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.     

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.     

Evaluating strength of thermally sprayed Al2O3 on aluminum

We evaluated the strength of thermally sprayed Al₂O₃ on aluminum. The thermally sprayed Al₂O₃ films were processed using low-pressure plasma spraying. The thickness of the thermally sprayed Al₂O₃ was 0.3 mm and 0.7 mm. We arranged a 4-point bending test and a heating test to evaluate the strength of the thermally sprayed Al₂O₃. We also investigated the effect of residual stress on the strength by measuring deformation of the thermally sprayed Al₂O₃ after removing the aluminum substrates. The bending strength was 120 MPa, regardless of thickness. We assumed that the bending strength would be equal to the tensile strength because the thermally sprayed Al₂O₃ films were very thin. A crack was generated at 433 K, regardless of thickness. The thermal stress was 160 MPa when the crack was generated. It was 40 MPa higher than we estimated. We found that the residual stress was compression stress that measured 40 MPa, which contributed to the prevention of the crack generation. We presume that the tensile strength was lower than the thermal stress because the residual stress was reduced by stress-relaxing of the aluminum near the interface in the bending test. The influence of heat-resisting strength is dominant over residual stress. Therefore, strength design should take into account residual stress.     

Impact of the non-homogenous temperature distribution and the coatings process modeling on the thermal barrier coatings system

The present study deals with a numerical investigation of the residual stresses arising during the plasma-sprayed coatings process and their effects on the final stress state of the thermal barrier coatings system (TBCs) during service. A new thermo-mechanical finite element model (FEM) has been designed to function using a non-homogenous temperature distribution. Several phenomena are taken into account in the model such as: residual stresses generated during the spraying of coatings, morphology of the top-coat/bond-coat interface, oxidation at the top-coat/bond-coat interface, thermal mismatch of the material components, plastic deformation of the bond-coat and creep of all layers during thermal cycling. These phenomena induce local stresses in the TBCs that are responsible of micro-crack propagation during cooling and thermal cycling, specifically near the ceramic/metal interface.     

Correlation of processing technique and microstructure of platinum aluminide bond coats with performance of thermal barrier coatings deposited on nickel base superalloy

Abstract

We show that the performance of thermal barrier coating systems is critically dependent upon the processing technique and microstructure of platinum aluminides utilised as bond coats. It is demonstrated by thermal exposure tests at 1150°C in air with 24 h cycling period to room temperature that the average useful life of a coating system employing zirconia–7 wt-% yttria as top coat and alloy MAR M002DS as substrate is increased from 192 to 480 h by replacing a three-layer bond coat aluminised by conventional pack cementation with a two-layer bond coat aluminised by chemical vapour deposition. Before each aluminising process, the superalloy has been electroplated with a platinum layer about 7 μm in thickness. Microstructural characterisation using scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron-probe microanalysis, transmission electron microscopy and X-ray diffraction indicates that the superior performance provided by the two-layer bond coat is related to its higher thermal stability enhancing the adhesion of the thermally grown oxide. However, both coating systems are found to fail by the same mechanism involving loss of adhesion between the thermally grown oxide and bond coat.     

Corrosion resistance of the Al2O3+ZrO2 thermal barrier coatings on stainless steel substrates

Al₂O₃ + ZrO₂ composite thermal barrier coatings (TBCs) were deposited on SUS304 substrates using the gas tunnel type plasma spraying method. Groups of coating samples with different mixing ratios of Al₂O₃ and different thicknesses were respectively obtained. The graded microstructure of the coatings was examined using optical microscopy (OM) and a scanning electron microscope (SEM). It is well known that the thermal oxidation of interfaces is the main reason for the spallation of TBC coatings, because sprayed TBCs contain micropores and microcracks. The anodic polarization characterization of sprayed TBCs provides a way, to some extent, to investigate the mechanism of the interface oxidation. In this research, anodic polarization was performed to investigate the effect of coatings on the corrosion resistance. The results showed that a higher alumina mixing ratio and thicker coatings lead to higher corrosion resistance. The corrosion potential and deactivated corrosion current of the samples were correlated and analyzed according to the coating porosity, because the through pores in the coatings provided the way for the oxidants of the ambient solution to access the interface. The analysis indicated that lowering the porosity and increasing the gradient of coating porosity help lowering the oxidation.     

界面形貌对热障涂层残余应力分布的影响

用ABAQUS有限元软件建立了热障涂层模型,计算得出,在热载荷作用下,界面形貌对热障涂层材料残余应力分布影响很大,σ22主要集中在热生长氧化层和过渡层波峰处,随着热生长氧化层变厚、波长变小、振幅变大,σ22变大,且最大值产生在热生长氧化层/过渡层界面的波峰处.     

Effects of TGO Roughness on Indentation Response of Thermal Barrier Coatings

In this paper, an axisymmetric indentation model is set up to calculate the effects of the roughness of the thermally grown oxide (TGO) layer, which was modeled as a sinusoidal wave, on the indentation response of the thermal barrier coatings. It is found that the amplitude, wavelength, and thickness of the thermally grown oxide layer have obvious influences on the indentation response, while the effect of the indenter position can be neglected. In the top coating layer, residual stress mainly occurs below the indenter and around the nearest two peaks of the thermally grown oxide layer to the indenter. Only when the indentation depth is less than 10% of the thickness of the top coating layer, the influence of TGO roughness on the force versus displacement curves of the indentation can be ignored. Correlating this work with the experimental data from indentation test may lead to improved characterization of the mechanical properties of TBC systems.     

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.     

Finite element analysis of the effects of comers on residual stresses in protective oxide scales

Finite element simulations are used to examine residual thermal stresses and strains in corner regions of protective Al₂O₃ scales on Fe₃Al specimens, both during cooling from oxide formation temperatures and during subsequent thermal cycling. The effects of a corner's radius of curvature and oxide thickness, as well as the impact of aluminide plasticity, are considered. Localized plasticity is found to have a major influence on net deformation and on the magnitude and location of maximum stress. As the ratio of corner curvature to oxide thickness (r_s/t) is reduced, stresses within the oxide corner shift from highly compressive to tensile and the location of the maximum principal stress moves from the substrate to the oxide scale. Based on these stress distributions prior to the development of any flaws, key implications about the tendencies for damage are addressed. The stress evolution during cooling and thermal cycling is presented; these results demonstrate the effects of temperature-dependent material properties. For the material behavior assumed in this study, thermal cycling does not cause significant stress relaxation.     

The role of the surface roughness on the integrity of thermally generated oxide scales. Application to the Al2O3/MA956 system

This work deals with the effect of the metal roughness on the integrity of thermally generated oxide scales. For illustrative purposes, experimental evidence is shown for the alumina forming MA 956 alloy. The experimental results reveal that scale spallation occurs more readily in scales with rough surfaces than in scales with smooth surfaces, preferentially at the crests of the scale profile. In order to explain this feature, the effect of the roughness of both the gas/scale and scale/metal interfaces on the thermal stress distribution was analyzed by the finite element method. This analysis shows that with increasing roughness a gradient of compression stresses develops in the scale, being the maximum value of stresses located near the gas/scale interface. In general, the higher the roughness the higher the difference between the maximum and minimum values of the stresses. However, the average value of the stress distribution through the scale thickness decreases with increasing surface roughness. The effect of a planar gas/scale and a rough scale/metal interfaces was also modelled. In this case, the stress gradient in the scale was found to monotonically increase with increasing roughness although in a lower extension than when a rough gas/scale interface was also considered. On the basis of the experimental results and the stress distribution analyses a sequence of the scale failure during the cooling stage are proposed for both cases. It is concluded that the stress component that is normal to the interface and the shear stress play a key role on the scale integrity.     

SiO2 and Al2O3/SiO2 coatings for increasing emissivity of Cu(In, Ga)Se2 thin-film solar cells for space applications

In this study, optical coatings were investigated as substitutes for the coverglass on flexible thin-film space solar cells. The inherent low emissivity of copper-indium-gallium-diselenide (CIGS) thin-film solar cells was increased using optical coatings for thermal balance in space. Evaporated silicon dioxide (SiO₂) and an additional aluminum oxide (Al₂O₃) coating on the CIGS solar cell increased the emissivity from 0.18 to 0.77. Higher emissivity was realized with the Al₂O₃/SiO₂ double-layer coating than with the SiO₂ single-layer coating. The straightforward double-layer coating gives the CIGS solar cells appropriate radiative properties for keeping the cell within a permissible temperature range in space.     

Y2O3 stabilized ZrO2 thin films deposited by electron-beam evaporation: Optical properties, structure and residual stresses

This paper describes the preparation and the characterization of Y₂O₃ stabilized ZrO₂ thin films produced by electric-beam evaporation method. The optical properties, microstructure, surface morphology and the residual stress of the deposited films were investigated by optical spectroscopy, X-ray diffraction (XRD), scanning probe microscope and optical interferometer. It is shown that the optical transmission spectra of all the YSZ thin films are similar with those of ZrO₂ thin film, possessing high transparency in the visible and near-infrared regions. The refractive index of the samples decreases with increasing of Y₂O₃ content. The crystalline structure of pure ZrO₂ films is a mixture of tetragonal phase and monoclinic phase, however, Y₂O₃ stabilized ZrO₂ thin films only exhibit the cubic phase independently of how much the added Y₂O₃ content is. The surface morphology spectrum indicates that all thin films present a crystalline columnar texture with columnar grains perpendicular to the substrate and with a predominantly open microporosity. The residual stress of films transforms tensile from compressive with the increasing of Y₂O₃ molar content, which corresponds to the evolutions of the structure and packing densities.     

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.     

Y2O3与Gd2O3共掺杂SrZrO3热障涂层材料的热物理性能

采用固相反应法合成了5mol%Y2O3与5mol%Gd2O3共掺杂SrZrO3(Sr(Zr0.9Y0.05Gd0.05)O2.95,SZYG)粉末.采用X射线衍射(XRD)和差示扫描量热仪(DSC)分别研究了SZYG粉末在1450℃长期热处理后以及200~1400℃范围内的相稳定性.采用高温热膨胀仪测量了SZYG块材的热膨胀系数,结果表明:通过Y2O3与Gd2O3共掺杂改性可以明显抑制SrZrO3的相转变.在1000℃下SZYG块材的热导率是～1.36 W/(m.K),与SrZrO3和8YSZ块材相比降低~35%SZYG分别与8YSZ和Al2O3在1250℃热处理24 h表现出很好的化学相容性.     

Microstructural characterization and properties of ZrO2/Al2O3 thermal barrier coatings by gas tunnel-type plasma spraying

Spraying condition plays an important role in the plasma-sprayed coating process and affects the final properties of the coatings. Zirconia, alumina and zirconia/alumina composite coatings were prepared on a stainless-steel substrate (SUS304) by the gas tunnel-type plasma spraying. Effects of different alumina mixing ratios on the coating properties were investigated. The results indicated that the mixing ratio of powders and the traverse number of substrate had an influence on the hardness, porosity and wear weight loss of composite coatings. The hardness increased while the porosity decreased with the increase in alumina mixing ratio. The porosity that was less than 10% and a hardness about Hv=1400 was obtained for the alumina coating. The adhesive strength and wear weight loss of the composite coatings were also clarified at different alumina mixing ratios.     

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.     

Influence of residual water on magnetron sputter deposited crystalline Al2O3 thin films

The effects of residual water on the phase formation, composition, and microstructure evolution of magnetron sputter deposited crystalline alumina thin films have been investigated. To mimic different vacuum conditions, depositions have been carried out with varying partial pressures of H₂O. Films have been grown both with and without chromia nucleation layers. It is shown that films deposited onto chromia nucleation layers at relatively low temperatures (500 °C) consist of crystalline α-alumina if deposited at a low enough total pressure under ultra high vacuum (UHV) conditions. However, as water was introduced a gradual increase of the γ phase content in the film with increasing film thickness was observed. At the same time, the microstructure changed drastically from a dense columnar structure to a structure with small, equiaxed grains. Based on mass spectrometry measurements and previous ab initio calculations, we suggest that either bombardment of energetic negative (or later neutralized) species being accelerated over the target sheath voltage, adsorbed hydrogen on growth surfaces, or a combination of these effects, is responsible for the change in structure. For films containing the metastable γ phase under UHV conditions, no influence of residual water on the phase content was observed. The amounts of hydrogen incorporated into the films, as determined by elastic recoil detection analysis, were shown to be low. Overall, the results demonstrate that residual water present during film growth drastically affects film properties, also in cases where the hydrogen incorporation is found to be low.     

Deformation and damage in Al/Al2O3

Al/Al₂O₃-composites as an example of light weight materials are very interesting for many industrial applications because of a favourable combination of low density and improved mechanical properties. The prediction of the macroscopic mechanical behaviour of these materials related to their microstructure requires the knowledge of damage initiation and crack development under external loading conditions and, if present, residual stresses have to be taken into consideration. Different types of material degradation like particle/matrix-debonding, particle failure and matrix cracking can be observed in these types of metal matrix composites. The aim of the present work is to introduce damage criteria into a FE-microstructural model in order to foresee the degradation process in an Al₂O₃-particle reinforced Al(6061) composite during mechanical loading. Presently, the conventional fabrication route of the Al/Al₂O₃-composite is a metallurgical method with extrusion for homogenisation of the microstructure and final heat treatment to achieve a defined precipitation state. The influence of thermal residual stresses due to cooling down from annealing temperature on the deformation and damage initiation of Al(6061)/10vol%Al₂O₃-composites is investigated through finite element analyses using the experimentally mapped real microstructure as binary data. Especially, the stresses in the ceramic particles which are responsible for particle cracking and the hydrostatic stresses in the Al-matrix making the particle–matrix interface prone to debonding were analysed. The results show the importance of the thermal residual stresses with respect to damage criteria as obtained by micromechanical FE-calculations.     

Influence of Al2O3 particle pinning on thermal stability of nanocrystalline Fe

Second-phase particle pinning has been well known as a mechanism impeding grain boundary (GB) migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline (NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al₂O₃, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al₂O₃ nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures (T_ann) for different time periods (t_ann). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al₂O₃ nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al₂O₃ particles on the NC Fe grains.