高温合金热障涂层的氧化和失效研究

评述了热障涂层发展的现状，研究了电子束物理气相沉积（EB－PVD）热障涂层的恒温氧化和循环氧化行为。结果表明：热障涂层 1000℃氧化稳定后，基本遵循抛物线氧化规律。循环氧化过程中，微裂纹优先沿陶瓷层柱状晶界形成，并逐渐沿横向及纵向扩展。热障涂层热循环过程中产生的热应力、氧化物长大应力等引起金属氧化物（TGO）／粘结层分界面多处开裂，最终导致热障涂层失效于TGO中或TGO／粘结层的分界面。     

基于内聚力单元与XFEM的热障涂层失效分析

为了更好的理解热障涂层的失效机理,文中运用ABAQUS有限元软件来分析热障涂层的失效情况,使用内聚力单元和扩展有限元（XFEM）两种方法研究热障涂层TGO界面开裂与陶瓷涂层（TC）和氧化层（TGO）内随机裂纹的萌生与扩展,研究竖直裂纹与水平裂纹的关系.结果表明,热障涂层TGO界面的开裂首先出现在TGO/TBC波谷处.陶瓷涂层和氧化层内随机裂纹的萌生同样发生在TGO/TBC波谷处.竖直裂纹的存在可以抑制水平裂纹的萌生与扩展,且其在TGO/TBC波谷处的扩展长度比在TGO/TBC波峰处的扩展长度更长,说明TGO/TBC波谷区域是个危险区域,在此区域容易引发裂纹的萌生与扩展.     

热障涂层系统的热梯度机械疲劳应力分析

基于IN738高温合金基体上涂覆的热障涂层系统(Thermal barrier coating system,TBCs),分析热循环和热梯度机械疲劳加载条件下涂层的应力分布及演变。通过有限元分析研究了热生长氧化层(Thermally growth oxidation,TGO)的应力分布,以预测不同载荷作用下TBCs的失效行为。结果可知,在热循环的基础上施加应变载荷会造成TGO应力性质及大小的改变。只施加温度载荷,在加热过程中TGO/粘结层(Bond coat,BC)界面波峰位置会承受轴向较大的拉伸应力,裂纹多会在此处萌生,且以层间开裂的方式失效。而在温度与机械载荷的共同作用下,冷却过程中会承受较大的拉伸应力,显著增大的轴向应力与径向应力共同作用,使垂直于TGO/BC界面的裂纹沿着界面方向扩展,从而造成陶瓷层(Top coat,TC)剥落。进一步对比分析了同相和反相加载时的应力分布,结果表明反相加载时一次循环周期内会产生拉伸平均应力,更易发生TBCs的失效。     

应变幅和相角度对热障涂层材料系统热机械疲劳性能的影响（英文）

热障涂层作为燃气轮机高温部件的关键材料,其服役过程中的脱落与失效机理一直是研究的热点问题。研究了应变幅和相角度对含热障涂层的镍基高温合金热机械疲劳性能的影响。研究结果表明,在相同相角度下,热机械疲劳寿命随应变幅的增大而降低。固定应变幅,同相位下样品的热机械疲劳寿命要高于反相位样品。所有样品中,裂纹萌生于热生长氧化物层,在粘结层与陶瓷层界面扩展形成分层裂纹,分层裂纹与陶瓷层内贯穿裂纹连接起来导致大面积的陶瓷层剥落,从而导致TBC层失效。另外,分析了热障涂层中的应力分布,初步建立了含热障涂层的镍基高温合金热机械疲劳寿命模型,发现含热障涂层的镍基高温合金热机械疲劳寿命与涂层中的最大应力呈指数关系。     

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。     

热障涂层高温TGO生长变化

通过Abaqus有限元分析软件对热障涂层在高温氧化过程中的热氧化物层(themally growth oxide,TGO)生长机制进行研究.结果表明,当高温氧化到100 h时,TGO厚度由初始的0.5μm生长至6.7μm且在不同位置TGO的厚度略微不同.随着高温时间的增加,热障涂层在TGO的波峰、波谷以及涂层边界处容易出现应力较大值,且和周围材料相比应力明显较大,此时,这些位置容易达到材料开裂临界应力,形成裂纹萌生点,使得涂层失效.在高温氧化过程中,涂层吸收总能量为43.6 J,其中少部分转化为涂层变形所消耗的能量,剩下的能量为高温氧化过程中涂层成分改变,微观组织改变以及裂纹萌生扩展提供能量.     

应变幅和相角度对热障涂层材料系统热机械疲劳性能的影响

热障涂层作为燃气轮机高温部件的关键材料,其服役过程中的脱落与失效机理一直是研究的热点问题。本文主要研究了应变幅和相角度对含热障涂层的镍基高温合金热机械疲劳性能的影响。研究结果表明,在相同相角度下,热机械疲劳寿命随应变幅的增大而降低。固定应变幅,同相位下样品的热机械疲劳寿命要高于反相位样品。所有样品中,裂纹萌生于热生长氧化物层,在粘结层与陶瓷层界面扩展形成分层裂纹,分层裂纹与陶瓷层内贯穿裂纹连接起来导致大面积的陶瓷层剥落,从而导致TBC层失效。另外,本文分析了热障涂层中的应力分布,初步建立了含热障涂层的镍基高温合金热机械疲劳寿命模型,发现含热障涂层的镍基高温合金热机械疲劳寿命与涂层中的最大应力呈指数关系。     

热障涂层陶瓷层/TGO界面开裂行为的有限元模拟

目的更好地理解热障涂层在热循环条件下的失效行为。方法采用有限元方法引入了内聚力模型,研究热障涂层在多次热循环条件下的界面开裂行为,并且考虑了陶瓷层厚度和粘结层厚度对界面开裂行为的影响。结果涂层最先在陶瓷层/TGO层界面的波峰与波谷之间开裂,此外在界面波谷处也存在开裂现象。当陶瓷层厚度在300~500μm范围内,界面裂纹的平均长度随陶瓷层增厚而增长,裂纹密度也随之增加。粘结层厚度为50μm时,界面裂纹的平均长度为15μm;当厚度增加到100μm时,界面裂纹平均长度减少到10μm;而厚度为150μm时,界面裂纹平均长度又提高至12μm。当粘结层与陶瓷层厚度比在0.2~0.4的范围内时,陶瓷层/TGO层界面上的最大拉应力最小。结论陶瓷层厚度和粘结层厚度对热障涂层界面开裂行为的影响极大,小厚度陶瓷层以及当粘结层与陶瓷层厚度比在0.2~0.4的范围内时,热障涂层具有更好的抗界面开裂能力。粘结层厚度不宜过大,超过一定厚度时反而会降低涂层的抗界面开裂能力。计算结果与文献报道的结果相近,证明了模拟结果的准确性。     

粘结层粗糙度对YSZ涂层热震性能的影响

目的 提高YSZ涂层的服役寿命.方法 采用火焰喷涂和等离子喷涂的方法,在GH4169高温合金上分别制备NiCoCrAlY粘结层和8YSZ陶瓷层.在制备陶瓷层之前,通过低压喷砂的方法对粘结层表面进行处理,改变粘结层的表面粗糙度.利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、3D测量激光显微镜,表征涂层的物相、微观形貌和表面粗糙度.使用马弗炉进行1100℃的热震实验,表征YSZ涂层的热震性能.结果 通过喷涂方法制备的粘结层,表面较为粗糙,喷砂处理后,有效地改善了表面情况,表面粗糙度下降了1～3μm.随着热震实验的进行,陶瓷层和粘结层中间形成的热生长氧化物(TGO)不断增厚.喷砂处理后,涂层的氧化速率降低.热震实验后,涂层边缘处会萌生微裂纹,随着裂纹的累积扩展,最终贯通,形成局部的涂层剥离.失效前后,YSZ没有相变发生.结论 涂层失效多发生在TGO界面处,粘结层的高温氧化引起TGO层生长增厚,会导致涂层的失效.平整的TGO生长界面可以减少接触面,降低粘结层的氧化速率.平整界面可以避免局部起伏造成的TGO过度生长.经过表面喷砂处理改善粘结层粗糙度的涂层,具有更优异的抗热震性能.TGO层的形成和生长,容易导致微裂纹的萌生和扩展.粘结层表面处理后,能够为TGO的形成和生长提供相对平整的界面,有效提高TGO的质量,进而提高涂层的抗热震性能.     

热障涂层TGO界面应力分布及裂纹扩展行为的研究进展

热障涂层以其优异的抗氧化? 隔热? 耐腐蚀性而广泛应用于热端部件表面.在超过1000℃高温的服役环境下,外界的氧元素通过陶瓷层扩散到粘接层界面,与其中的金属元素发生氧化反应生成一层热生长高温氧化物(TGO)? 随着服役时间的增加,TGO不断生长,TGO界面产生较大的热应力,导致裂纹的萌生与扩展,使得涂层大面积剥落,因此TGO氧化失效的研究既是难点也是热点问题.总结了研究TGO界面建立的主要几种模型,例如同心圆界面模型? 曲线弦界面模型和真实界面模型,其中曲线几何结构通常只需要振幅和波长即可很好地描述界面,因此大多数模型建立采用了曲线模型.在此基础上,重点综述了界面形貌、界面粗糙度和TGO厚度对应力分布的影响,以及从应变能释放率和裂纹路径两个角度探讨了界面损伤行为.上述研究很好地阐明了TGO生长过程,但是仅考虑了形态特征对应力分布的影响,接下来的研究应考虑真实TGO界面并完善模拟的准确性,同时发展有限元技术实现在实际条件下裂纹扩展路径和使用寿命的预测.     

On the interfacial degradation mechanisms of thermal barrier coating systems: Effects of bond coat composition

Thermal barrier coating (TBC) systems based on an electron beam physical vapour deposited, yttria-stabilized zirconia (YSZ) top coat and a substrate material of CMSX-4 superalloy were identically prepared to systematically study the behaviour of different bond coats. The three bond coat systems investigated included two β-structured Pt–Al types and a γ–γ′ type produced by Pt diffusion without aluminizing. Progressive evolution of stress in the thermally grown aluminium oxide (TGO) upon thermal cycling, and its relief by plastic deformation and fracture, were studied using luminescence spectroscopy. The TBCs with the LT Pt–Al bond coat failed by a rumpling mechanism that generated isolated cracks at the interface between the TGO and the YSZ. This reduced adhesion at this interface and the TBC delaminated when it could no longer resist the release of the stored elastic energy of the YSZ, which stiffened with time due to sintering. In contrast, the TBCs with Pt diffusion bond coats did not rumple, and the adhesion of interfaces in the coating did not obviously degrade. It is shown that the different failure mechanisms are strongly associated with differences in the high-temperature mechanical properties of the bond coats.     

Thermal Failure of Nanostructured Thermal Barrier Coatings with Cold-Sprayed Nanostructured NiCrAlY Bond Coat

Nanostructured thermal barrier coatings (TBCs) were deposited by plasma spraying using agglomerated nanostructured YSZ powder on Inconel 738 substrate with cold-sprayed nanostructured NiCrAlY powder as bond coat. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured TBCs. For comparison, the TBC consisting of conventional microstructure YSZ and conventional NiCrAlY bond coat was also deposited and subjected to the thermal shock test. The results showed that nanostructured YSZ coating contained two kinds of microstructures; nanosized zirconia particles embedded in the matrix and microcolumnar grain structures of zirconia similar to those of conventional YSZ. Although, after thermal cyclic test, a continuous, uniform thermally grown oxide (TGO) was formed, cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of nanostructured and conventional TBCs mainly occurred through spalling of YSZ. Compared with conventional TBCs, nanostructured TBCs exhibited better thermal shock resistance.     

On the role of imperfections in the failure of a thermal barrier coating made by electron beam deposition

An impression test has been used to explore the remnant toughness and the delamination characteristics of thermal barrier coatings (TBCs) after extended thermal exposure. The delamination trajectory is found to change as the thermally grown oxide (TGO) thickens. At small thicknesses, delamination occurs predominantly within the TGO and TBC. With a thicker TGO, developed after 100 h exposure at 1100°C, the delamination extends predominantly along the TGO/bond coat interface, but with small oxide domains remaining embedded in the bond coat. The changes in the interface adhesion and the mechanics responsible for this transition are addressed, along with a discussion of the role of morphological imperfections in the TGO in failure nucleation. A method for determining the effective in-plane modulus of the TBC from the curvature of decohered TGO/TBC bilayers is also presented.     

Failure Mechanism for Thermal Fatigue of Thermal Barrier Coating Systems

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partially stabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). The thermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to the specifications of an original equipment manufacturer (OEM). The morphology, residual stresses, and micromechanical properties (microhardness, indentation fracture toughness) of the TBC systems before and after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to be similar to that of conventional thin TBC systems made by APS.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

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.     

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.     

Investigation of TBCs on turbine blades by photoluminescence piezospectroscopy

Thermal barrier coating (TBC) blade specimens with Pt diffusion bond coats were subjected to thermal cycling with periodic measurements of the residual stress in the thermally grown oxide (TGO) using photoluminescence piezospectroscopy. Two distinct stress levels were generally found to coexist in the probed volume, i.e. a high stress (～4 GPa) and a low stress (～500 MPa) level. Both the high and low stress levels were independent of the curvature of the blade surface, in agreement with numerical modelling based on a composite cylinder stress model. The relative contributions of the two stress levels appear to be correlated with the θ-Al₂O₃ content of the TGO, which was dependent on the position on the blade. The TBCs tended to fail along the TGO/bond coat interface in thermal cycling. This was modified by the presence of mixed transition metal oxides in the TGO. The results are interpreted in terms of a likely failure mechanism.     

Characterisation of residual stress and interface degradation in TBCs by photo-luminescence piezo-spectroscopy

Residual stress in the TGO in two different TBC systems, one with a Pt aluminide (β structure) bond coat and another with a Pt diffusion (γγ′ structure) bond coat, were studied using photo-luminescence piezo-spectroscopy (PLPS). The luminescence spectra and TGO morphology were investigated progressively with thermal cycling at 1135 °C. The two TBC systems were found to have distinctly different TGO residual stresses and different failure modes. Several stress relaxation mechanisms were found to be operative in the Pt aluminide system, while no stress relaxation was evident in the Pt diffusion system until close to the end of life (spallation). Luminescence spectral shape evolution has been quantitatively analysed and correlated with TBC system degradation processes. Both TBC systems showed clear spectral shape changes as a result of the formation of interface cracks when they reached approximately 75% lifetime. Characteristic spectral shape changes in response to different types of interface crack were demonstrated experimentally. The correlation between spectral shape evolution and interface degradation opens a new avenue for studies of degradation and lifetime assessment of TBCs.     

TGO Growth and Crack Propagation in a Thermal Barrier Coating

In thermal barrier coating (TBC) systems, a continuous alumina layer developed at the ceramic topcoat/bond coat interface helps to protect the metallic bond coat from further oxidation and improve the durability of the TBC system under service conditions. However, other oxides such as spinel and nickel oxide, formed in the oxidizing environment, are believed to be detrimental to TBC durability during service at high temperatures. It was shown that in an air-plasma-sprayed (APS) TBC system, postspraying heat treatments in low-pressure oxygen environments could suppress the formation of the detrimental oxides by promoting the formation of an alumina layer at the ceramic topcoat/bond coat interface, leading to an improved TBC durability. This work presents the influence of postspraying heat treatments in low-pressure oxygen environments on the oxidation behavior and durability of a thermally sprayed TBC system with high-velocity oxy-fuel (HVOF)-produced Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat. Oxidation behavior of the TBCs is evaluated by examining their microstructural evolution, growth kinetics of the thermally grown oxide (TGO) layers, and crack propagation during low-frequency thermal cycling at 1050 °C. The relationship between the TGO growth and crack propagation will also be discussed.