Study of the coating stability of a TBC system

Abstract

The degradation of thermal barrier coatings is closely linked to their spalling resistance which depends on the stability of the protective oxide scale produced by oxidation of the bond-coat. This study deals with a characterisation of the oxide layer, its interfaces with the bond-coat and the ceramic and their evolution during isothermal oxidation at 1,100°C. The microstructural evolution of the bond-coat, the outer part of the superalloy and the ceramic layer was also studied.     

Microstructural evolution and degradation modes in cyclic and isothermal oxidation of an EB-PVD thermal barrier coating

Abstract

Thermal barrier coatings constituted of a partially stabilised zirconia layer (～100 μm thick) deposited by EB-PVD on top of a platinum-modified aluminide bondcoat to protect a single crystal superalloy substrate, have been studied in cyclic and isothermal oxidation at 1,100°C in air. Close examination by high resolution SEM (with field emission gun) of the oxide scale and of the metallic surfaces after various durations, up to 600 hours at 1,100°C, show several morphological features which may affect the integrity of the systems: partial decohesion regions with large grain areas at the inner surface of the scale, subsequently replaced by a cellular morphology, cavities within the oxide scale as well as at the zirconia–alumina interface. The nature and distribution of these features depend on the type of test (isothermal or cyclic oxidation) and on the systems studied (standard superalloy or modified to lower the sulphur content). Scenarios are proposed to explain the observations reported.     

Boundary element prediction of TBC fracture resistance

The present study investigates the transient behaviour of interfacial cracks in thermal barrier coatings. It is assumed that a TBC withstands a thermal shock in the presence of external mechanical load acting on its surface. Crack closure takes place while the thermal contact resistance as well the friction between the crack faces is considered. The dependence of the thermal resistance on the contact pressure invokes coupling between the temperature and displacement fields. An appropriate boundary element procedure based on two‐dimensional time‐dependent thermoelasticity, is utilized for the numerical solution. A series of parametric analyses examines the impact of the coefficient of friction, thermal contact resistance and coefficient of heat convection on the crack severity.     

Analysis of interfacial cracks in a TBC/superalloy system under thermal loading

Thermal barrier coatings (TBCs) provide thermal insulation to high temperature superalloys. Residual stresses develop in TBCs during cool down from processing temperatures and subsequent thermal cyclic loading due to the thermal expansion mismatch between the different layers (substrate, bond coat, and TBC). These residual stresses can initiate microcracks at the bond coat/TBC interface and can lead to debonding at the bond coat/TBC interface. The highest residual stresses occur at the interfaces. The effect of voids or crack like flaws at the interface can be responsible for initiating debonding and accelerate the oxidation process. The effect of interfacial microcracks has been investigated using the fracture mechanics approach. In particular, J-integral and the energy release rate G, for both mode I and mode II using the virtual crack extension method were evaluated. Two types of specimens were studied. The specimens were cooled down from processing temperature of 1000°C to 0°C. The variation of the properties as a function of temperature were used for the analysis. It was found that the use of temperature dependent properties in contrast to constant properties provide significantly different values of J-integral and G. For the stepped-disc specimen with an edge crack, crack growth is only due to mode II, while for the cylinder specimen with an internal crack, crack growth is due to mixed-mode loading. An important implication of this result is that edge delaminations in a disk specimen may only grow due to mode II conditions under pure thermal loading. Shear fracture characteristics of interfacial crack thus become important in the failure of the TBC.     

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.     

由多孔型阳极氧化铝制备纳米氧化铝纤维

利用磷酸溶液浸渍具有独特的六边形结构和组成的多孔型阳极氧化铝(PAAO), 获得了带状、棒状、管状等不同形貌的纳米氧化铝纤维. 用扫描电镜(SEM)、能谱仪和透射电镜(TEM)等手段对其形貌和组成进行了分析. 结果表明, 纳米氧化铝纤维是在阳极氧化铝的多孔层形成的, 且在浸渍过程中阻挡层和多孔层表现出完全不同的溶解趋势. PAAO孔壁的特殊结构和组成上的差异造成的择优溶解是不同形貌纳米氧化铝纤维形成的主要原因.     

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.     

Laser Modification of Plasma Sprayed Alumina Coating

An alumina coating was developed on mildcarbon steel by plasma spraying and then modifiedby laser remelting.Some characteristics of plasmasprayed ceramic coating before and after laserremelting,such as microstructure,wear resistance,heat shock resistance,hot corrosion property andbond strength,have been investigated in this paper.     

低压等离子喷涂氧化铝涂层的特性

以细小的氧化铝为热喷涂粉末,采用低压等离子喷涂制备了沉积率高于50%,孔隙率低于2%的氧化铝涂层.研究了不同工艺下低压等离子喷涂氧化铝涂层的沉积率、相组成和显微结构,并对低压等离子功率和真空室压力工艺参数对涂层的影响进行了分析.研究结果表明,所制备的涂层以α-Al₂O₃和γ-Al₂O₃相并存;随着功率和压力提高,涂层的孔隙率有明显的降低,但压力达到23.7kPa时功率影响较小.此外,还对等离子焰流中的粒子温度和速度进行了计算.结果表明,在23.7kPa压力下保证粒子充分熔融的前提下使粒子具有较高的运动速度.     

钛表面阴极微弧电沉积制备氧化铝涂层

采用阴极微弧电沉积在钛表面生成了厚度达100μm的氧化铝涂层,研究了不同电压下涂层的结构和组成,分析了涂层的生长规律和形成过程.结果表明：阴极微弧电沉积过程包括火花前、微弧和局部弧光三个阶段,期间伴随有Al（NO3）3的离解、Al（OH）3的沉积与高温烧结等反应,微弧区产生的高温高压是形成Al2O3涂层的关键.涂层主要由γ-Al2O3和α-Al2O3组成,随电压升高,α-Al2O3的含量逐渐增加,在400V时其含量达76%.     

氧化镧掺杂氧化铝的高温物相转变

本文以碳酸钠和氯化铝为原料,采用沉淀法制备活性氧化铝,对氧化镧改性的氧化铝的高温相变过程进行了研究。运用差热分析法对样品进行热分析,运用XRD和TEM进行物相分析和形貌表征。结果表明:掺杂的镧能够有效的抑制氧化铝在升温过程中的相变,同时能够抑制氧化铝粉体的团聚,保持其高比表面积。     

热障涂层（TBC）稳定性的研究EI

将ＺｒＯ_２·８％Ｙ_２Ｏ_３用射频磁控溅射技术沉积在ＮｉＣｒＡｌＹ底涂层上，进行ｌｌ００℃→室温，１１００℃→冷水和９００℃→室温等热周期和熔盐作用的试验。随后进行Ｘ－射线衍射（ＸＲＤ）分析和扫描电镜（８ＥＭ）观察。结果表明，沉积态的氧化锆层主要为立方相和少量单斜相及四方相。热周期及熔盐Ｎａ_２ＳＯ_４－５％ＮａＣｌ与ＺｒＯ_２·８％Ｙ_２Ｏ_３的化学作用都促使立方相→单斜相的转变。熔盐对晶界和底涂层的化学和物理的作用是影响涂层稳定性的另一因素。     

莫来石/氧化铝预应力涂层增强氧化铝的弯曲强度和抗热震性能

在陶瓷表面引入含压应力的涂层是一种有效的增强技术。本研究将氧化铝和石英粉混合浆料涂覆在预烧后的氧化铝坯体上,无压共烧原位合成了热膨胀系数较低的莫来石–氧化铝涂层。利用降温过程中涂层内形成的残余压应力实现了氧化铝陶瓷的预应力强化。结果表明：随着涂层中石英掺量增加,预应力氧化铝的强度出现先增大后减小的趋势;当涂层中掺入石英的质量分数为15%时,预应力增强效果最好,涂层与基体界面结合紧密,预应力氧化铝陶瓷的弯曲强度达到(549.44±27.2)MPa,比普通氧化铝的强度提高了37.19%;当涂层中掺入石英的质量分数增大到15%以上,由于烧结收缩不匹配反而引起强度下降;这种预应力增强效果会随着温度升高逐渐减弱,当测试温度达到并超过1000℃时,预应力氧化铝和普通氧化铝会具有大致相等的抗弯强度。由于表层压应力的存在,预应力氧化铝还展现出更好的抗热震性能和损伤耐受性。     

氧化铝气凝胶的合成与性能

氧化铝气凝胶具有大比表面积、高孔隙率以及比氧化硅气凝胶更优异的热稳定性, 在高温隔热和催化领域有广阔的应用前景, 但其复杂的合成工艺和较低的强度限制了其应用。本文首先介绍氧化铝气凝胶合成工艺, 指出溶胶-凝胶过程中存在的问题, 不同干燥方式的特点以及最新发展的干燥工艺; 然后介绍了近年来氧化铝气凝胶在比表面积、热稳定性、强度、导热系数等性能改善方面所取得的代表性研究成果; 最后介绍了氧化铝气凝胶在高温催化、隔热领域的应用, 并展望了未来氧化铝气凝胶的发展方向以及应关注的问题。     

TGO对热障涂层失效的作用分析

热生长氧化物(TGO)的形成与长大是热障涂层失效的根本原因。先在IC10高温合金基体上超音速火焰喷涂(HVOF)NiCoCrAlTaY粘结层(BC层),再等离子喷涂二元稀土氧化物稳定氧化锆Sc2O3-Y2O3-ZrO2,喷涂样在1 100℃恒温氧化,利用扫描电镜(SEM)、能谱仪对断面形貌、成分进行分析,讨论了TGO的形成机理及其与热障涂层失效的关系。结果表明:随着恒温氧化时间增加,TGO层底部的Al含量下降,上部、中间弥散颗粒及底部的Ni含量均增加,上部、中间弥散颗粒中Cr含量均减少;喷涂样氧化140 h后,TGO层由靠近陶瓷层的富(Cr,Al)2O3层、弥散其间的富Ni颗粒和靠近BC层的Al2O3层组成;TGO的生长速度先由Al与O2化学反应速度决定,接着受BC层金属元素扩散速度影响,最后由化学反应速度和扩散速度共同控制;减少TGO中的有害氧化物含量以降低涂层内的应力,可有效提高涂层的使用寿命。     

The effect of gas composition and contaminants on scale growth rates of FeCrAl alloys

Abstract

Fe–20Cr–5Al alloys were exposed to several environments crucial for industrial applications. Experiments in a simulated fuel-rich exhaust gas, in combustion gas, in air with additions of SO₂ and HCl, and a N₂–NO mixture were compared to the oxidation kinetics in air.

While the addition of HCl and SO₂ enhances oxide growth via increased spallation, the N₂–NO mixture and the simulated exhaust gas seem to act as a shield gas due to the reduced oxygen partial pressure. As the local geometry can also change the gas composition during oxidation, e.g. inside crevices, results from an experiment with a deep bore are presented, simulating such a situation.     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

The influence of laboratory test procedures on scale growth kinetics and microstructure during steam oxidation testing

Abstract

Laboratory exposures cannot reproduce all the features present in service conditions. The experimentalist is faced with the conflict between increasing the complexity of laboratory tests to replicate service more closely and keeping testing costs low by maintaining a simple procedure. The influence of various experimental parameters, which can be controlled in the laboratory, on the steam oxidation response of materials is reviewed and recommendations for best practice are proposed.     

单相双稀土改性SrZrO3热障涂层的热物理性能

由于SrO和ZrO₂的蒸气压不同, 造成等离子喷涂SrZrO₃涂层组分偏离原始粉末化学计量比, 从而导致制备态涂层中出现第二相ZrO₂。为了获得高相稳定性的单相涂层, 实验采用固相合成法合成并经过喷雾造粒制备了双稀土改性Sr过量SrZrO₃(Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05)热喷涂粉末, 采用大气等离子喷涂方法制备了相应的涂层, 研究了单相双稀土改性SrZrO₃热障涂层的热物理性能及其热循环寿命。研究结果表明, 制备态Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05涂层中无第二相产生, 1600 ℃热处理360 h后Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05保持单相SrZrO₃结构, 高温相稳定性良好。Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05涂层的烧结系数为7.27×10 ^-6 s ^-1, 热处理360 h后该涂层的热膨胀系数为(9.0~11.0)×10 ^-6 K ^-1 (200~1400 ℃), 热导率为2.83 W/(m?K) (1000 ℃)。Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05/YSZ双层涂层的火焰循环次数为1000次, 失效区域主要发生在Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05陶瓷层内。在喷涂粉末中增加SrO的含量能够弥补在大气等离子喷涂过程中Sr元素过量挥发的问题, 成功制备了单相双稀土改性Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05热障涂层。双稀土掺杂能够明显提高涂层的热膨胀系数, 且单相双稀土改性Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05涂层的抗烧结性能明显优于SrZrO₃涂层, 但单相Sr_1.1(Zr_0.9Yb_0.05Gd_0.05)O_3.05涂层热导率比含有第二相的SrZrO₃涂层高。     

Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders

Abstract

The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O₂ and Ar-4%H₂-2%H₂O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H₂–H₂O than in Ar–O₂. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H₂/H₂O containing gas.