热障涂层的制备及其失效的研究现状

热障涂层作为航空发动机和燃气轮机高温部件的保护涂层,其抗高温失效能力直接决定了部件的工作效率和寿命.回顾热障涂层的发展历史及研究现状,着重介绍了热障涂层的主要制备方法及其相应涂层的结构特征,综述了各类热障涂层失效的影响因素和失效机理.     

电子束物理气相沉积热障涂层的高温氧化行为

采用电子束物理气相沉积（EB-PVD）在Ni-Cr-Al-Y粘结层上沉积Y2O3部分稳定的ZrO2陶瓷层（YSZ），进行了900，1000，1100℃的恒温氧化和1050℃的循环氧化实验，用扫描电镜（SEM），能谱（EDS）和X射线衍射（XRD）对样品进行观察分析，结果表明，恒温氧化最初20h，1100℃的增重比900℃和1000℃的都要小，热循环过程中产生的热应力和氧化物生长应力等导致粘结层氧化物（TGO）/NiCrAlY粘结层界面开裂，引起EB-PVD热障涂层失效。     

ZrO2陶瓷热障涂层的研究

用等离子涂技术制备了ＺｒＯ２结构梯度和成份梯度涂层,并试验测定了各涂层的隔热效果及抗热震性,涂层试片在２．５Ｍａ、１８００℃、５８００ＫＷ／ｍ＾２的热流冲刷５ｓ后,结果表明：涂层表面仍较光滑,与烧蚀前无明显差别,更无剥落,崩裂出更,平均每毫米隔热涂层降温约４０２．２℃。     

高性能陶瓷涂层及其制备工艺发展趋势

从功能、用途方面分类综述了耐磨、耐蚀以及热障三大类高性能陶瓷涂层,在此基础上,介绍了高性能陶瓷涂层常用的制备工艺,重点探讨了不同制备方法的特点和适用场合,指出了制备方法的发展方向。物理气相沉积技术(PVD)制备的陶瓷涂层纯度高、致密性好,并且与基体结合牢固,但其生产成本高,生产效率低,因此物理气相沉积技术向着高效率、低成本的方向发展。化学气相沉积技术(CVD)制备的陶瓷涂层涂覆率高、致密性好,但其反应温度高,并且伴随着有毒有害气体产生,因此化学气相沉积技术向着低温、环保的方向发展。等离子喷涂技术(PS)制备陶瓷涂层成本低、效率高、适应性强,但涂层孔隙率高,并且涂层与基体的结合强度低,因此等离子喷涂技术向着高致密、高结合强度的方向发展。激光熔覆技术制备的陶瓷涂层组织细小、力学性能优良,但其操作工艺复杂,产品质量很难控制,因此激光覆熔技术向着工艺简单、质量可控的方向发展。最后,展望了高性能陶瓷涂层及其制备工艺的发展方向和可能的研究内容。     

热障涂层的CMAS腐蚀失效及对策研究综述

研制具有长寿命、高性能的热障涂层（TBCs）,是制造我国大功率航空发动机、发展新一代超音速战机的一项十分紧迫的任务。由于外来沉积物CaO-MgO-Al2O3-SiO2（CMAS）渗入涂层而导致TBCs失效的现象越来越受到人们的重视,在过去的一段时间里,很多学者对CMAS渗入后涂层的失效机理进行了大量的研究并尝试了一些缓解CMAS渗入涂层的方法。本文针对当前应用最为广泛的“层状多孔结构”的等离子喷涂涂层及“细长柱晶结构”的电子束辅助物理气相沉积涂层,系统地梳理了近年来国内外学者对TBCs在CMAS渗入条件下的失效机制及涂层缓解CMAS渗入方面的最新研究成果,为制备高性能的TBCs提供帮助。     

EB-PVD热障涂层高温氧化过程中的显微结构和相分析

采用磁控溅射方法在镍基单晶高温合金基体上沉积NiCrAlY粘结层，电子束物理气相沉积方法（EB－PVD）沉积陶瓷层（7mass%Y2O3-ZrO2)。用X－射线衍射（XRD）、扫描电镜（SEM）等研究了EB－PVD热障涂层热循环过程中的相变，同时观察了样品的形貌变化。结果表明，EB－PVD方法沉积的陶瓷层，其柱状晶粒簇拥成团，表面比较致密，然而晶粒簇间存在间隙，允许柱状晶横向伸缩，使基体能在相对大的范围内自由膨胀。经高温氧化后，陶瓷层表面变得疏松，柱状晶粒簇间距增大，随着热循环的继续进行，相邻较大的间隙互相连接而形成微裂纹，并逐渐横向及纵向扩展，循环氧化比恒温氧化更易于产生显微裂纹。1050℃热循环过程中，ZrO2正方相的c/a轴比值逐渐增加，Y2O3含量逐渐减少，非平衡正方相逐渐分解成平衡正方相和立方相。循环300次后，观察到少量从平衡正方相转变而来的单斜相。     

电子束物理气相沉积热障涂层抗高温腐蚀性能的研究

对采用电子束物理气相沉积方法制备的MCrAlY涂层、双层结构热障涂层以及新型梯度结构热障涂层的高温氧化性能以及热腐蚀性能进行了研究。3种涂层均具有良好的抗高温氧化性能，但在950℃等温热腐蚀环境下，涂层的氧化速率加快，梯度热障涂层的增重小于NiCoAlY涂层，表明YSZ陶瓷层有助于提高NiCoAlY涂层的抗热腐蚀性能，但热循环寿命降低。     

未来航空发动机热障涂层材料及制备技术

概述了未来航空发动机热障涂层最有前景的新材料、结构和制备工艺。新材料主要有改进型氧化钇稳定的氧化锆、A2B2O7型材料;新结构主要有双陶瓷层;新工艺主要为制备含垂直裂纹的热障涂层的改进大气等离子体喷涂、等离子喷涂-物理气相沉积、悬浮液等离子喷涂、电子束直接气相沉积。这些相互结合,必将促进高性能热障涂层的快速发展和应用,使其在未来航空发动机中发挥重要作用。     

垂直裂纹对EB-PVD热障涂层热循环失效模式的影响

对电子束物理气相沉积（EB-PVD）双层结构热障涂层在热循环过程中形成的陶瓷层垂直裂纹对涂层失效模式的影响进行了研究.结果表明,陶瓷层表面垂直裂纹出现在热循环初期,但并未造成涂层的早期剥落;随着热循环次数增加,垂直裂纹网格变密.有限元计算表明,垂直裂纹的形成在试样中部产生了类似于试样边缘的剪应力集中效应.经过长时间热循环后,当陶瓷层等轴晶区的强度或者热氧化生长层（TGO）的强度小于边缘效应产生的剪应力时,涂层在试样的中部以垂直裂纹网格形状发生剥落失效.     

等离子喷涂和激光熔覆热障涂层隔热性能比较

采用多种方法制备不同类型的Al2O3-13％TiO2热障涂层,即等离子喷涂常规涂层、纳米结构涂层及激光熔覆纳米结构涂层．在分析三类涂层微观组织的基础上,对其隔热性能进行了比较．结果表明,即等离子喷涂常规陶瓷涂层呈典型的层状堆积特征,纳米结构涂层都为特殊的两相结构,其中部分熔化区由类似的残留纳米粒子组成,等离子喷涂纳米结构涂层的完全熔化区为片层状结构,而相应的激光熔覆涂层的完全熔化区则为细小等轴晶．在相同条件下,等离子喷涂纳米结构热障涂层具有最好的隔热性能,而激光熔覆纳米结构涂层的隔热性能要好于等离子喷涂常规涂层．     

等离子喷涂耐磨涂层及热障涂层的新进展

综述了等离子喷少技术在耐磨损涂层及热障涂层方面的研究进展,分析了等离子喷涂技术与新设备,新工艺融合后两种涂所展现出的更为优越的性能;最后指出了两种涂层的发展趋势。     

Influence of substrate strain anisotropy in TBC system failure

The lifetimes of thermal barrier coatings (TBC) are known to show considerable scatter. Experimental original data have revealed the strong correlation between spallation patterns and local strain field. This strain field was shown to be a function of the substrate single crystal orientation. Thus, this study addresses the strain variation in the single crystal substrate and its influence on TBC system life. This task was achieved using both finite element analysis (FEA) to take into account crystal plasticity in the substrate single crystal and a recent TBC life model [1].     

Effects of deposition conditions on composition and thermal cycling life of lanthanum zirconate coatings

Lanthanum zirconate (La₂Zr₂O₇, LZ) coatings were prepared under four different deposition conditions by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphology, cyclic oxidation behavior of these coatings were studied. Elemental analysis indicates that the coating composition has partially deviated from the stoichiometry of pyrochlore, and the existence of excess La₂O₃ is also observed. The deviation could be reduced by properly controlling the electron beam current or by changing the ingot composition. Meanwhile, when the electron beam current was 500 - 600 mA, the thermal cycling life of the coating is superior to other coatings.     

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.     

激光熔覆技术研究现状及展望

通过研究激光熔覆技术的现状,综述了激光表面熔覆的特点、工艺方法、材料体系以及当前存在的问题,并展望其应用前景和未来的研究重点.     

Non-contact sensing of TBC/BC interface temperature in a thermal gradient

Luminescence lifetimes of rare-earth ions in yttria-stabilized zirconia have been shown to exhibit temperature sensitivity from 500-1150 °C [Gentleman, M.M. and Clarke, D.R. (2005) Surface and Coatings Technology 200, 1264; Gentleman, M.M. and Clarke, D.R. (2004) Surface and Coatings Technology 188-189, 93.]. These doped zirconias can be deposited along with standard thermal barrier coatings to create thin temperature sensing layers within the coating. Of particular interest is the temperature at the coating/bond coat interface as the oxidation life of a TBC system is exponentially dependent on this temperature. In this study, thin (∼ 10 μm) layers of europia-doped yttria-stabilized zirconia were deposited by EB-PVD onto bond-coated CMSX-4 superalloy buttons to achieve sensor layers located next to the TBC/BC interface. These coatings were then used to measure the interface temperature in a thermal gradient. Combined with pyrometric measurements of the coating-surface temperature and metal-surface temperature, the thermal conductivity of the coating (1.5 W/mK) and heat flux (∼ 1 MW/m²) in the tests were calculated.     

Non-destructive thermal property measurements of an APS TBC on an intact turbine blade

The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilized ZrO₂ (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm²/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity.     

Abrasive wear behaviour of laser clad and flame sprayed-melted NiCrBSi coatings

In this work, the influence of the processing conditions on the microstructure and abrasive wear behaviour of a NiCrBSi hardfacing alloy is analysed. The hardfacing alloy was applied in the form of coatings onto a mild steel substrate (Fe–0.15%C) by different techniques: laser cladding (LC) and flame spraying (FS) combined with surface flame melting (SFM). In both cases, the appropriate selection of the process parameters enabled high-quality, defect-free NiCrBSi coatings to be obtained. The microstructure of the coatings was analysed by scanning electron microscopy (SEM), with attached energy dispersive spectroscopy (EDS) microprobe, and by X-ray diffraction (XRD). Their tribological properties were evaluated by micro-scale ball cratering abrasive wear tests using different abrasives: diamond, SiC and WC. Microstructural characterisation showed that both coatings exhibit similar phases in their microstructure, but the phases present differ in morphology, size distribution and relative proportions from one coating to another. Wear tests showed that in three-body abrasive conditions, despite these microstructural differences, the wear behaviour is comparable for both coatings. Conversely, in two-body wear conditions with diamond particles as the abrasive, it was observed that the specific wear rate of the material is sensitive to microstructural changes. This fact is particularly apparent in LC coatings, in which the zones of the layers with higher proportions of very small hard particles present a lower wear resistance. These results indicate that it is important to have good microstructural control of this material, in order to obtain coatings with an optimized and homogeneous tribological behaviour.     

Dry coatings and ecodesign: Part. 2 — Tribological performances

The goal of this study is to find an industrial alternative application to hard chromium plating. The first part presents the appropriate environmental and chemical behaviour of NiCrBSi alloy and AISI 316L stainless steel coated by thermal spraying and laser cladding. This paper will concern the wear resistance of these dry coatings. However, stainless steel has awful performance in friction contacts, and this material cannot be suggested as a substitute solution to hard chromium coating for applications where the wear resistance is an important issue. Therefore the tribological pin-on-disc tests will only concern the NiCrBSi coatings. Consequently in the second part, tribological characterizations were carried out in order to determine the alternative potential of NiCrBSi coatings compared to electrolytic hard chromium plating, in applications to resist wear. The coatings are sprayed by Atmospheric Plasma Spraying (APS) and laser cladding (with a diode laser). Especially, the dry coated samples present a good tribological behaviour, whereas the wear mechanisms are different between APS and laser cladding coated samples. Laser cladding allows the manufacture of denser coatings, thus the mechanical properties are increased while the wear rate was reduced. At the same time, APS coated samples show a lower value of shear strength in comparison with laser cladding. That is the reason why fatigue is the main wear mechanism, and this process is called splat delamination.