Fracture mechanics analysis of microcracks in thermally cycled thermal barrier coatings

The effects from thermal shock loading on pre-existing microcracks within thermal barrier coatings (TBCs) have been investigated through a finite element based fracture mechanical analysis. The TBC system consists of a metallic bond coat and a ceramic top coat. The rough interface between the top and bond coats holds an alumina oxide layer. Stress concentrations at the interface due to the interface roughness, as well as the effect of residual stresses, were accounted for. At the eventual closure between the crack surfaces, Coulomb friction was assumed. To judge the risk of fracture from edge cracks and centrally placed cracks, the stress intensity factors were continuously monitored during the simulation of thermal shock loading of the TBC. It was found that fracture from edge cracks is more likely than from centrally placed cracks. It was also concluded that the propagation of an edge crack is already initiated during the first load cycle, whereas the crack tip position of a central crack determines whether propagation will occur.     

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.     

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.     

Laser cycling and thermal cycling exposure of thermal barrier coatings on copper substrates

Thermal barrier coatings (TBCs) are successfully applied in turbines and could also protect combustion chambers in rocket engines. Apart from different loading conditions, the main difference between these applications is the substrate material, which is nickel-based for turbines and copper-based for rocket engines. To optimize the coating system, more knowledge of possible failure modes is necessary.In this work a standard coating system was applied by atmospheric plasma spraying to copper specimens. These specimens were exposed to thermal cycling with different cooling rates and to laser shock testing. A laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3 kW) which provides high heating rates to up to 1500 °C. Laser shock testing has proven to be a suitable alternative to burner rig testing.The results were different to the common failure modes for TBCs on nickel substrates as the coatings system does not fail at the interface between top coat and bond coat, but at the interface between substrate and bond coat. Two failure modes were observed: copper oxide was undermining the coatings at the substrate/bond coat-interface in the case of thermal cycling experiments, and complete delamination occurred at the same interface in the case of laser shock testing. Consequently, this interface is critical in the investigated material system.     

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.     

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.     

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.     

Comparison of thermal shock behaviors between plasma-sprayed nanostructured and conventional zirconia thermal barrier coatings

NiCoCrAlTaY bond coat was deposited on pure nickel substrate by low pressure plasma spraying(LPPS), and ZrO2-8%Y2O3 (mass fraction) nanostructured and ZrO2-7%Y2O3 (mass fraction) conventional thermal barrier coatings(TBCs) were deposited by air plasma spraying(APS). The thermal shock behaviors of the nanostructured and conventional TBCs were investigated by quenching the coating samples in cold water from 1 150, 1 200 and 1 250 ℃, respectively. Scanning electron microscopy(SEM) was used to examine the microstructures of the samples after thermal shock testing. Energy dispersive analysis of X-ray(EDAX) was used to analyze the interface diffusion behavior of the bond coat elements. X-ray diffractometry(XRD) was used to analyze the constituent phases of the samples. Experimental results indicate that the nanostructured TBC is superior to the conventional TBC in thermal shock performance. Both the nanostructured and conventional TBCs fail along the bond coat/substrate interface. The constituent phase of the as-sprayed conventional TBC is diffusionless-transformed tetragonal(t′). However, the constituent phase of the as-sprayed nanostructured TBC is cubic(c). There is a difference in the crystal size at the spalled surfaces of the nanostructured and conventional TBCs. The constituent phases of the spalled surfaces are mainly composed of Ni2.88Cr1.12 and oxides of bond coat elements.     

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。     

Thermal Shock Resistance of APS 8YSZ Thermal Barrier Coatings on Titanium Alloy

Thermal barrier coatings (TBCs) with a typical 8YSZ ceramic top coat and CoNiCrAlY bond coat were deposited on titanium alloy substrate (Ti-6Al-4V in wt.%) by air plasma spraying. Thermal insulation and thermal shock resistance of the TBCs at different temperatures as well as their failure behavior were investigated. The results showed that the test temperature had a significant effect on thermal shock life of the TBCs. Failure of the TBCs systems was caused by the formation of crack, bond coat oxidation and elemental diffusion. The vertical cracks induced by thermal shock cycles were probably responsible for the enhancement in thermal shock resistance of the TBCs. Furthermore, elemental diffusion had a great effect on the acceleration of the TBCs failure. The TBCs could provide a good thermal protection for the titanium alloy substrate.     

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。     

Influence of thermal shock on insulation effect of nano-multilayer thermal barrier coatings

Thermal barrier coatings (TBCs) with nano-multilayer structure were investigated by thermal shock test. The change of insulation effect during thermal shock test was studied by in-situ temperature monitor with a thermal couple set into the substrate. Microstructure and electrical properties of TBCs were characterized by SEM and Impedance Spectroscopy, respectively. Initial increase in insulation effect was observed and related to the formation and growth of perpendicular microcracks in top coat and transversal microcracks in TGO. With thermal shock, the insulation effect decreased due to the further growth of microcracks in top coat and TGO which induced the failure of TBCs.     

The spalling modes and degradation mechanism of ZrO2-8 wt.% Y2O3/CVD-Al2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

State-of-the-art conventional thermal-barrier coatings consist of a thermalinsulating, partially-stabilized ZrO₂ top coat and a bond coat. In this study, a continuous alumina-diffusion-barrier layer was deposited and interposed between the top coat and bond coat by chemical-vapor deposition (CVD). Both the conventional and the experimental TBC systems were cyclically tested at 1000°C, 1050°C, 1100°C, and 1150°C to evaluate and compare oxidation, performance, and fracture behavior. Introduction of the intermediate CVD-Al₂O₃ layer effectively suppressed the oxidation rate of the bond coat and sufficiently altered its oxidation behavior. The thermal-cyclic life of TBCs was improved by the new system. The failure of the ZrO₂-8 wt.% Y₂O₃/CVD-Al₂O₃/Ni-22Cr-10Al-1Y TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, and secondarily along the top coat/CVD-Al₂O₃ interface.     

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.     

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。     

等离子喷涂Al2O3与ZrO2复合热障涂层的高温性能

采用等离子喷涂(PS)方法，在GH536高温合金基材上制备了传统的双层热障涂层(TBCs)和2种含有Al2O3与ZrO2陶瓷复合层的3层热障涂层。传统TBC8结构为Ni22Cr10AlY合金连接层和8％Y2O3部分稳定的ZrO2(8YPSZ)陶瓷顶层；3层TBCs中分别采用置于8YPSZ陶瓷层内层及外层的Al2O3与8YPSZ复合层。3种类型试样的100h。1050℃静态氧化试验及1050℃热震试验结果表明：3层涂层较双层涂层的氧化阻力提高，双层涂层的热震阻力最佳，氧化阻力最差。不同复合层形式试样的热振失效方式和寿命也有差别，复合层置于陶瓷层外层热震寿命略高，但100h氧化增重有明显提高，抗氧化性降低。     

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.     

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.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.     

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.