Degradation of thermal barrier coatings due to thermal cycling up to 1150°C

The degradation of thermal barrier coatings (TBCs) due to thermal cycling up to 1150°C has been studied. During thermal cycling, the bond coat in the TBCs was oxidised to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and the bond coat of MCrAlY alloy. The mixed oxide layer mainly consists of -Cr₂O₃ and (Ni,Co)(Cr,Al)₂O₄ spinel phases, which are formed above the -alumina layer. Interestingly, the alumina layer gradually disappeared during the oxidation while the content of chromium in the mixed oxide increased with increasing oxidation time. As the oxidation accelerated after the disappearance of the alumina layer, cracks initiated and propagated in the mixed oxide layer near the YSZ. Eventually, the crack propagation induced the spallation of some YSZ top coatings after the 2000 h oxidation.     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

High temperature oxidation interfacial growth kinetics in YSZ thermal barrier coatings with bond coatings of NiCoCrAlY with 0.25% Hf

The results of an experimental study of the high-temperature isothermal oxidation behavior and microstructural evolution in two variations of air plasma sprayed ceramic thermal barrier coatings (TBCs) are discussed in the paper. Two types of TBC specimens were produced for testing. These include a standard and vertically cracked APS. High temperature oxidation was carried out at 900, 1000, 1100 and 1200 °C. The experiments were performed in air under isothermal conditions. At each temperature, the specimens were exposed for 25, 50, 75 and 100 h. The corresponding microstructures and microchemistries of the TBC layers were examined using scanning electron microscopy and energy dispersive X-ray spectroscopy. Changes in the dimensions of the thermally grown oxide layer were determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide layers were compared in the TBC specimens with standard and vertically cracked microstructures.     

氧化锆陶瓷四方相以及晶界热导率的定量计算

氧化钇部分稳定的氧化锆(YSZ)涂层是应用广泛的热障涂层材料。为了更好地研究各种因素对热障涂层热导率的影响, 使用压制烧结的方法制备基本致密的氧化锆陶瓷, 研究相组成和晶粒大小对热导率的定量影响。在不同的烧成制度下制备出不同晶粒大小的氧化锆陶瓷。用电子背散射衍射(EBSD)图像研究氧化锆陶瓷材料的相组成以及晶界的分布情况。综合有限元模拟的方法以及傅立叶传导方程, 计算出四方相和晶界的热导率分别为2.65 W/(m·K)和1.54 W/(m·K)。研究表明, 四方相的热导率比氧化锆陶瓷的热导率高, 而晶界的热导率比氧化锆陶瓷的低。     

Relation of microstructural and compositional features to the electrical properties in degraded thermal barrier coating systems

Degraded thermal barrier coating samples cut from different after-service gas turbine components are examined by both electron microscopy and impedance spectroscopy. There is a relationship between the microstructural and compositional features of the thermally grown oxide (TGO) and its electrical properties. The resistance of the TGO decreases with the TGO evolution from alumina to porous mixed oxides composed probably of NiO, spinel, Cr₂O₃, and Al₂O₃, while the relaxation frequency corresponding to the TGO increases. For seriously degraded TBCs, there is an additional semicircle in the impedance spectra in the extremely low frequency range, possibly arising from cracking in the vicinity of YSZ-TGO interface regions.     

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.     

Degradation of Thermal Barrier Coated Superalloy Component During Service

The time-dependent degradation of first-stage vanes for gas turbines was investigated. The specimens were prepared from the operating facilities in the Ulsan combined-cycle plant. A microstructural investigation of the thermal barrier coatings (TBCs) showed that spallation was occurring at inner root area of vanes during operation. Microstructural comparison of coating surfaces showed that exposure to the environment increased the thickness of thermally grown oxide (TGO) and that the β-depletion region also increased with operation time. After spallation of the top coat, mechanical thinning and the development of fine grains by recrystallization at the bond coat also occurred. The substrates of welded and normal regions were compared and showed that the difference in aluminum diffusion rate and crack density resulted from chemical and mechanical differences in the two areas.     

Einfluss einer PVD‐Al‐Zwischenschicht auf die Eigenschaften eines thermisch gespritzten Wärmedämmschichtsystems nach Temperaturwechselbelastung

Thermal barrier coatings (TBC) generally consist of a metallic bond coat (BC) and a ceramic top coat (TC). Co–Ni–Cr–Al–Y metallic super alloys and Yttria stabilised zirconia (YSZ) have been widely used as bond coat and top coat for thermal barrier coatings systems, respectively. As a result of long‐term exposure of thermal barrier coatings systems to oxygen‐containing atmospheres at high temperatures, a diffusion of oxygen through the porous ceramic layer occurs and consequently an oxidation zone is formed in the interface between ceramic top coat and metallic bond coat. Alloying components of the BC layer create a so‐called thermally grown oxides layer (TGO). One included oxide type is α‐Al₂O₃. α‐Al₂O₃ lowers oxygen diffusion and thus slows down the oxidation process of the bond coat and consequently affects the service life of the coating system positively. The distribution of the alloying elements in the bond coat layer, however, generally causes the formation of mixed oxide phases. The different oxide phases have different growth rates, which cause local stresses, micro‐cracking and, finally, delamination and failure of the ceramic top coat layer. In the present study, a thin Al inter‐layer was deposited by DC‐Magnetron Sputtering on top of the Co–Ni–Cr–Al–Y metallic bond coat, followed by thermal spraying of yttria‐stabilised zirconia (YSZ) as a top coat layer. The deposited Al inter‐layer is meant to transform under operating conditions into a closed layer with high share of α‐Al₂O₃ that slows down the growth rate of the resulting thermally grown oxides layer. Surface morphology and microstructure characteristics as well as thermal cycling behaviour were investigated to study the effect of the intermediate Al layer on the oxidation of the bond coat compared to standard system. The system with Al inter‐layer shows a smaller thermally grown oxides layer thickness compared to standard system after thermal cycling under same conditions.     

Isothermal oxidation behaviour of EB-PVD MCrAlY bond coat

The isothermal oxidation behaviour of EB-PVD NiCoCrAlY bond coat on nickel based superalloy at different treatment conditions were studied at 1100 °C up to 100 h. The reaction products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS). It is found that after 10 h oxidation, while a protective alumina layer was developed on pre-oxidized specimens, a mixed oxide layer was observed on non-pre-oxidized specimens. Only after 50 h, spallation was observed on pre-oxidized specimens but did occur after 10 h for a non-pre-oxidized one, in both cases it is believed that the diffusion of the substrate element Ti to the surface of the bond coat deteriorates the adhesion of protective oxide to the bond coat and hence leads to spallation. Oxide rich in Ti was found to be located on the top of column grains rather than in the column grain boundaries.     

Adhesion of thermal barrier coatings – the development of metastable alumina

Abstract

The adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC and the corrosion resistant bond coat. Work has been carried out to investigate the properties of the TGO as a function of ageing treatments using piezospectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC which showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with a alumina (R1 and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable θ alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.     

Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

Thermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

Investigation on the thermo-chemical reaction mechanism between yttria-stabilized zirconia (YSZ) and calcium--magnesium--alumino-silicate (CMAS)

Thermal barrier coatings (TBCs) with Y₂O₃-stabilized ZrO₂ (YSZ) top coat play a very important role in advanced turbine blades by considerably increasing the engine efficiency and improving the performance of highly loaded blades. However, at high temperatures, environment factors result in the failure of TBCs. The influence of calcium--magnesium--alumino-silicate (CMAS) is one of environment factors. Although thermo-physical effect is being paid attention to, the thermo-chemical reaction becomes the hot-spot in the research area of TBCs affected by CMAS. In this paper, traditional two-layered structured TBCs were prepared by electron beam physical vapor deposition (EB-PVD) as the object of study. TBCs coated with CMAS were heated at 1240°C for 3 h. Additionally, 15 wt.% simulated molten CMAS powder and YSZ powder were mixed and heated at 1240°C or 1350°C for 48 h. SEM and EDS were adopted to detect morphology and elements distribution. According to XRD and TEM results, it was revealed that CMAS react with YSZ at high temperature and form ZrSiO₄, Ca_0.2Zr_0.8O_1.8 and Ca_0.15Zr_0.85O_1.85 after reaction, as a result, leading to the failure of TBCs and decreasing the TBC lifetime.     

Thermally grown oxide scales on γ-TiAl coated with thermal protection systems

Abstract

Thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on gamma TiAl samples using electron-beam physical vapour deposition. The specimens were coated with intermetallic Ti –Al – Cr layers and CrAlYN/CrN nanoscale multilayer coatings. The lifetime of the TBC systems was determined performing cyclic oxidation tests in air at temperatures between 850 and 950–C. The TBC systems with Ti –Al – Cr and CrAlYN/CrN layers did not fail at 850 and 900–C during the maximum exposure time period of 1000 cycles of 1 h dwell time at high temperature. No spallation of the thermal barrier coatings was observed. As revealed by post-oxidation microstructural analysis, the protective coatings were severely degraded when exposed at 900–C, resulting in growth of mixed oxides on the substrate. Underneath the thermal barrier coating an outer oxide scale with a columnar structure was observed, consisting of rutile and α-Al₂O₃. Energy-dispersive X-ray spectroscopy analysis revealed zirconia and chromia being dissolved in the outer oxide scale. The columnar structure and the presence of zirconia indicated an effect of the TBC on the morphology of the outer oxide scale. The zirconia top coat exhibited an excellent adherence to this oxide scale formed on the protective layers when degraded, and at defects like cracks. When thermally cycled at 950–C, the TBC system on specimens coated with Ti –Al – Cr failed by spallation of the thermally grown mixed oxides, whereas the thermal barrier coating was well adherent to the outer oxide scale at this temperature, too.     

Evidence for Cr-carbide formation at the scale/metal interface during oxidation of FeCrAl alloys

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm, respectively) were investigated in respect of their oxidation behaviour at 1200 °C in air. Both alloys exhibited formation of grain boundary Cr-carbides after 1200 °C exposure, whereas the high-carbon content alloy additionally showed carbide formation at the interface between alloy and alumina surface scale. The extent of the carbides formed at both this interface and the alloy grain boundaries was dependent on the cooling rates of the oxidized samples. The presence of the carbides may decrease the adherence of the oxide scale to the metallic substrate and explain the decreased time to breakaway oxidation of the high-carbon content alloy.     

燃气热循环下7YSZ热障涂层的微结构演变与阻抗谱特征

在1250℃燃气热循环条件下,测试热障涂层抗冷热冲击性能,以模拟发动机叶片的启动升温与关闭降温循环过程。采用电化学阻抗谱测试和扫描电镜(SEM)系统研究热循环过程中热生长氧化物(TGO)生长与YSZ陶瓷层微结构演变。结果表明:随着热循环次数增加,热障涂层内TGO不断生长变厚,在中频阶段的阻抗谱响应越来越显著。YSZ陶瓷层内部经历了微裂纹的萌生与扩展两个阶段。经过100次热循环后的YSZ层表现出与喷涂态涂层相似的阻抗特征,表明高温下烧结会使YSZ层产生的微裂纹在短时间内愈合。但经过300次热循环后的YSZ层表现出与喷涂态完全不同的阻抗谱,并随热循环次数增加,YSZ颗粒间隙阻抗值不断增加,表明YSZ内层产生了不可愈合的微裂纹,是导致YSZ层最终失效的主要因素。     

La2(Zr0.7Ce0.3)2O7——新型高温热障涂层

采用电子束物理气相沉积技术(EB-PVD)制备了新型La2(Zr0.7Ce0.3)2O7 (LZ7C3)热障涂层.研究了涂层的组分、显微结构、表面和横截面形貌以及恒温氧化行为.结果表明:涂层中La2O3/ZrO2/CeO2的相对含量偏离了化学计量比,但X 射线衍射(XRD)相结构与靶材非常相似.通过CeO2 掺杂后,LZ7C3体材料的热膨胀系数比La2Zr2O7 (LZ)大;在1100℃恒温氧化890h的条件下,LZ7C3涂层的抗氧化增重性能明显优于传统的Y2O3部分稳定化的ZrO2(8YSZ)涂层.此外,热膨胀不匹配、黏结层氧化和陶瓷涂层内部微观裂纹的出现可能是导致LZ7C3涂层恒温氧化失效的主要原因.     

粗糙度对热障涂层冲蚀失效的影响及模型建立

采用气体喷砂试验机研究了大气等离子喷涂(APS) ZrO₂-7%Y₂O₃(7YSZ)热障涂层的冲蚀失效机理, 分析了陶瓷层表面粗糙度对热障涂层冲蚀磨损率及失效行为的影响, 研究了基于多孔层状的热障涂层在常温高速粒子90°攻角下的冲蚀失效特征, 探讨了多孔层状结构对涂层冲蚀失效演变机制的作用, 此外, 建立了基于粗糙度的冲蚀失效数学模型。研究结果表明: 冲蚀磨损率与陶瓷层表面粗糙度呈线性递增关系; 涂层在高速粒子冲蚀下多孔层状结构加剧了涂层的冲蚀失效; 涂层表面在粒子冲蚀下承受着非均匀、非连续的压-压脉动循环载荷, 在微凸粗糙表面承载着正应力和切应力的相互作用; 正应力迫使涂层出现凹坑, 切应力易导致涂层出现沟槽, 并同时以原喷涂态表面网状裂纹为策源地诱发裂纹在粒子晶界和层间界面萌生扩展导致涂层产生粒子和片状剥落, 最终涂层显示出典型的磨粒磨损和低周疲劳失效形式。     

On the opening of a class of fatigue cracks due to thermo-mechanical fatigue testing of thermal barrier coatings

The evolution of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to an accelerated test scheme is investigated via numerical simulations. The TBC system consists of a NiCoCrAlY bond coat and partially yttria stabilized zirconia top coat with a thermally grown oxide (TGO) between these two coatings. The cracks of interest evolve in the bond coat parallel and near the interface with the TGO during thermo-mechanical fatigue testing. In their final stage, the cracks lead to partial spallation of the TBC. This study focuses on why the cracks open to their characteristic shape. To this end, finite element simulations are utilized. The crack surface separation is monitored for a range of material properties and oxidation rates. The simulations show that the inelastic response of the bond coat and the oxidation rate of the TGO govern the crack surface separation. Most interestingly, permanent separation of the crack surfaces is caused by a structural ratcheting in the vicinity of the crack.     

High spatial resolution imaging of the segregation of reactive elements to oxide grain boundaries in alumina scales

Abstract

Although it is well established that reactive elements such as yttrium and hafnium can segregate to oxide/metal interfaces and oxide grain boundaries in thermally grown oxides, their distribution and role at these sites are less certain. For example, their effect on oxide growth, scale plasticity and spallation is still debated. It has also been reported that hafnium and yttrium rich oxide particles can be present within growing alumina scales and that the growth or shrinkage of these particles can affect the Y and Hf distributions in the aluminium oxide grain boundaries in their vicinity. Hence, we now report the use of very high spatial resolution imaging in the SuperSTEM electron microscope to investigate the distribution of Y and Hf in aluminium oxide grain boundaries at the atomic level.

The oxide scales studied were detached from Fe – 20Cr – 5Al alloy substrates doped with Y and Hf, which had been oxidised for up to 100 h at 1250°C in laboratory air. The scales were ion beam thinned prior to examination in the STEM, and a series of tilting experiments and through focal series were used to map out the distributions of the reactive elements. The influence of electron beam/sample interactions was also studied and some evidence for the movement of Hf and Y atoms along the grain boundaries to the surfaces of the thin oxide foils is also reported.