Simulation of the effect of material properties and interface roughness on the stress distribution in thermal barrier coatings using finite element method

A Finite Element Model (FEM) was developed to evaluate the stresses induced by the thermal cycling in a typical plasma-sprayed thermal barrier coating system (TBCs). The thermo-mechanical model of this multi-layer system takes into account the effects of thermal and mechanical properties, morphology of the top-coat/bond-coat interface and oxidation on the local stresses that are responsible for the micro-crack nucleation during cooling, especially near the metal/ceramic interface.Two top-coat/bond-coat geometries corresponding to different interfacial asperity morphologies (semicircle or sinusoidal) are modeled considering a two dimensional and periodic geometry. The effect of the geometry and the amplitude of asperities on stress distribution are examined to study the cause of the subsequent delamination of the TBCs system. Moreover, the effect of the creep in all layers and plastic deformation in the bond-coat as well as the oxidation in the perpendicular direction of the top-coat/bond-coat interface are examined toward the stress development and critical sites with respect to possible crack paths. In addition, crack initiation and propagation at the system was predicted.     

Measurements of the thermal gradient over EB-PVD thermal barrier coatings

Conventional two-layered structure thermal barrier coatings (TBCs), graded thermal barrier coatings (GTBCs) and graded thermal barrier coatings with micropores were prepared onto superalloy DZ22 tube by electron beam physical vapor deposition (EB-PVD). Thermal gradient of the TBCs was evaluated by embedding two thermal couples in the surfaces of the tube and the top coat at different surrounding temperatures with and without cooling gas flowing through the tube. The results showed that higher thermal gradient could be achieved for the GTBCs with micropores compared to the two-layered structure TBCs and GTBCs. However, after the samples were heated at 1050°C, the thermal gradient for the GTBCs with or without micropores decreased with the increase of heating time. On the other hand, the thermal gradient for the TBCs increased with the increase of heating time. Cross-section observations by scanning electron microscopy showed that the change in microstructure was the main reason for the change of the thermal gradient.     

Modeling of residual stresses variation with thermal cycling in thermal barrier coatings

Thermal barrier coatings (TBCs) are commonly used as protective coatings for engine metal components to improve performance. Many investigations have shown that residual stresses in TBCs applications play an important role, but the residual stresses are mainly obtained by simulation method. As we know, there are a few analytical solutions of residual stress in TBCs system. In this paper, a new two-dimensional analytical solution has been obtained under the condition of non-linear coupled effects of temperature gradient, thermal fatigue, deposited residual stress, thermally grown oxide (TGO) thickening, elasto-plasticity deformation and creep deformation of TBC. Moreover, the influences of bending moment and curvature on stress variation in TBCs are considered during thermal cycling. The calculated results are in agreement with the prior experimental results.     

Effects of oxide thickness, Al2O3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al₂O₃ interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

Analysis of crack nets development in thermal barrier coatings

After a relatively short time in service, components with thermal barrier coatings (TBCs) protection typically develop a system of cracks that propagate from the coated surface toward the interface. Usually these cracks propagate across the thickness of the protective coating and branch along the interface between the coating and the bank metal. The presence of these crack nets is a concern for the durability of the components with TBCs. In the study of thin TBCs by Rubinstein and Tang (Int J Solids Struct 42:5831–5847, 2005), it was found that in a number of cases, these components may still serve for a long time because of crack growth resistance development for cracks growing along the interface, which was found to be the most stable crack path under thermal loading conditions. One of the aims of this study is to determine whether similar fracture resistance is typical for thick TBC coatings as well. The emphases of the analysis presented here are on cases when the coating thickness is comparable to the thickness of the bank material, and on the effect of heat conduction changes due to branching of the developing cracks in a direction parallel to or along the interface. These items were not addressed in sufficient detail in the previous investigations.     

Influence of pores on the thermal insulation behavior of thermal barrier coatings prepared by atmospheric plasma spray

The defects in materials play very important role on the effective thermal conductivity. Especially, the spatial and geometrical characteristics of pores are significant factors for the thermal insulation behavior of thermal barrier coatings (TBCs). In this paper, finite element method was employed to simulate the thermal transfer behavior of TBCs with different spatial and geometrical characteristic of pores. The simulation results indicate that the thermal insulation effect of TBCs would be enhanced when the pore size, pore volume fraction and pore layers which are perpendicular to the thickness direction increase and the space between the adjacent pores decreases. It is predicted that the effective thermal conductivity is different at different directions for the atmospheric plasma spray (APS) TBCs. A novel method, Computational Micromechanics Method (CMM), was utilized to depict the thermal transferring behavior of actual coatings. At the same time, model with different kinds of defects were established, and the effective thermal conductivity as the function of defect orientation angle, defect volume fraction and defect shape coefficient was discussed in detail. The simulation results will help us to further understand the heat transfer process across highly porous structures and will provide us a powerful guide to design coating with high thermal insulation property.     

Adhesive strength of new thermal barrier coatings of rare earth zirconates

La₂Zr₂O₇ (LZ) and La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) as novel candidate materials for thermal barrier coatings (TBCs) were prepared by electron beam-physical vapor deposition (EB-PVD). The adhesive strength of the as-deposited LZ and LZ7C3 coatings were evaluated by transverse scratch test. Meanwhile, the factors affecting the critical load value were also investigated. The critical load value of LZ7C3 coating is larger than that of LZ coating, whereas both values of these two coatings are lower than that of the traditional coating material, i.e. 8 wt% yttria stabilized zirconia (8YSZ). The micro-cracks formed in the scratch channel can partially release the stress in the coating and then enhance the adhesive strength of the coating. The width of the scratch channel and the surface spallation after transverse scratch test are effective factors to evaluate the adhesive strength of LZ and LZ7C3 coatings.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Femtosecond laser machining of single-crystal superalloys through thermal barrier coatings

Femtosecond laser machining of single-crystal superalloys coated with thermal barrier coatings (TBCs) has been investigated. The investigations were carried out in air using a titanium:sapphire laser system (λ = 780 nm) operating at a repetition rate of 1 kHz and delivering individual pulses of 150 fs in duration. The ablation threshold of 7 wt.% yttria stabilized zirconia (7YSZ) has been measured to be 1.52 ± 0.21 J/cm². Microstructural investigations indicated a complete absence of conventional processing defects such as recast layers and microcracking in the vicinity of the machining area. The absence of machining-induced melting or delamination along interfaces of the TBC system demonstrates a significant advantage in comparison with conventional laser machining.     

Bending fatigue failure of atmospheric-plasma-sprayed CoNiCrAlY + YSZ thermal barrier coatings

Bending fatigue failure of conventional atmospheric-plasma-sprayed CoNiCrAlY + ZrO₂–8 wt.% Y₂O₃ thermal barrier coatings with/without the thermally grown oxide layer generated between the bond coat and the top coat was experimentally studied at room temperature. Microscopical and profilometrical characterization of as-received and fractured specimens and a simplified finite element study of cooling thermal stresses show that the same fatigue strength of both the as-coated and the oxidized specimens (i.e. its insensitivity to the presence of the thermally grown oxide) is most likely caused by a preferential through-the-thickness cracking of the thermally grown oxide layer. Moreover, the bond-coat/substrate interface is identified as the weakest part of the studied thermal barrier system under both low and high crack growth rates.     

Finite element simulation of residual stress of double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings using birth and death element technique

In this paper, the residual stress of double-ceramic-layer (DCL) La₂Zr₂O₇/8YSZ thermal barrier coatings (TBCs) fabricated by atmospheric plasma spraying (APS) was calculated by finite element simulation using birth and death element technique. The residual stress was composed of two parts, i.e. the quenching stress and the thermal stress. The simulation results indicated that the surface and the edge of interface are often the positions of stress concentration. The DCL La₂Zr₂O₇/8YSZ has lower residual stress compared with that of the single-ceramic-layer (SCL) 8YSZ TBCs with the same thickness. In addition, the influence of defects on the residual stress has been calculated and discussed using finite element method combined with Computational Micro-Mechanics (CMM). As the DCL TBCs has better thermal insulation effect, sintering resistance ability and lower residual stress compared with that of the SCL 8YSZ at the same time, it was expected to be an ideal candidate material for the application in the future.     

High temperature oxidation interfacial growth kinetics in YSZ thermal barrier coatings with bond coatings of NiCoCrAlY with 0.25% Hf

The results of an experimental study of the high-temperature isothermal oxidation behavior and microstructural evolution in two variations of air plasma sprayed ceramic thermal barrier coatings (TBCs) are discussed in the paper. Two types of TBC specimens were produced for testing. These include a standard and vertically cracked APS. High temperature oxidation was carried out at 900, 1000, 1100 and 1200 °C. The experiments were performed in air under isothermal conditions. At each temperature, the specimens were exposed for 25, 50, 75 and 100 h. The corresponding microstructures and microchemistries of the TBC layers were examined using scanning electron microscopy and energy dispersive X-ray spectroscopy. Changes in the dimensions of the thermally grown oxide layer were determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide layers were compared in the TBC specimens with standard and vertically cracked microstructures.     

Understanding of degradation-resistant behavior of nanostructured thermal barrier coatings with bimodal structure

Nanostructured thermal barrier coatings (TBCs) often provide high degradation resistance, as well as extended lifetime. However, the underlying mechanism has not been fully understood. In this study, the sintering characteristics of nanostructured yttria-stabilized zirconia (YSZ) coatings were investigated, and compared with those of the conventional YSZ coatings. Multiscale characterizations of the changes in microstructures and properties were performed. Results showed that the enhanced high-performance durability was mainly attributed to different sintering mechanisms of lamellar zones and nanozones. Sintering characteristics of the lamellar zones were similar to those of the conventional coatings. Stage-sensitive healing of two-dimensional (2D) pores dominated the sintering behavior of the lamellar zones. However, the differential densification rates between nanozones and lamellar zones of the nanostructured TBCs led to the formation of coarse voids. This counteractive effect, against healing of 2D pores, was the main factor contributing to the retardation of the performance degradation of bimodal TBCs during thermal exposure. Based on the understanding of the performance-degradation resistance, an outlook towards TBCs with higher performances was presented.     

Modeling of micro-crack growth during thermal shock based on microstructural images of thermal barrier coatings

Based on digital image processing theory and finite element mesh generation principle, a methodology is proposed to model the micro-crack growth of thermal barrier coatings (TBCs) during thermal shock with the aid of finite element program. Firstly, a microstructural image of plasma sprayed TBCs is transferred to digital image; secondly, a finite element grid model is generated by thresholding segmentation according to the actual microstructure; finally, based on the finite element grid model, the Tuler–Butcher failure criterion is employed to model the micro-crack growth of TBCs during thermal shock. The numerical simulation result agrees well with the experimental result, and the methodology presented in this paper is found to be effective to model the micro-crack growth.     

Composite ceramics thermal barrier coatings of yttria stabilized zirconia for aero-engines

Composite ceramics thermal barrier coatings(TBCs) are widely used in the aero-engines field due to their excellent thermal insulation, which improves the service life and durability of the inherent hot components. The most typical, successful and widely used TBCs material is yttria stabilized zirconia(YSZ). In this paper, fabrication methods, coating structures, materials, failure mechanism and major challenges of YSZ TBCs are introduced and reviewed. The research tendency is put forward as well. This review provides a good understanding of the YSZ TBCs and inspires researchers to discover versatile ideas to improve the TBCs systems.     

热传导对双陶瓷热障涂层隔热效果影响研究

为了更好地设计双陶瓷热障涂层结构,考察在制备和服役过程中热导率的变化对隔热效果的影响,建立了双陶瓷热障涂层半透明数学模型,采用有限元ANSYS软件模拟了稳态隔热效果.结果表明:顶层陶瓷层的热导率增大降低了隔热效果,且随顶层厚度增加隔热效果降低幅度增大;第2层陶瓷层的热导率增大降低了隔热效果,且随顶层厚度增加隔热效果降低幅度减小;陶瓷层半透明且衰减系数很小时,顶层厚度增加,隔热效果先快速后缓慢增加至不变甚至略有降低,且远低于相同条件下不透明时.顶层陶瓷层热导率变化对隔热效果影响大于第2层陶瓷层.     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

The role of transient oxides during deposition and thermal cycling of thermal barrier coatings

Abstract

High temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC) and a partially yttria stabilised zirconia (PYSZ) thermal barrier coating (TBC), were thermally cycled to failure for three different pre-oxidation treatments performed for 1 h at 1373 K and a partial oxygen pressure (pO₂) of 20 kPa, 100 Pa and 0.1 Pa, respectively. These pre-treatments resulted in the formation of different thermally grown oxide (TGO) layers prior to TBC deposition with respect to the presence of the transient oxides NiAl₂O₄, θ-Al₂O₃, and Y₃Al₅O₁₂ at the TGO surface. The TGO microstructures after TBC deposition and thermal cycling were investigated with a variety of analytical techniques and compared with those after pre-oxidation. For all pre-oxidation treatments, a double-layered TGO developed on the BC during thermal cycling. The TGO adjacent to the TBC consisted of small Zr-rich oxide crystallites embedded in an Al₂O₃ matrix when the TGO surface after pre-oxidation comprised of Y₃Al₅O₁₂ plus α-Al₂O₃. When the TGO surface constituted of θ-Al₂O₃, the Zr-rich oxide crystallites were embedded in a NiAl₂O₄ spinel layer after thermal cycling. Zr was absent in the oxide layer when the TGO surface prior to TBC deposition was composed of NiAl₂O₄ spinel. The TGO contiguous to the BC consisted in all cases of α-Al₂O₃ with Y₃Al₅O₁₂ crystallites. The roughness of the α-Al₂O₃/BC interface increased for a higher density of Y-rich oxide protrusions (i.e. pegs) along this interface.     

Residual stress measurement by Piezospectroscopy of cross sections through thermal barrier coatings

Abstract

Piezo-spectroscopic measurements of the residual stress in the TGO have been demonstrated on cross sections through thermally cycled TBC systems with high spatial resolution (approximately 2 × 2 × 5 µm). The residual stress is perturbed by relaxation at the free surface, but this can be taken into account in an approximate way. This relaxation has a range approximately equal to the YSZ thickness indicating that the YSZ imposes significant mechanical constraint on the TGO despite its low modulus.

The measurements have shown that the non-planar morphology of the TGO induces large deviations from the thermo-elastic equi-biaxial stress expected for a planar TGO. The mean level of compressive residual stress is reduced by relaxation due to bending of the non-planar TGO, in agreement with elastic FEM analysis of sinusoidal TGO morphology. However, the real morphology is not sinusoidal and in some locations the local curvature is extremely high. In these regions the residual stress is observed to become tensile and as high as 1 GPa. The failure mechanism is by nucleation and growth of local damaged regions caused by these tensile stresses (which are evident as low stress regions on analysis through the YSZ) into larger regions that eventually become unstable to large-scale buckling and spalling.