航空发动机涡轮叶片热障涂层研究进展

涡轮叶片的热障涂层技术是保障和提升航空发动机性能的关键技术之一，涡轮叶片的工作环境要求热障涂层需要具备隔热性能好、热膨胀系数与基材相匹配、抗氧化性能好、抗熔盐腐蚀性能好等一系列特点，这对热障涂层的材料、结构以及制备工艺提出了巨大的挑战，是当前航空发动机领域的热点研究之一。本文对构成热障涂层的金属粘结层和陶瓷层材料，以及热障涂层体系结构的研究现状做了详细介绍，并简要介绍了常用的热障涂层制备方法，展望了金属粘结层和陶瓷层材料体系和制备技术的发展趋势，以期为未来航空发动机涡轮叶片热障涂层体系的构建提供有益参考。     

高炉风口上超音速火焰喷涂金属-陶瓷梯度层的可行性

在高炉风口表面制备一层适宜的金属-陶瓷梯度热障涂层,是提高风口寿命的一种有效的方法.本文旨在验证在风口上制备金属-陶瓷梯度保护层,提高高炉风口寿命的可行性.通过大型的有限元仿真软件AN-SYS,模拟分析了在紫铜表面喷涂陶瓷热障层的隔热效果,并且探讨了HVOF涂层的性能.结果发现,仅仅0.4 mm厚的ZrO2涂层就可以使铜基体温降200℃左右,而且超音速火焰喷涂打底的金属-陶瓷涂层的抗氧化性、热震性优于其他喷涂方法,这表示能够从材料和工艺两方面解决风口喷涂陶瓷层易脱落的问题.证明了在风口上喷涂金属-陶瓷梯度热障涂层的可行性以及利用HVOF打底层制备金属-陶瓷热障层的优势.     

MCrAlY金属黏结层高温失效行为研究进展

随着航空发动机与燃气轮机涡轮进口温度的不断提高,MCrAlY(M=Ni,Co或NiCo)包覆型涂层因具有抗高温氧化以及高的热膨胀系数等优点,成为广泛应用的热障涂层金属黏结层材料。然而,高温服役环境下热障涂层中金属黏结层与陶瓷面层界面应力分布状态愈加复杂,黏结层界面失效导致陶瓷面层的剥落,限制了其在热防护涂层领域的发展。本文简述了黏结层的发展进程,重点阐述高温相转变、热应力和生长应力增加以及S元素扩散等因素导致的黏结层界面的失效行为,分析黏结层界面失效机理,归纳总结了国内外针对金属黏结层界面失效的改进研究工作,并在此基础上提出采用稀土及纳米颗粒协同强化MCrAlY材料,为未来热障涂层体系的优化设计提供了研究方向。     

高温热处理对等离子体喷涂的纳米氧化锆热障涂层的影响

热障涂层已广泛用于燃气发动机燃烧室等高温零部件上。纳米热障涂层韧性改善,厚度可以增加,能够提高零部件使用温度和使用寿命。首先利用低压等离子体在镍基体上喷涂制备NiCoCrAlYTa金属中间结合层和大气等离子体喷涂制备Y_2O_3部分稳定的ZrO_2纳米陶瓷面层,然后将纳米氧化锆热障涂层样品在大气中于1050～1250℃温度范围内煅烧处理2～20h。通过扫描电镜和X射线衍射仪分析纳米氧化锆热障涂层高温煅烧前后的组织结构变化和相组成变化,并与常规微米氧化锆热障涂层进行比较。研究结果表明:经高温煅烧后,纳米氧化锆热障涂层中晶粒大小和在陶瓷面层/金属中间结合层界面上形成的TGO的厚度随煅烧温度升高和时间的延长而增大;纳米热障涂层中TGO的增长速度比常规微米热障涂层快;纳米热障涂层经高温煅烧空气中冷却后,主要由四方相组成;与常规微米热障涂层的相组成比较,纳米氧化锆热障涂层中的四方相为低稳定剂四方相。     

热障涂层的研究进展与发展趋势

热障涂层一般由金属粘结层和具有低热导率的陶瓷顶层组成,应用于涡轮发动机的热端部件可显著提高其使用温度,延长部件的使用寿命,提高发动机的效率.综述了热障涂层的成分选择、制备方法及等离子喷涂和电子束物理气相沉积2种热障涂层的典型结构,分析了热障涂层的剥落失效机理,并简单介绍了热障涂层的寿命预测模型和隔热特性的研究.     

等离子喷涂热障涂层隔热性能分析方法

热障涂层材料已成为现代高性能航空发动机的关键材料,而隔热性能一直是评价热障涂层性能的一个重要指标。首先基于傅里叶导热定律,推导出一维稳态温度场的解析表达式,并讨论了陶瓷层厚度、陶瓷层上表面工作温度和金属基底下表面工作温度对热障涂层系统隔热性能的影响。设计了一种比较新颖的实验测试方法,成功实现了对热障涂层系统内部不同位置的温度进行实时测试和保存实验数据。结果表明,各个温度采集点的实验测试结果与理论预测结果吻合很好,说明提出的实验测试方法可以有效评估不同类型的热障涂层材料体系的隔热性能。     

防钛火金属/陶瓷阻隔层设计与制备方法

金属/陶瓷阻隔层不仅可用于防钛火可磨耗封严涂层起阻燃作用,还可用于航空发动机热部件的热障涂层.以航空发动机压气机防钛火涂层的应用为主要背景,对金属/陶瓷阻隔层体系结构设计、涂层材料以及制备方法进行了系统的阐述.在涂层材料层面,分别介绍了陶瓷层与金属粘结层的材料成分设计、性能特点及其合成方法.在涂层制备技术层面,主要阐述了等离子喷涂(PS)、电子束物理气相沉积(EB-PVD)以及新型的等离子物理气相沉积(PS-PVD)3种方法.最后,面向防钛火涂层的发展需求,提出了金属/陶瓷阻隔层未来在材料升级、工艺优化、性能表征以及涂层技术理论体系等方面的发展方向.     

陶瓷/金属功能梯度热障材料研究进展

梯度层中金属相在高温环境下的氧化是导致陶瓷／金属功能梯度热障材料失效的主要原因,限制了功能梯度热障材料的应用,叙述了陶瓷／金属功能梯度热障材料的研究进展,并讨论了其发展前景。     

热障陶瓷涂层的最新发展

综述了现代航空发动机用热障陶瓷涂层的最新发展，着重介绍了双陶瓷层，电子束物理气相沉积（EB－PVD）和溶液等离子喷涂（SPS）纳米热障陶瓷涂层的性能和特点。     

ZrO2陶瓷-金属梯度热障涂层的显微结构特征

利用等离子喷涂技术制备了ZrO2陶瓷和NiCrAl金属的梯度热障涂层，利用扫描电镜（SEM）、能谱仪（EDS）和电子探针（EPMA）研究了涂层的显微结构和化学成分分布特征。实验发现：梯度过渡层的显微结构明显不同，以金属为主要组分的过渡层主要为层状结构，以陶瓷为主要组分的过渡层层状特征不明显；过渡层化学元素分布不均匀，其中Zr和Mg分布特征相同，Zr与Ni分布位置互补，Cr、Al主要以氧化物形式位于晶粒边界处。     

On debonding of graded thermal barrier coatings

Thermal barrier coatings generally consist of a metallic substrate which is the primary structural component, a metallic bond coat which serves as oxygen diffusion barrier, a very thin layer of thermally grown oxide and a ceramic top coat that provides the main thermal shielding. Homogeneous ceramic coatings as top coats appear to have certain undesirable features such as high residual and thermal stresses, generally low toughness and relatively poor bonding strength. The new concept of compositional grading of the top coat may help to overcome some of these shortcomings by eliminating the material property discontinuities. A common mode of failure in thermal barrier coatings seems to be the debonding of the top coat. In this study the related interface crack problem for a graded ceramic/metal top coat is considered. It is assumed that the thermophysical properties of the top coat continuously vary between that of the bond coat at the top coat-bond coat interface and that of the ceramic at and near the free surface. The main objective of the study is to examine the influence of the material nonhomogeneity parameters and relative dimensions on the stress intensity factors and the crack opening displacements.     

热障涂层材料研究进展

简要概述了热障涂层材料的基本要求,介绍了国内外热障涂层材料近年来的研究状况和发展趋势.目前广泛使用的是T2O3稳定ZrO2热障陶瓷材料及其粘结层材料,而稀土锆酸盐和稀土氧化物是非常有前景的隔热材料.     

Effect of the thickness of plasma-sprayed coating on bond strength and thermal fatigue characteristics

NiCrAlY bond coat and ZrO2–8 wt% Y2O3 top coat with various thicknesses were deposited on Hastelloy X by plasma spraying. Residual stress was calculated by the finite element method (FEM) to explain the variations in the bond strength and thermal fatigue characteristics with the thickness of the bond coat and top coat. The bond strength of thermal barrier coatings (TBCs) increased with decreasing maximum residual stress in the y-direction of the top coat. The thermal fatigue characteristics increased with decrease of the maximum principal residual stress of the top coat and the thickness of oxidation layer of the bond coat.     

A simple method for the preparation of plasma-sprayable powders based on ZrO2

Plasma-sprayable powders of calcia, magnesia and yttria-stabilized zirconia have been prepared by using polyvinyl alcohol binders. The powders have been characterized for sprayability by spray coating on steel plates previously coated with an NiAl bond coat. The suitability of these coatings for thermal barrier applications have been examined. Thermal barrier and related properties, along with phase stability and mechanical properties, have been found to be good. Failure of the thermal barrier coating has been observed to occur at the interface between the bond coat and the substrate, due to the formation of a pile-up layer consisting of Fe-Zr-Al-O compound.     

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.     

Modeling of elastic thermal stresses in two materials joined by a graded layer

The distribution of elastic thermal stresses in two dissimilar materials joined by a graded layer is a critical issue and has been analyzed previously. However, numerical methods have often been used. The difficulty in obtaining an analytical solution in existing analyses is discussed, and an exact closed-form solution is derived in the present study. To illustrate applications of the present solution, results are calculated for elastic thermal stresses in thermal barrier coatings, which consist of substrate, graded bond coat, and top coat. Different profiles of the thermomechanical properties in the bond coat are considered to examine how the graded interlayer modifies the elastic thermal stress distribution in the system.     

Improving thermal barrier coatings by laser remelting

Thermal barrier coatings are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. These coatings are usually processed by thermal spraying techniques and the resulting microstructure includes thin and large splats, associated with the deposition of individual droplets, with porosity between splats. This porosity reduces the oxidation and corrosion resistance favouring the entrance of aggressive species during service. To overcome this limitation, the top coat could be modified by laser glazing reducing surface roughness and sealing open porosity. ZrO2(Y2O3) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. This layer inhibited the ingress of aggressive species and delayed bond coat oxidation.     

Crack propagation studies and bond coat properties in thermal barrier coatings under bending

Ceramic based thermal barrier coatings (TBC) are currently considered as a candidate material for advanced stationary gas turbine components. Crack propagation studies under bending are described that were performed on plasma sprayed ZrO₂, bonded by MCrAlY layer to Ni base superalloy. The crack propagation behaviour of the coatings at room temperature in as received and oxidized conditions revealed a linear growth of the cracks on the coating till the yield point of the super alloy was reached. High threshold load at the interface between the ceramic layer and the bond coat was required to propagate the crack further into the bond coat. Once the threshold load was surpassed the crack propagated into the brittle bond coat without an appreciable increase in the load. At temperatures of 800°C the crack propagated only in the TBC (ceramic layer), as the ductile bond coat offered an attractive sink for the stress relaxation. Effects of bond coat oxidation on crack propagation in the interface region have been examined and are discussed.     

Application Study on Ceria Based Thermal Barrier Coatings

The industrial application of APS sprayed YPSZ coatings for thermal insulation is established in several branches. As the main potential to increase the efficiency of combustion processes is thermal efficiency and the state‐of‐the‐art systems are limited to surface temperatures below 1200°C for long term applications, there is interest in concepts, that allow an increase of the process temperature. Ceria and ceria based ceramics show an outstanding potential for use at temperatures exceeding 1200°C. A triple‐layer thermal barrier system in consideration of the established system – MCrAlY bond coat and YPSZ – and an additional ceria based top coating are investigated. TBC systems with two different ceria powders are produced by APS and HVOF spraying and evaluated with concern to the microstructure, bond strength, thermal shock behaviour and long term compatibility of the constituents. HVOF sprayed coatings contain more oxygen, are more dense than APS sprayed coatings and do not show segmentation due to cracks perpendicular to the surface. APS sprayed pure ceria coatings show a columnar morphology inside single splats forming the coating. The bond between YPSZ and ceria and the total bond strength of the thermal barrier system exceeds the cohesion inside the ceria coating. The thermal shock resistance of ceria coatings with high silica and sulphur content is low. Long term sintering investigations prove the compatibility of ceria and YPSZ at 1150°C.     

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.