EB-PVD热障涂层热循环过程中粘结层的氧化和相结构

采用磁控溅射方法在镍基单晶高温合金基体上沉积Ni-30Cr-12Al-0.3Y（质量分数，％）粘结层，采用电子束物理气相沉积方法（EB－VPD）沉积7％Y2O3（质量分数）－ZrO2陶瓷顶层，结果表明，在热循环过程中，非平衡相t′-ZrO2中的Y2O3含量逐渐减少，t′－ZrO2相逐渐分解成平衡相t-ZrO2（冷却时变转变成斜相）和立方组ZrO2,1050℃循环200次，粘结层氧化物（Al2O3)厚度约为3μm，表明Ni-Cr-Al-Y达宜作粘结层，继续热循环，陶瓷层中出现单斜阳，粘结层中Al贫化，氧化层中出现NiO及尖晶石等，引起应力集中，导致涂层失效。     

等离子喷涂ZrO2—NiCoCrAlY梯度热障涂层的抗热震性及失效机理

研究了ZrO2-NiCoCrAlY热障涂层的抗热震性和热震失效机理。实验结果表明,梯度热隙涂层能明显延缓热震裂纹的形成和扩展,具有较高的抗热震性。热震裂纹形成与扩展主要在粘结层与基体的界面处。随热循环次数的增加,热震裂纹可在表面陶瓷层内和陶瓷层与过渡层的界面处形成。实验表明热障涂层热震失效的过程主要是裂纹形成、扩展及涂层剥落,粘结层的氧化是导致涂层剥落失效的重要原因。     

Effect of Particle Size on the Micro-cracking of Plasma-Sprayed YSZ Coatings During Thermal Cycle Testing

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service or thermal cycle testing usually results from internal cracking in the top coat, erosion and CMAS (calcium-magnesium-alumina-silicate)-induced damage, etc. The microstructure of ceramic coatings affects their durability and other properties of TBCs. In the present study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. In addition, the effect of particle size on damage evolution in the top coat was investigated. It is found that the coatings deposited using coarse particles show the higher thermal cycle life. Crack length grew with increasing numbers of thermal cycles. The faster crack growth rate can be found for the coatings deposited from fine particles. The porosity of the coating made from the coarse powder is larger than the porosity of the coating made from fine powder both in the as-sprayed condition and after thermal cycling. The changes in crack growth rate and the porosity are related to the effect of sintering and stress evolution in coatings during the thermal cyclic tests.     

Thermal stability and mechanical properties of thick thermal barrier coatings with vertical type cracks

The thermal stability and failure mechanism of thick thermal barrier coatings (TBCs) with and without vertical type cracks were investigated through the cyclic thermal exposure and thermal-shock tests. The TBC systems with thickness of about 2000 µm in the top coat were prepared by an air plasma spray (APS) on the bond coat of about 150 µm in thickness prepared by APS. The adhesive strength values of the as-prepared TBCs with and without vertical type cracks were determined to be 24.7 and 11.0 MPa, respectively, indicating the better interface stability in the TBC with vertical type cracks. The TBC with vertical type cracks shows a better thermal durability than that without vertical type cracks in the thermal cyclic exposure and thermal-shock tests. The hardness values of the as-prepared TBCs with and without vertical type cracks were found to be 6.6 and 5.3 GPa, respectively, which were increased to 9.5 and 5.5 GPa, respectively, after the cyclic thermal exposure tests. These results indicate that the vertical type cracks developed in the top coat are important in improving the lifetime performance of thick TBC in high temperature environment.     

等离子喷涂热障涂层热震失效过程及残余应力分析

采用等离子喷涂技术在高温合金上制备了热障涂层(粘接层为NiCoCrAlY,陶瓷层为ZrO2-8%Y2O3),利用扫描电镜(SEM)、拉曼光谱(RFS)等试验手段研究了热障涂层热震失效的过程及残余应力大小和分布状态。结果表明:150次热循环后,陶瓷层和热生长氧化物(TGO)生成裂纹,其中陶瓷层的裂纹已扩展至TGO;350次热循环后,出现贯通陶瓷层与金属过渡层的纵向裂纹,涂层局部出现剥离,剥离位置位于TGO与陶瓷层界面;拉曼光谱(RFS)分析结果显示TGO内应力水平分布不均,局部厚大区和凸凹处残余应力较大,是裂纹萌生、扩展的主要部位。     

Study of microstructure and thermal shock behavior of two types of thermal barrier coatings

Gas turbines provide one of the most severe environments challenging material systems nowadays. Only an appropriate coating system can supply protection particularly for turbine blades. This study was made by comparison of properties of two different types of thermal barrier coatings (TBCs) in order to improve the surface characteristics of high temperature components. These TBCs consisted of a duplex TBC and a five layered functionally graded TBC. In duplex TBCs, 0.35 mm thick yittria partially stabilized zirconia top coat (YSZ) was deposited by air plasma spraying and ～0.15 mm thick NiCrAlY bond coat was deposited by high velocity oxyfuel spraying. ～0.5 mm thick functionally graded TBC was sprayed by varying the feeding ratio of YSZ/NiCrAlY powders. Both coatings were deposited on IN 738LC alloy as a substrate. Microstructural characterization was performed by SEM and optical microscopy whereas phase analysis and chemical composition changes of the coatings and oxides formed during the tests were studied by XRD and EDX. The performance of the coatings fabricated with the optimum processing conditions was evaluated as a function of intense thermal cycling test at 1100 °C. During thermal shock test, FGM coating failed after 150 and duplex coating failed after 85 cycles. The adhesion strength of the coatings to the substrate was also measured. Finally, it is found that FGM coating has a larger lifetime than the duplex TBC, especially with regard to the adhesion strength of the coatings.     

地面重型燃气轮机及其热障涂层的研究进展与发展趋势

国际公认的重型燃气轮机制造尖端技术之一—热障涂层技术,高温下通常面临CMAS(CaO-MgO-Al₂O₃-SiO₂)腐蚀、氧化、相变与烧结等问题,其抗CMAS腐蚀性等关键性能极大地影响涂层寿命,提高热障涂层的性能刻不容缓。对重型燃气轮机用热障涂层的研究进展与发展趋势进行全面总结与分析。首先介绍国内外重型燃气轮机的现状及发展趋势、热障涂层的系统结构、材料和几种典型的制备工艺,然后针对高温下燃气轮机热障涂层遇到的一些问题,对其隔热性、抗氧化性及抗热震性等关键性能的研究进展进行综述,最后分类详述热障涂层的CMAS腐蚀机理及其防护研究进展。综述热障涂层的几种关键性能,提出热障涂层的性能与其材料、结构及制备工艺密切相关,据此总结归纳提高热障涂层性能的方法,为热障涂层性能的提高提供参考依据,以弥补燃气轮机热障涂层领域目前缺乏这类综述文章的不足。     

Thermal cyclic performance of NiAl/alumina-stabilized zirconia thermal barrier coatings deposited using a hybrid arc and magnetron sputtering system

In the present work, a hybrid arc/sputter deposition system is used to deposit alumina-stabilized zirconia (ASZ) thermal barrier coatings. An initial NiAl bond coat is deposited on the nickel super alloy substrates, in the same coating system, by arc ion plating alone. The as-deposited ASZ coatings have a dense columnar structure. From X-ray diffractometry (XRD), it is found that the as-deposited ASZ films are almost entirely tetragonal. This is believed to result from the relatively high concentration of aluminum (10 at.%) in the ASZ films. Thermal testing is carried out over numerous cycles, each consisting of 30 and 10 min at 1100 °C and ambient, respectively. During thermal testing, a significant amount of the tetragonal phase is transformed to monoclinic. Furthermore a significant number of cracks, both parallel and perpendicular to the substrate surface, are found to occur in the ASZ layer as a result of tensile and compressive thermal stresses. Although a thermally grown oxide (TGO) layer between the top and bond coat, and Kirkendall voids in the interfacial region between substrate and bond coat occur in this coating system, the failure occurs principally as a result of parallel cracks which form just above the TGO. After 200 thermal cycles, a great portion of the topcoat has failed. The formation of cracks, and the subsequent coating failure, is likely to result from the dense coatings being unable to accommodate the thermal stresses parallel to the surface during the cycling tests.     

Microstructure and Thermal Cycling Behavior of Air Plasma-Sprayed YSZ/LaMgAl11O19 Composite Coatings

Yttria-stabilized zirconia (YSZ) based composite coatings with the addition of LaMgAl₁₁O₁₉ (LaMA) as the secondary phase, were prepared by air plasma spraying in order to improve the performances of the traditional YSZ coating. Results indicate that the newly developed composite coating shows increased vertical crack density with the enhancement of the LaMA content during thermal cycling process, which results in increased strain tolerance and service lifetime. However, such composite coatings about 200 ??m thick, exhibit inferior thermal cycling lifetimes with respect to the typical YSZ coating for surface temperatures above 1400 °C. The presence of amorphous LaMA phase in the composite coating results in increased thermal conductivity and a relative thin top coat leading to a reduced thermal insulation efficiency. These are believed to be responsible for the premature degradation of bond coat and final top coat spallation failure. Such an investigation gives useful guidelines to develop advanced composite coatings based on YSZ/LaMA systems.     

Microstructural analysis of YSZ and YSZ/Al2O3 plasma sprayed thermal barrier coatings after high temperature oxidation

Air plasma sprayed TBCs usually include lamellar structure with high interconnected porosities which transfer oxygen from YSZ layer towards bond coat and cause TGO growth and internal oxidation of bond coat.The growth of thermally grown oxide (TGO) at the interface of bond coat and ceramic layer and internal oxidation of bond coat are considered as the main destructive factors in thermal barrier coatings.Oxidation phenomena of two types of plasma sprayed TBC were evaluated: (a) usual YSZ (yttria stabilized zirconia), (b) layer composite of (YSZ/Al₂O₃) which Al₂O₃ is as a top coat over YSZ coating. Oxidation tests were carried out on these coatings at 1100°C for 22, 42 and 100h. Microstructure studies by SEM demonstrated the growth of TGO underneath usual YSZ coating is higher than for YSZ/Al₂O₃ coating. Also cracking was observed in usual YSZ coating at the YSZ/bond coat interface. In addition severe internal oxidation of the bond coat occurred for usual YSZ coating and micro-XRD analysis revealed the formation of the oxides such as NiCr₂O₄, NiCrO₃ and NiCrO₄ which are accompanied with rapid volume increase, but internal oxidation of the bond coat for YSZ/Al₂O₃ coating was lower and the mentioned oxides were not detected.     

Experimental and numerical life prediction of thermally cycled thermal barrier coatings

This article addresses the predominant degradation modes and life prediction of a plasma-sprayed thermal barrier coating (TBC). The studied TBC system consists of an air-plasma-sprayed bond coat and an air-plasma-sprayed, yttria partially stabilized zirconia top layer on a conventional Hastelloy X substrate. Thermal shock tests of as-sprayed TBC and pre-oxidized TBC specimens were conducted under different burner flame conditions at Volvo Aero Corporation (Trollhättan, Sweden). Finite element models were used to simulate the thermal shock tests. Transient temperature distributions and thermal mismatch stresses in different layers of the coatings during thermal cycling were calculated. The roughness of the interface between the ceramic top coat and the bond coat was modeled through an ideally sinusoidal wavy surface. Bond coat oxidation was simulated through adding an aluminum oxide layer between the ceramic top coat and the bond coat. The calculated stresses indicated that interfacial delamination cracks, initiated in the ceramic top coat at the peak of the asperity of the interface, together with surface cracking, are the main reasons for coating failure. A phenomenological life prediction model for the coating was proposed. This model is accurate within a factor of 3.     

大气等离子喷涂锆酸镧热障涂层

利用自制的稀土元素复合掺杂锆酸镧热喷涂粉末,采用大气等离子喷涂技术,在镍基高温合金表面制备La1.6Nd0.4Ce1.0Zr1.0O7(LNCZ)热障涂层,研究了喷涂参数对涂层沉积率、显微结构、结合强度及抗热震性能的影响。结果表明,增大喷涂功率,降低送粉速率可以有效地提高涂层的沉积率,但对涂层结合和抗热震性能不利。减小喷涂距离不仅提高沉积率,还有利于获得孔径细小、孔隙率适中的层状组织结构,该结构对提高涂层的抗热震性能极为有利。采用DH-1080大气等离子喷涂设备制备LNCZ涂层较好的喷涂条件为:功率40 kW,喷涂距离9 cm,送粉速率12 g/min。     

Cross-Sectional Residual Stresses in Thermal Spray Coatings Measured by Moiré Interferometry and Nanoindentation Technique

A plasma-sprayed thermal barrier coating (TBC) was deposited on a stainless steel substrate. The residual stresses were firstly measured by moiré interferometry combined with a cutting relaxation method. The fringe patterns in the cross-section of the specimen clearly demonstrate the deformation caused by the residual stress in thermal spray coatings. However, restricted by the sensitivity of moiré interferometry, there are few fringes in the top coat, and large errors may exist in evaluating the residual stress in the top coat. Then, the nanoindentation technique was used to estimate the residual stresses across the coating thickness. The stress/depth profile shows that the process-induced stresses after thermal spray are compressive in the top coat and a tendency to a more compressive state toward the interface. In addition, the stress gradient in the substrate is nonlinear, and tensile and compressive stresses appear simultaneously for self-equilibrium in the cross-section.     

Behavior of plasma-sprayed thermal barrier coatings during thermal cycling and the effect of a preoxidized NiCrAlY bond coat

The failure mechanisms of thermal barrier coatings (TBCs) subjected to a thermal load are still not entirely understood. Thermal stresses and/or oxidation cause the coating to fail and hence must be minimized. During the present investigation, TBCs up to 1.0 mm were sprayed and withstood high thermal stresses during thermal testing. Owing to the substantial thickness, the temperature at the top coat/bond coat interface was relatively low, resulting in a low oxidation rate. Furthermore, bond coats were preoxidized before applying a top coat. The bond strength and the behavior during three different thermal loads of the preoxidized TBCs were compared with a standard duplex TBC. Finite-element model (FEM) calculations that took account of bond coat preoxidation and interface roughness were made to calculate the stresses occurring during thermal shock. It is concluded that the thick TBCs applied during this research exhibit excellent thermal shock resistance and that a preoxidizing treatment of the bond coat increases the lifetime during thermal loading, where oxidation is the main cause of failure. The FEM analysis gives a first impression of the stress conditions on the interface undulations during thermal loading, but further development is required.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

The effect of silicon on thermal shock performance of aluminide-thermal barrier coatings

The failure of air-plasma-sprayed thermal barrier coatings (APS TBCs) with conventional pack aluminide and slurry Si-modified aluminide bond coats on superalloy In-738LC was investigated during a thermal-shock test. Thermal shock experiments consisted of rapid thermal cycling between 1100 °C and 300 °C for 120 times. It was found that the lifetime of APS TBCs on aluminide bond coats can be extended by introducing silicon into aluminide structure. Silicon improved the bond coat oxidation resistance as well as the stability of β-NiAl phase, which is critical to the coating life enhancement.     

Effect of Bond Coats on Thermal Shock Resistance of Thermal Barrier Coatings Deposited Onto Polymer Matrix Composites Via Air Plasma Spray Process

Thermal barrier coating systems with different bond coats were fabricated on polymer matrix composites via the air plasma spray process. During a thermal shock test at 400 °C, Zn and Al interlayers were helpful in improving the thermal shock resistance of coatings due to the low melting point. The coating system consisted of a soft zinc layer as a bond coat, and YSZ as a top coat exhibited the best thermal shock resistance, attributed to the lower residual stress and lower thermal stress in the Zn interlayer. The failure mechanism of the coating system was mainly ascribable to the residual stress derived from the deposition process, thermal stress, and further damage of the substrate.     

Oxidation Resistance of Thermal Barrier Coatings Based on Hollow Alumina Particles

Thermal barrier coatings were synthesized in a single step process from a slurry containing Al microspheres onto different Ni-based superalloys. Upon growth of the coating a top coat of hollow alumina spheres linked to an aluminium diffused coating through an alumina TGO formed. The isothermal and cyclic oxidation tests at different temperatures (900 till 1100 °C) up to 1000 h or 1500 cycles revealed progressive growth of different thermal oxides depending on the substrate composition. Faster degradation of the coatings occurred in the titanium-rich substrates (e.g. IN-738LC and PWA1483) compared to the titanium-poor ones (CM-247LC and René N5). By comparing with conventional low activity aluminide coatings, it appeared that the incorporation of alloying elements (notably Ti and Ta) to the diffused layers upon the high activity slurry coating process is responsible for such fastest degradation.     

Thermal shock testing of burner cans coated with a thick thermal barrier coating

Thick (1.8 mm) thermal barrier coatings were air-plasma-sprayed onto two different substrate geometries, including small circular substrates and burner cans. Two different top-coating spray parameters were used, where the settings of the substrate temperature and the lamella thickness were varied. A segmentation crack network was found in the top coatings sprayed using a high substrate temperature and a high lamella thickness. The density of segmentation cracks was found to be independent of substrate geometry. No segmentation cracks were found in the top-coatings when a low substrate temperature and a low lamella thickness were used. In the segmented burner can, after 1000 thermal shock cycles, the segmentation crack network was still stable and no severe cracks had formed in the top coating. In the nonsegmented burner can, cracks were formed after only 35 thermal shock cycles. Among the crack types, horizontally oriented cracks were found in the top coating close to, and sometimes reaching, the bond coating. Cracks of this type are not tolerated in thermal barrier coatings because they can cause failure of the coating. Regarding the lifetime of the segmented burner can, it is believed the failure will be dependent on other mechanisms, such as bond-coating oxidation or top-coating decomposition.