting and drying cycle. These tests give support to the thesis that supercontraction helps to keep the spider webs tight and opens new applications for synthetic analogs.     

等离子喷涂Sm_2Zr_2O_7热障涂层残余热应力

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7热障涂层在金属基体材质类型、厚度、半径变化时涂层的残余热应力进行了分析。结果表明,金属基体的热膨胀系数对Sm2Zr2O7热障涂层的残余热应力有着显著影响,金属粘结层与表面陶瓷层界面处的残余应力及其应力梯度随着金属基体热膨胀系数增加而增大。金属基体厚度和半径不是影响Sm2Zr2O7热障涂层残余应力的主要因素。