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.     

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.     

A physics-based life prediction methodology for thermal barrier coating systems

A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC’s morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents’ elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.     

Delamination resistance of thermal barrier coatings containing embedded ductile layers

Micromechanical models are developed to explore the effect of embedded metal layers upon thermal cycling delamination failure of thermal barrier coatings (TBCs) driven by thickening of a thermally grown oxide (TGO). The effects of reductions in the steady-state (i.e. maximum) energy release rate (ERR) controlling debonding from large interface flaws and decreases in the thickening kinetics of TGO are investigated. The models are used to quantify the dependence of the ERR and delamination lifetime upon the geometry and constitutive properties of metal/TBC/TGO multilayers. Combinations of multilayer properties are identified which maximize the increase in delamination lifetime. It is found that even in the absence of TGO growth rate effects, the delamination lifetime of TBC systems with weak TGO/bond coat interfaces can be more than doubled by replacing 10–20% of the ceramic TBC layer with a metal whose ambient temperature yield stress is in the ～100–200 MPa range.     

A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system.: Part I: model formulation

The effect of the oxidation induced degradation of a typical plasma-sprayed thermal barrier coating (PS-TBC) system on the local ceramic–metal interfacial stresses responsible for the nucleation of mesoscopic cracks is investigated. A coupled oxidation-constitutive approach is proposed to describe the effect of the phase transformations caused by local internal and external oxidation processes on the constitutive behaviour of the metallic coating. The coupled constitutive framework is implemented into the finite element method and used in parametric studies employing periodic unit cell techniques. The effects of service, microstructural and ceramic–metal interface parameters on the peak interfacial stresses during service and cooling to room temperature are quantified. The results of the parametric unit cell FE analyses revealed a strong dependency of the local stresses responsible for mesoscopic crack nucleation and growth on the local morphology of the oxidised interface, the sintering of the ceramic coating, stress relaxation effects due to creep, the thickness of the thermally grown oxide (TGO), and the applied mechanical loads. When no mechanical straining of the TBC system is considered, local tensile stresses normal to the coating surface within the ceramic top coating reach values of up to 330 MPa at room temperature for a critical TGO thickness of approx. 3 μm.     

Thermal shock testing of thermal barrier coating/bondcoat systems

Various methods of thermal shock testing are used by aircraft and industrial gas turbine engine (IGT) manufacturers to characterize new thermal barrier coating systems in the development stage as well as for quality control. The cyclic furnace oxidation test (FCT), widely used in aircraft applications, stresses the ceramic/bondcoat interface, predominantly through thermally grown oxide (TGO) growth stress. The jet engine thermal shock (JETS) test, derived from a burner rig test, creates a large thermal gradient across the thermal barrier coating (TBC), as well as thermomechanical stress at the interface. For IGT applications with long high-temperature exposure times, a combination of isothermal preoxidation and thermal shock testing in a fluidized bed reactor may better represent the actual engine conditions while both types of stress are present. A comparative evaluation of FCT, JETS, and a combined isothermal oxidation and fluidized bed thermal shock test has been conducted for selected ceramic/bondcoat systems. The results and the failure mechanisms as they relate to the TBC system are discussed. A recommendation on the test method of choice providing best discrimination between the thermal shock resistance of the ceramic layer, the ceramic/bondcoat interface, and even substrate related effects, is given. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 520–29.     

重型燃机喷嘴壳体及遮热板热障涂层剥落机制

目的 探究重型燃机喷嘴壳体及遮热板热障涂层剥落机制,为该部件的全寿命管理提供参考。方法 采用等离子喷涂方法,分别制备以06Cr25Ni20不锈钢和Hastelloy X合金为基材的热障涂层试验件,并结合水淬热冲击表征方法与瞬态热力耦合仿真方法,表征热障涂层水淬后的剥落状态,获得热障涂层残余剪应力的分布状态随基材和服役工况的变化行为,揭示热障涂层在多层热失配工况下的剥落机制。结果 在水淬热冲击条件下,2种不同基材的热障涂层试验件表现出类似的剥落行为,但由于基材热膨胀系数的差异,以06Cr25Ni20不锈钢为基材的热障涂层的残余剪应力(70.1 MPa)比Hastelloy X合金基材的热障涂层(52.7 MPa)更大,热冲击寿命更短。在梯度温度载荷下,2种不同基材热障涂层试验件的失效模式不同,前者的最大残余剪应力为39.2 MPa,后者为25.7 MPa。结论 在2种温度载荷下,以Hastelloy X合金为基材的热障涂层具有较低的残余应力和较长的服役寿命。此外,水淬热冲击可以快速表征热障涂层的寿命行为,但其失效模式与实际梯度温度载荷下的失效模式仍有一定区别。     

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.     

金属/陶瓷发热体直接钎焊接头的应力分析

采用热弹塑性有限元方法,在考虑了材料性能参数随温度变化的情况下,分析了采用Ag-Cu-Ti钎料钎焊Al2O3陶瓷与镍金属丝的钎焊接头,在钎焊和随后再次加热过程中产生的应力大小和分布情况,计算中着重考虑了钎料对接头残余应力的影响.结果表明,在钎料与陶瓷的界面处存在着较大的残余拉应力,影响了钎焊接头的连接强度,并可能在界面的陶瓷侧产生裂纹.通过试验对比,认为在此类连接结构中,钎料是造成接头形成较大残余应力的主要因素.并指出钎料性能参数是决定有限元计算精度的主要因素,要使计算结果与实际情况尽量符合,钎料性能参数的正确选择是关键.     

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.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Effect of surface preparation on the durability of NiCoCrAlY coatings for oxidation protection and bond coats for thermal barrier coatings

A principal concern with alumina‐forming coatings for high‐temperature oxidation protection and bond coats (BCs) for ceramic thermal barrier coatings (TBCs) used in gas turbines is the spalling of the alumina scales during service. This paper describes the effects of BC surface preparation on the durability of NiCoCrAlY coatings exposed under thermal cycling conditions. State‐of‐the‐art TBC systems deposited by electron beam physical vapor deposition (EBPVD) with NiCoCrAlY overlay BCs were found to fail as the result of defects which included transient oxides, defects in the BC surface, defects in the as‐deposited microstructure of the TBC, and excessive oxidation of reactive element additions. In some instances, the TBC life was greatly extended by surface treatments, such as fine polishing. The oxidation behavior of NiCoCrAlY coatings, absent a TBC, was found to be sensitive to Y content and to surface preparation. This paper describes how a variety of surface treatments affected coating lives and failure mechanisms.     

Thermal fracture mechanisms in ceramic thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) represent an attractive method of increasing the high-tempera-ture limits for systems such as diesel engines, gas turbines, and aircraft engines. However, the dissimilari-ties between ceramics and metal, as well as the severe temperature gradients applied in such systems, cause thermal stresses that can lead to cracking and ultimately spalling of the coating. This paper reviews the research that has considered initiation of surface cracks, initiation of interfacial edge cracks, and the effect of a transient thermal load on interface cracks. The results of controlled experiments are presented together with analytical models. The implications of these findings to the differences between diesel en-gines and gas turbines are discussed. The importance of such work for determining the proper design cri-teria for TBCs is underlined.     

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。     

High-temperature oxidation and hot corrosion behavior of two kinds of thermal barrier coating systems for advanced gas turbines

High-temperature oxidation and hot corrosion tests were conducted at 800 to 1100 °C under isothermal and thermal-cycle conditions for two kinds of thermal barrier coating (TBC) systems with different compositions of ceramic top coat: Y₂O₃-stabiIized zirconia (YSZ) and CaO-SiO₂-ZrO₂ (C₂S-CZ). Qualitative and quantitative failure analyses were carried out to clarify the failure mechanisms of TBC systems. In high-temperature oxidation up to 1100 °C, the YSZ-TBC system was subjected more easily to spalling of the ceramic top coat. This is attributed to the localized oxidation along the ceramic top coat/metallic (NiCrAlY) bond coat interface, as compared with the case of the C₂S-CZ-TBC system. Thus, the most significant oxidation damage resulted in the YSZ system under the thermal-cycle condition. On the other hand, for hot corrosion by Na₂SO₄-NaCI molten salt up to 1000 °C, the C₂S-CZ system was more reactive with the molten salt to form a new phase layer composed of both the metallic bond coat constituents, such as aluminum and chromium, and corrosive species such as oxygen at the inner region of the ceramic top coat. Furthermore, effects of both the heat treatment, in particular the atmosphere after plasma spraying, and the chromium content of the bond coat were investigated for each coating system.     

Estimation of spallation life of thermal barrier coating of gas turbine blade by thermal fatigue test

Plasma-sprayed thermal barrier coatings (TBCs) are applied to protect the blades of a gas turbine system from high-temperature gas and to lower the surface temperature of the blades. The failure of TBC is directly connected to the failure of the blades because the spallation of a ceramic layer leads to the acceleration of local corrosion and oxidation at the location of failure. Therefore, the spallation life of TBC is very important in the evaluation of the reliability of a gas-turbine blade.In this study, thermal fatigue tests were performed at 1100 °C and 1151 °C. Then, c-scanning and bond strength tests were performed for TBC specimens that were thermally aged by thermal fatigue tests. From the results, an empirical equation based on the ratio of the delamination area and the thermal cycle number was presented and the spallation life of a TBC specimen could be roughly estimated using the relationship between the delaminated area and the number of cycles.     

Finite element analysis of stress distribution in thermal barrier coatings

A numerical simulation of crack development within APS TBC systems is presented. The TGO thickening and creep deformation of all system constituents is modelled. Two dimensional periodic unit cell is used to examine the effect of interfacial asperity on stress distribution and subsequent delamination of APS TBC. A study of cyclic loading and of creep of the base material on the stress distribution close to the asperity at the TGO/BC interface is made, revealing a small in?uence influence of both on the stress state in the thermal barrier coating system subjected to temperature loading. Cohesive zone elements at the oxide/ceramic interface model the development of the interfacial micro-crack. The finite element analysis shows that the development of the interfacial crack allows for a micro-crack formation within APS TBC. Subsequent TGO growth results in a tensional zone within the oxide layer. Linking of the micro-cracks at the interface and within TBC through TGO could lead to a coating delamination in the unit cell.     

Investigation of residual stress effects in an alloy reinforced ceramic/metal composite

This study aims at investigating the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al₂O₃/A356 CMCs composites with an interpenetrating network structure and varying metal content, ranging from 10 to 40 vol.%, were produced using the pressure infiltration technique of Squeeze casting. Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. Residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic/metal interface of the Al₂O₃/A356 composites. The relationship of residual stresses and the contact area of the ceramic/metal interface are also discussed.     

基于遗传算法的热障涂层寿命微观影响因素

热障涂层寿命受到界面波长和幅值等微观因素的影响,但对其影响机制并不清楚。首先,基于Manson-Coffin公式和累计损伤理论,建立热障涂层寿命预测模型,并将拟合问题转化为优化问题,采用遗传算法求解寿命模型中的系数。然后,基于涂层试验数据建立热障涂层二维轴对称有限元模型,研究并确定可用于准确预测涂层寿命的应力应变信息类型。最后,采用响应面法选取陶瓷层厚度、黏结层厚度、界面波长和幅值作为影响因素,开展涂层寿命的微观影响因素研究。结果表明,使用循环等效应变范围进行涂层寿命预测的最大误差和平均误差最小,分别为50%和21%;涂层寿命随陶瓷层厚度的增加略微上升,随黏结层厚度的增加先下降后上升,随界面波长的增加先上升后下降,随界面幅值的增加而下降,且界面幅值对涂层寿命的影响最大;最优组合的涂层寿命为947次循环,与初始值相比提高了163.1%。给出不同涂层厚度下使涂层寿命达到极值的波长与幅值选择公式,研究成果可为热障涂层的寿命预测和结构优化设计提供方法与理论指导。