Neodymium-cerium oxide as new thermal barrier coating material

Neodymium-cerium oxide (Nd2Ce2O7) was proposed as a new thermal barrier coating material in this work. Monolithic Nd2Ce2O7 powder was prepared by the solid-state reaction at 1400 degrees C. The phase composition, thermal stability and thermophysical properties of Nd2Ce2O7 were investigated. Nd2Ce2O7 with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd2Ce2O7 was higher than that of YSZ (6-8 Wt-% Y2O3 + ZrO2) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd2Ce2O7 is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd2Ce2O7 was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.

A New Analytical Model for Residual Stresses due to Thermal Surface Coating

发展了一种新的分析涂层结构（平板、梁）热残余应力的模型,可以研究骤冷过程(Quenching)和冷却过程(Cooling)在涂层结构内引发的残余应力分布。与以往模型相比,其优势在于：它可以考虑源于喷涂过程的涂层孔隙率、温度梯度等因素对于涂层和基底内残余应力的影响。其中孔隙率和温度分布由计算机模拟涂层沉积过程得到。另外,当基底的材料和几何参数被固定时,我们分析了诸如涂层的理想模量、厚度、热膨胀系数等参数,对于涂层结构中最终残余应力分布的改变机理。

Interface delamination of the thermal barrier coating subjected to local heating

To investigate the possible failure modes of the thermal barrier coating (TBC) used to protect the scramjet combustion chamber, the local heating via laser beam irradiation was utilized to simulate the service condition of high thermal flux and high temperature gradient. Firstly, the experimental method and process were described and the typical fracture morphology of the TBC under test were provided. Then, the theoretical and finite element modeling were carried out to study the temperature, deformation and stresses of the specimen when the top ceramic coat was subjected to local heating, and to demonstrate the mechanism on the failure of the TBC. It is revealed that the interface delamination shall appear and ultimately lead to the failure of the TBC under such thermal loading of local quick heating. According to the outcome of this study, the driving force of the interface delamination is influenced greatly by the key structural parameters and performance matching. Moreover, by utilizing the rules of the effects of these parameters on the fracture driving force, there is some possibility for the designer to optimize the performances of the TBC.

Thermal barrier coating of lanthanum-zirconium-cerium composite oxide made by electron beam-physical vapor deposition

Lanthanum-zirconium-cerium composite oxide (La-2(Zr0.7Ce0.3)(2)O-7, LZ7C3) as a candidate material for thermal barrier coatings (TBCs) was prepared by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, thermophysical properties, surface and cross-sectional morphologies and cyclic oxidation behavior of the LZ7C3 coating were studied. The results indicated that LZ7C3 has a high phase stability between 298 K and 1573 K, and its linear thermal expansion coefficient (TEC) is similar to that of zirconia containing 8 wt% yttria (8YSZ). The thermal conductivity of LZ7C3 is 0.87 W m(-1) K-1 at 1273 K, which is almost 60% lower than that of 8YSZ. The deviation of coating composition from the ingot can be overcome by the addition of excess CeO2 and ZrO2 during ingot preparation or by adjusting the process parameters.

Application of rare earths in thermal barrier coating materials

Rare earths are a series of minerals with special properties that make them essential for applications including miniaturized electronics, computer hard disks, display panels, missile guidance, pollution controlling catalysts, H-2-storage and other advanced materials. The use of thermal barrier coatings (TBCs) has the potential to extend the working temperature and the life of a gas turbine by providing a layer of thermal insulation between the metallic substrate and the hot gas. Yttria (Y2O3), as one of the most important rare earth oxides, has already been used in the typical TBC material YSZ (yttria stabilized zirconia). In the development of the TBC materials, especially in the latest ten years, rare earths have been found to be more and more important. All the new candidates of TBC materials contain a large quantity of rare earths, such as R2Zr2O7 (R=La, Ce, Nd, Gd), CeO2-YSZ, RMeAl11O19 (R=La, Nd; Me=Mg, Ca, Sr) and LaPO4. The concept of double-ceramic-layer coatings based on the rare earth materials and YSZ is effective for the improvement of the thermal shock life of TBCs at high temperature.

The thermal cycling behavior of Lanthanum-Cerium Oxide thermal barrier coating prepared by EB-PVD

Bulk material and coatings of Lanthanum-Cerium Oxide (La2Ce2O7) with a fluorite structure were studied as a candidate material for thermal barrier coating (TBC). It has been showed that such material has the properties of low thermal conductivity about four times lower than YSZ, the difference in the thermal expansion coefficient between La2Ce2O7 and bond coat is smaller than that of YSZ in TBC systems, high phase stability between room temperature and 1673 K, about 300 K higher than that of the YSZ. The coating prepared by electron beam physical vapor deposition (EB-PVD) showed that it has good thermal cycling behavior, implying that Such material can be a promising thermal barrier coating material. The deviation of coating composition from ingot can be overcome by the addition of excess La2O3 during ingot preparation and/or by adjusting the process parameters.

热喷涂涂层界面断裂韧性的反向三点弯曲法试验

以热喷涂NiCrBSi涂层/钢基体为材料模型,利用将涂层置于受压侧的反向三点弯曲法试验,对热喷涂涂层界面断裂现象进行分析并建立相应的界面断裂韧性计算模型.结果表明,界面裂纹起始于三点弯曲试样中部,对应于加载压头与涂层接触区域正下方的界面上,并向两侧扩展,伴随涂层屈曲,形成分层屈曲的破坏形貌.根据分层屈曲形貌建立计算模型,通过分层屈曲几何参数与屈曲临界应力、涂层内真实应力以及界面断裂韧性的关系,获得界面断裂韧性值.

FEM Simulations and Optimization about Residual Stresses in Coating Structures with Functionally Graded Materials Layer

An elasto-plastic finite element method is developed to predict the residual stresses of thermal spraying coatings with functionally graded material layer. In numerical simulations, temperature sensitivity of various material constants is included and mix

Partial Evaporation of Strontium Zirconate During Atmospheric Plasma Spraying

Perovskite-type SrZrO3 has been investigated as a candidate material for thermal barrier coating application. During plasma spraying of SrZrO3, SrO volatilized more than ZrO2 and the coating composition deviates from initial stoichiometry. In this investigation, partial evaporation was investigated by spraying SrZrO3 powders into water. The influences of spraying current, distance and particle size of the powder on the partial evaporation were also investigated in a quantitative way. With optimized spraying parameters, stoichiometric SrZrO3 coating was produced by adding an excess amount of Sr in the precursors before plasma spraying to compensate for the volatilized component.

Effect of Process Conditions on Microstructure and Corrosion Resistance of Cold-Sprayed Ti Coatings

Ti and Ti alloys can be applied to steels as a protective coating in view of its excellent resistance to corrosive environment. Cold spraying, as a new coating technique, has potential advantages in fabrication of Ti coating in comparison with conventional thermal spraying techniques. In this study, Ti coatings were prepared on carbon steel substrates by cold spraying via controlling the process conditions. The microstructure of coatings was observed by SEM. The porosity of coatings was estimated by image analysis and the bond strength was tested for comparison of the process conditions. Potentiodynamic polarization and open-circuit potential (OCP) measurements were performed to understand the corrosion behavior of the coatings. The SEM examination shows that the coatings become more compact with the increases of pressure and temperature of driving gas. The potentiodynamic polarization curves indicate that the coating which has lower porosity has lower corrosion current. The polarization and OCP measurement reveal that cold-sprayed Ti coating can provide favorable protection to carbon steel substrate. The polishing treatment of coating surface polishes the rough outer layer including the small pores as well as decreases the actual surface area of the coating, leading to the considerable improvement of corrosion resistance.

What Do We Know about Long Laminar Plasma Jets?

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

热喷涂构件中残余应力的理论分析

本文发展一种新的分析涂层结构（平板、梁）热残余应力的模型，可以研究骤冷过程（Quenching）和冷却过程（Cooling）在涂层结构内引发的残余应力分布。与以往模型相比，其优势在于：它可以考虑源于喷涂过程的涂层孔隙率、温度梯度等因素对于涂层和基底内残余应力的影响。其中孔隙率和温度分布由计算机模拟涂层沉积过程得到。另外，当基底的材料和几何参数被固定时，我们分析了诸如涂层的理想模量、厚度、热膨胀系数等参数，对于涂层结构中最终残余应力分布的改变机理。

激光辐照热障涂层中平面应变问题的热弹性变分分析

针对激光辐照热障涂层材料的平面应变问题,提出热障涂层热弹性分析的基本方程,对定常温度场给出级数形式解析解,并用最小余能原理和变分法分析了结构的热弹性应力场,研究了最大应力和界面应力的分布特征,并就一些物理参数的影响进行了讨论.结果表明,热障涂层的主要破坏因素为表面拉伸应力,界面应力相对较小,但在自由边界有集中现象,剥落应力大于剪切应力,是导致涂层破坏的重要原因.涂层厚度增加会改变厚度方向上的应力分布,界面应力向中心集中.

What Do We Know about Long Laminar Plasma Jets?

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.