Numerical modeling of short crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of microstructurally short inherent cracks within a preoxidized thermal barrier coating system upon thermal shock loading is considered. A thin alumina oxide layer holding residual stresses was induced at the ceramic/metal interface to simulate thermally grown oxide on the bond coat. Undulation of the oxidized bond coat was modeled as a sinusoidal surface. The variations of the stress-intensity factors of inherent centrally located cracks and of edge cracks were calculated during the thermal cycling. The instant crack shapes during the first thermal cycle and at steady state were investigated. It was found that oxide layer thickness, crack tip location, as well as interfacial undulation are factors influencing the risk of crack propagation. It was also found that an edge crack constitutes a greater threat to the coating durability than a central crack. The propagation of an edge crack, if it occurs, will take place during the first load cycle, whereas for a central crack, crack tip position decides the risk of crack propagation.     

Evolution of Lamellar Interface Cracks During Isothermal Cyclic Test of Plasma-Sprayed 8YSZ Coating with a Columnar-Structured YSZ Interlayer

The failure of plasma-sprayed thermal barrier coatings (TBC) usually occurs through spalling of ceramic coating. The crack evolution during thermal cycling of TBC is directly associated with its spalling. In this paper, the cracks in TBC along the direction of the interface between ceramic coating and bond coat were examined from cross-section of TBC experienced different numbers of thermal cycle, and crack number and the total length of cracks were measured to aim at understanding the failure mechanism. TBC consists of cold-sprayed NiCoCrAlTaY bond coat on IN738 superalloy and double layered plasma-sprayed 8YSZ with a columnar grain structured YSZ interlayer of about 20 μm thick and about 230 μm lamellar YSZ. With each isothermal cyclic test, the TBC samples were kept at 1150 °C for 26 min hold and then cooled down to a temperature less than 80 °C in 4 min by air forced cooling. Results showed that cracks propagated primarily within lamellar-structured YSZ over the columnar YSZ along lamellar interface. The measurement from the cross-section revealed that crack number and total crack length apparently increased with the increase of the number of thermal cycle. It was found that cracks with a length less than a typical size of 200 μm accounted for the majority of cracks despite the number of thermal cycle during the test. A crack initiation and propagation model for plasma-sprayed TBC is proposed with a uniform distribution of circular cracks. The propagatable cracks form homogeneously within plasma-sprayed porous YSZ coating at the early stage of thermal cycling and propagate at an identical rate during thermal cycling. Only a few of large cracks are formed before most cracks reach to the critical size for multi-cracks linking-up. The propagation of most cracks to the critical size will leads to the rapid crack bridging and subsequent spalling of top ceramic TBC.     

Long crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of macroscopic long cracks in the ceramic top coat of a thermal barrier coating (TBC) system subjected to thermal shock loading and the influence of the cracks on the coating durability were investigated experimentally and numerically. Thermal shock testing was conducted until coating failure. Comparisons were made with coating samples without macroscopic cracks. The experimental results revealed that the presence of macroscopic cracks reduces the life of the TBC. The finite-element method, with a fracture mechanics approach, was applied to analyze preexisting long cracks, and the calculations correlate well with the experimental findings. It was found that the life of the coating is reduced with crack length as well as with maximum cycle temperature. It was also found that the stress-intensity factors for long cracks are initially high and decrease with the number of temperature cycles, which indicates that rapid crack growth occurs during the first number of cycles.     

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.     

界面粗糙度对热障涂层残余应力和裂纹演化的影响

由于残余应力的作用是造成热障涂层失效剥落的主要因素之一,本工作采用不同幅值的正弦曲线来模拟粗糙度对陶瓷层(TBC)-结合层(BC)界面处残余应力分布的影响;以内聚力模型模拟TBC-BC界面,研究了在外加机械载荷作用下粗糙度对界面裂纹萌生和扩展的影响。结果表明,粗糙度对残余应力分布以及裂纹的形核与扩展有很大的影响。随着粗糙度的增大,陶瓷层和结合层靠近界面的波峰波谷处最大拉/压应力也增大。当施加一定拉伸位移载荷时,最大损伤与裂纹首先在幅值最小的波峰波谷处产生。     

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.     

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.     

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。     

高温拉压环境下DZ125高温合金的热障涂层失效

采用电子束物理气相沉积法（EB-PVD）在定向凝固Ni基高温合金DZ125基体上制备了NiCoCrAlY粘结层和YSZ陶瓷层，研究了高温拉压环境下热障涂层的失效模式，并对其进行了有限元分析。实验结果表明，热障涂层的失效与仅受热载荷作用下的有很大不同，仅有热载荷作用下的热障涂层裂纹多萌生于热氧化层（TGO）内部，进而扩展引起热障涂层的失效。而高温拉压试验后热障涂层体系存在两种裂纹，分别萌生于TGO／粘结层界面和粘结层／扩散层界面附近。有限元模拟结果显示TGO／陶瓷层和TGO／粘结层处存在应力状态的转变和应力值的突变，径向应力的突变导致了界面分离现象的产生，而轴向应力的突变加速了垂直于界面裂纹的扩展，并导致了试样的最终断裂。     

Influence of Thermal Cycle Frequency on the TGO Growth and Cracking Behaviors of an APS-TBC

The durability of thermal barrier coatings (TBCs) is controlled by fracture near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the influence of thermal cycle frequency on the oxidation performance, in terms of TGO growth and cracking behavior, of an air-plasma-sprayed (APS) Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat was studied. The results show that while TGO growth exhibited an initial parabolic growth behavior followed by an accelerated growth stage, higher cycle frequency resulted in a faster TGO growth and a higher crack propagation rate. It is found that a power-law relationship exists between the maximum crack length and the TGO thickness, which is independent of the cycle frequency. This relationship may warrant a TBC life prediction methodology based on the maximum crack length criterion.     

四点弯曲条件下热障涂层断裂模式的声发射表征

采用声发射技术实时监测喷涂态8% Y₂O₃稳定的ZrO₂（8YSZ）在四点弯曲载荷下的损伤断裂行为。采用特征参数分析、聚类分析和小波包变换分析声发射信号结合涂层的微观形貌和应力状态,从而推测出热障涂层系统的失效形式。结果表明：内弯和外弯两种加载模式下,均各有4种失效行为。宏观断裂对应的剥落信号无明显频带,而基底变形、表面垂直裂纹、张开型界面裂纹和剪切型界面裂纹信号对应的主频带可清晰区分为：0~156.25 kHz、156.25~234.375 kHz、312.625~390.625 kHz和390.625~468.75 kHz。热障涂层在外弯载荷下,表面垂直裂纹不断出现,随后扩展到粘结层-陶瓷层界面处并转化为张开型界面裂纹;而在内弯载荷下,则在粘结层-陶瓷层界面附近产生剪切型界面裂纹,仅出现少量的表面垂直裂纹。两种界面裂纹均会引起热障涂层的宏观裂纹和剥落。     

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

The mechanism responsible for the performance of a commercial thermal barrier system upon thermal cycling has been investigated. It comprises an electron beam physical vapor deposited (EB–PVD) yttria-stabilized zirconia thermal barrier coating (TBC) on a (Ni,Pt)Al bond coat. At periodic interfacial sites, the thermally grown oxide (TGO) that forms between the TBC and the bond coat at high temperature displaces into the bond coat with each thermal cycle. These displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO displaces, until those from neighboring sites coalesce. Once this happens, the system fails by large scale buckling. The displacements are accommodated by visco-plastic flow of the bond coat and “vectored” by a lateral component of the growth strain in the TGO. They depend upon the initial morphology of the metal/oxide interface. The observed responses are compared with the predictions of a ratcheting model.     

THE INFLUENCE OF Mo DIFFUSION ON THE THERMAL BEHAVIOR OF TBCs ON Ni_3Al BASED ALLOY IC-6

Conventional two-layered structure thermal barrier coatings (TBCs) were prepared onto γ'-Ni3Al based alloy IC-6 by electron beam physical vapor deposition (EB-PVD). Isothermal oxidation and thermal cycling tests were carried out to investigate the effect of Mo content at the interface between bond coat and ceramic top coat caused by diffusion. It has been found that the alloy coated with TBCs presented the lowest oxidation weight gain value for the reason that the ceramic top coat in TBC system can effectively stop Mo oxides evaporating. The life time of TBCs has close relation with Mo content at the interface between the bond coat and top coat. Spaliation of ceramic top coat occurred during thermal cyclic testing when Mo atoms accumulated at the interface up to certain amount to decline the combination between the bond coat and top coat.     

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.     

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。     

Analysis of thermoelastic characteristics for vertical-cracked thermal barrier coatings through mathematical approaches

Thermoelastic characteristics of thermal barrier coatings (TBCs) with vertical cracks were analyzed through mathematical approaches to investigate the thermoelastic behaviors of TBCs in a service temperature. TriplexPro?-200 system was applied to prepare the relatively dense TBC using METECO 204NS powder. The microstructure of top coat in the TBC was just controlled to create vertical type cracks by reheating without powder feeding in same equipment and rapid cooling process. A couple of governing partial differential equations were derived based on the thermoelastic theory, and a finite volume model was developed to the governing equations to evaluate the thermoelastic characteristics, such as temperature distribution profile, displacement, and stress, inducing a thermal fatigue. For the specimen with two or more vertical type cracks, smaller displacement appears to longitudinal direction and larger displacement to radial direction as the number of crack increases. In the longitudinal stress distribution profiles to z-direction, the tensile stress at the interface between the bond coat and the substrate converts into the compressive stress when the specimen has vertical cracks more than two, while larger magnitude undulation develops for the specimen with smaller number of crack in the radial stress distribution profiles. The results obtained demonstrate that multiple vertical cracks enhance the thermal durability and extend the lifetime of TBCs.     

Pre-oxidation and TGO growth behaviour of an air-plasma-sprayed thermal barrier coating

In thermal barrier coating (TBC) systems, spinel and nickel oxide formed in an oxidizing environment are believed to be detrimental to TBC durability during service at high temperatures. The present study shows that in an air-plasma-sprayed (APS) TBC with Co–32Ni–21Cr–8Al–0.5Y (wt.%) bond coat, pre-oxidation treatments in low-pressure oxygen environments can suppress the formation of the detrimental oxides by promoting the formation of an Al₂O₃ layer at the ceramic topcoat/bond coat interface. The development of the thermally grown oxide (TGO) layer generally exhibits a three-stage growth phenomenon that resembles high temperature creep. The pre-oxidation treatments reduce the growth rate and extend the steady-state growth stage, leading to an improved durability. Crack propagation in the TBC proceeds via opening and growth of pre-existing discontinuities in the ceramic topcoat, assisted by crack nucleation and growth associated with the TGO. Crack propagation during thermal cycling appeared to be controlled by TGO growth, and the maximum crack length and TGO thickness generally have a power law relationship.     

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.     

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.