A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate

The sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured.¹     

等离子喷涂ZrO2热障涂层的抗热震性能研究

采用等离子喷涂方法在304不锈钢表面喷涂NiCoCrAIY2O3+(ZrO2+7%Y2O3)陶瓷热障涂层,模拟航空发动机涡轮叶片工作环境,研究涂层在加热和空气冷却条件下的抗热震性能.结果表明:喷涂后的涂层内部产生大量微观裂纹,随着热震次数的增加,陶瓷层内部的纵向微观裂纹通过大孔隙的连通方式逐渐在涂层内部扩展.ZrO2陶瓷喷涂后涂层主要以四方相和部分立方相组成,当涂层在热震40次时,陶瓷中的部分四方相向单斜相转变,并伴随着体积的变化,易在涂层中形成新的裂纹并加快原有裂纹的扩展.     

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating system

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating (TBC) system was investigated. The impedance spectrum of the TBC system included impedance of the yttria stabilized zirconia (YSZ) grains, YSZ grain boundaries (negligible), the thermally grown oxide (TGO) and the electrode reaction. In the TBC system with a continuous TGO layer, the impedance of the YSZ grain was measured without influence of asymmetric electrode geometry above 100 kHz. In a frequency range below 100 kHz, asymmetric electrode geometry induced the spread of an electrical conduction region outside an electrode attached on the top coating surface. The impedance of the TGO and the electrode reaction was significantly affected by the asymmetric electrode geometry. The precise interpretation of an impedance spectrum of TBC systems measured under the condition of asymmetric electrode geometry requires further studies on the spread of an electrical conduction region due to decrease in frequency, increase in asymmetry of electrode geometry.     

A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings

A sintering model is presented for prediction of changes in the microstructure and dimensions of free-standing, plasma-sprayed (PS) thermal barrier coatings (TBCs). It is based on the variational principle. It incorporates the main microstructural features of PS TBCs and simulates the effects of surface diffusion, grain boundary diffusion and grain growth. The model is validated by comparison with experimental data for shrinkage, surface area reduction and porosity reduction. Predicted microstructural changes are also used as input data for a previously developed thermal conductivity model. Good agreement is observed between prediction and measurement for all these characteristics. The model allows separation of the effects of coating microstructure and material properties, and captures the coupling between densifying and non-densifying mechanisms. A sensitivity analysis is presented, which highlights the importance of the initial pore architecture. Predictions indicate that the microstructural changes which give rise to (undesirable) increases in thermal conductivity and stiffness are very sensitive to surface diffusion.¹     

A chromia forming thermal barrier coating system

Conventional thermal barrier coating (TBC) systems consist of an insulating ceramic topcoat, a bond coat for oxidation protection and the underlying superalloy designed to combat the oxidising conditions in aero‐ and land‐based gas turbines. Under high‐temperature oxidation, the use of an alumina forming bond coat is warranted, thus all current TBC systems are optimised for the early formation of a dense, protective thermally grown oxide (TGO) of alumina. This also offers protection against Type I hot corrosion but a chromia layer gives better protection against Type II corrosion and intermediate temperatures, the conditions found in land‐based gas turbines. In this paper the authors present the first known results for a chromia forming TBC system. Tests have been performed under oxidising conditions, up to 1000 h, at temperatures between 750 °C and 900 °C, and under Type I (900 °C) and Type II (700 °C) hot corrosion conditions up to 500 h. Under all these conditions no cracking, spallation or degradation was observed. Examination showed the formation of an adherent, dense chromia TGO at the bond coat / topcoat interface. These initial results are very encouraging and the TGO thicknesses agree well with comparable results reported in the literature.     

Damage mechanisms and lifetime behavior of plasma-sprayed thermal barrier coating systems for gas turbines — Part II: Modeling

A phenomenological lifetime prediction tool for plasma-sprayed ZrO₂ based thermal barrier coating systems with MCrAlY bondcoat is presented. The analytical model uses a two step approach for calculating the development of delamination cracks: The initial crack growth is considered to be proportional to the thickness of the thermally grown oxide (TGO) scale on the bondcoat. After exceeding a critical TGO thickness, crack propagation is governed by linear elastic fracture mechanics taking into account stresses induced by thermal mismatch and by TGO growth. Validation using experimental data from thermal cycling tests with high temperature dwell times from few seconds up to 96 h gave evidence of the good predictive quality of the model.     

An analytical model for simulation of heat flow in plasma-sprayed thermal barrier coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.     

Characterization of microstructural defects in plasma-sprayed thermal barrier coatings

Thermal barrier coatings with a NiCrAlY bond coating and a 1.5 mm thick zirconia top coating were air plasma sprayed onto a nickel-base substrate. The top coatings were deposited with the same spraying parameters except for the amount of external cooling, which varied from no cooling to the maximum available. This resulted in four sets of samples produced with different cooling conditions where substrate temperature varied from 100 to 830 °C. The coatings were examined by electron microscopy on polished surfaces and on fracture surfaces. The crack structure in the top coating was correlated to the substrate temperature. The density both of horizontal delaminations and of vertical microcracks was shown to decrease at higher substrate temperatures. The grain structure was columnar, and smaller grains were found at lower temperatures. Explanations for the differences in defect densities are discussed.     

Influence of dopant on the thermal properties of two plasma-sprayed zirconia coatings Part I: Relationship between powder characteristics and coating properties

Plasma- sprayed coatings produced with two zirconia powders (− 90 + 10 μm, spray dried and partially sintered) that were stabilized (9 wt %) with dysprosia (DSZ) and ytterbia (YbSZ) were compared to coat-ings sprayed with a yttria (7 wt %) stabilized zirconia (YSZ) powder (45 + 22 μm, fused and crushed). The YSZ particles in the coating were almost fully molten (less than 0.2 % monoclinic m- phase), with excellent contact between the layered splats (adhesion of 54 MPa). The DSZ particles were only partially melted (3.1 % m- phase), with coating adhesion greater than 34 MPa; the YbSZ particles were less melted (6.1 % m- phase), with coating adhesion of 27 MPa. The thermal properties (diffusivity, a; specific heat, c_p; and thermal conductivity, κ) of the coatings were about the same. Under thermal cycling (1 h heating at 1100 °C in a furnace followed by fast cooling for approximately 3 min by air jets) of the coatings sprayed on FeCrAl alloy manufactured by powder metallurgy, the behavior of the DSZ coating was simi-lar to that of the YSZ, whereas the YbSZ coating was partially detached. However, in all cases the percent-age of the monoclinic phase decreased and the ratio of the hexagonal structure increased to 1.013 of the nontransformable tetragonal phase t′.     

NDE of degradation of thermal barrier coating by means of impedance spectroscopy

Nondestructive evaluation of thermal barrier coating (TBC) degradation during service operation has received a wide attention for service life prediction of advanced gas turbines. In this work, TBC on nickel base superalloy degraded at various degrees by thermal aging is investigated. Particularly, TBC and Metal–Chromium–Aluminum–Yttrium alloy (McrAlY) (M indicates iron (Fe), cobalt (Co), nickel (Ni) or these combinations) bond coating interface where a detrimental reaction takes place and forms various reaction products during thermal aging, is studied in detail. Formation kinetics and physical properties of the reaction layer are evaluated by means of Impedance Spectroscopy (IS). Specimens aged at higher temperature and a longer aging time shows larger impedance. The impedance behavior of TBC and bond coating is found to be markedly changed by the formation of the reaction layer. Physical properties and thickness of the reaction layer are evaluated using the change of the impedance. A mechanistic interpretation of the cause of change in the physical properties and IS behavior are described.     

Damage mechanisms and lifetime behavior of plasma sprayed thermal barrier coating systems for gas turbines—Part I: Experiments

Detailed damage analyses of a plasma sprayed ZrO₂/8 wt.-% Y₂O₃-MCrAlY-CMSX-4 TBC system during isothermal and cyclic oxidation tests with different dwell times at high temperature have been performed. The resulting failure mode, i.e. the particular delamination crack path, is strongly dependent on the temperature cycle applied. Isothermal exposure promotes crack propagation within the TGO, whereas thermal cycling shifts the crack path towards the TBC. Thermal cycling with dwell time at high temperature leads to a mixed delamination crack path (partly within TBC and TGO). The respective correlation between TBC lifetimes and duration of high temperature dwell time per cycle (cycle frequency) is shown and discussed.     

Thermal conductivity of a zirconia thermal barrier coating

The conductivity of a thermal-barrier coating composed of atmospheric plasma sprayed 8 mass percent yttria partially stabilized zirconia has been measured. This coating was sprayed on a substrate of 410 stainless steel. An absolute, steady-state measurement method was used to measure thermal conductivity from 400 to 800 K. The thermal conductivity of the coating is 0.62 W/(m×K). This measurement has shown to be temperature independent.     

Anisotropic thermal conductivities of plasma-sprayed thermal barrier coatings in relation to the microstructure

Anisotropic thermal conductivities of the plasma-sprayed ceramic coating are explicitly expressed in terms of the microstructural parameters. The dominant features of the porous space are identified as strongly oblate (cracklike) pores that tend to be either parallel or normal to the substrate. The scatter in pore orientations is shown to have a pronounced effect on the effective conductivities. The established quantitative microstructure-property relations, if combined with the knowledge of the processing parameters-resulting microstructure connections, can be utilized for controlling the conductivities in the desired way.     

钛合金表面等离子喷涂Y2O3稳定的ZrO2涂层的 扫描热显微镜分析

以纳米结构Y2O3稳定的ZrO2热喷涂粉末为原料,采用等离子喷涂法在Ti-6Al-4V合金上制备了纳米结构的热障涂层。利用扫描电镜(SEM)及扫描热显微镜(SThM)对涂层的微观组织及热性能进行了分析。在实验基础上建立了理论模型,并对涂层及基体的热导率进行了估算。结果表明:采用SThM分析方法估算的涂层厚度及涂层上的缺陷尺寸与采用其它分析方法测得的结果一致;虽然热导率的估算结果与采用其它方法得出的结果差异较大,但显示出扫描热显微镜分析是估算材料热导率潜在的方法。     

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.     

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.     

High sintering resistance of a novel thermal barrier coating

Sintering resistance of a novel thermal barrier coating Nd_xZr_1 − xO_y with Z dissolved in, where 0 < x < 0.5, 1.75 < y < 2 and Z is an oxide of a metal selected from Y, Mg, Ca, Hf and mixtures thereof, was studied. The coatings of Nd_xZr_1 − xO_y and typical 7YSZ were deposited by electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). The samples with the coating system of EB-PVD Nd_xZr_1 − xO_y or 7YSZ overlaid onto a MCrAlY bond coat were cyclically sintered at 1107 °C for 706 hours. The freestanding coatings of EB-PVD Nd_xZr_1 − xO_y and 7YSZ were isothermally sintered at 1371 °C for 500 hours. The microstructure of EB-PVD Nd_xZr_1 − xO_y before and after the sintering was evaluated and compared with EB-PVD 7YSZ. The sintering resistance of freestanding APS Nd_xZr_1 − xO_y coating was also investigated after isothermal sintering at 1200 °C for 50 and 100 hours. The results demonstrated that the new coatings of Nd_xZr_1 − xO_y applied with both EB-PVD and APS have higher sintering resistance than EB-PVD and APS 7YSZ, respectively.     

Evaluation of microhardness,fracture toughness and residual stress in a thermal barrier coating system: A modified Vickers indentation technique

The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67 MPa m^1/2, and the corresponding residual stress near the indented region varies from − 36.8 to − 243 MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81 MPa m^1/2, and residual stress varies from − 5 to − 30 MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.     

Fracture toughness measurements of plasma-sprayed thermal barrier coatings using a modified four-point bending method

In this study, the adhesion strength of thermal barrier coatings 8YSZ (ZrO₂ + 8 wt.% Y₂O₃) deposited on NiCrAlY bond coats by atmospheric plasma spraying is investigated experimentally. A modified four-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing. The fracture surfaces were examined using a scanning electron microscope. Images show that cracks are initiated along YSZ/NiCrAlY interfaces, then kink and grow uniformly within the YSZ layer. The load-displacement curves obtained indicate three distinct stages in crack initiation and stable crack growth. Based on a microstructural model, finite element analyses were performed to extract the bonding strength of the thermal barrier coatings. The fracture toughness of the plasma-sprayed 8YSZ coatings, in terms of critical strain energy release rate G_c, can be reliably obtained from an analytical solution or from a numerical simulation of the cracking process using compliance methods.     

The effect of a high thermal gradient on sintering and stiffening in the top coat of a thermal barrier coating system

Superalloy substrates coated with plasma-sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats (TCs) have been subjected to a high heat flux under a controlled atmosphere. The sintering exhibited by the TC under these conditions has been studied and compared with the behavior observed during isothermal heating. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both the in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical model has been used to explore the stress state under isothermal and thermal gradient conditions. Dilatometry data indicate significant linear contractions during holding at elevated temperatures, particularly in the through-thickness direction. This is largely attributed to microstructural changes associated with sintering, with any volume changes due to phase transformations making relatively small contributions. Sintering proceeds faster at higher temperatures but is retarded by the presence of tensile stresses (from differential thermal expansion between the coating and substrate) within the TC. Thus, it occurs preferentially near the free surface of the TC under gradient conditions, not only due to the higher temperature, but also because the in-plane stress is more compressive in that region.