Thermal Barrier Coating System Research Articles

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.

PVD thermal barrier coating systems for Mo–Si–B alloys

Oxidation protective layers with chemical compositions of Mo–70Al, Mo–46Si–24B, Mo–37Si–15B and Mo–47Si–24Al (at.-%) were deposited on Mo–9Si–8B specimens by magnetron sputtering. After pre-oxidation of the coated samples, ceramic topcoats of yttria partially stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were applied using electron-beam physical vapour deposition. Both as-deposited YSZ and GZO topcoats exhibited good adhesion to the pre-oxidised bond coats. The different thermal barrier coating (TBC) systems were exposed to air at 1000 °C for periods between 20 and 100 h. The YSZ topcoat was tightly-adherent to the borosilicate scale grown on the Mo–46Si–24B bond coat after 20 h of exposure. Similar results were obtained for GZO topcoats deposited on Mo–46Si–24B and Mo–37Si–15B bond coats. The TBC system consisting of GZO topcoat and Mo–47Si–24Al bond coat, which formed a mixed scale of silica and mullite-like oxides, survived 100 h at 1000 °C. However, after this exposure time, the bond coats were approaching their lifetime due to the low layer thickness (5–10 μm). Oxidation of the Mo–Si–B substrate at unprotected areas around the suspension hole of the samples caused severe deterioration of the Mo–70Al bond coat and substantial degradation of the outer region of the GZO topcoat due to chemical reactions with MoO3.

Processing thermal barrier coatings via sol-gel route: Crack network control and durability

Thermal barrier coatings (TBC) processed by sol–gel route are deposited onto NiPtAl bond coated superalloy substrates. A crack microstructure, if well controlled, is adequate to get satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. This paper deals with the adjustment of the properties of the micro-cracked network which is inherent to the process by changing the formulation of the sol and by adding a reinforcement step. The objective is to reduce the size and depth of the surface cracks network. This network controls the release of thermo-mechanical stress in the layers and reduces detrimental propagation of cracks that could result in the spallation of the coatings during engine operation. Several physico-chemical characterizations were performed, associated to image analyses to (i) evaluate the cracks distribution (depth, length and width), in the case of two dispersants, and (ii) to estimate their influence on the performances of TBC systems. Characterizations by cyclic oxidation were carried out using a cyclic oxidation instrumented rig to monitor on a real time basis the crack propagation and spallation. Correlations between the cracked network parameters and the lifetime of the TBC are proposed in this paper.

Bending-driven failure mechanism and modelling of double-ceramic-layer thermal barrier coating system

Bending-driven failure test is an effective and efficient method to evaluate the quality and load-bearing capacity of a thermal barrier coating (TBC) system. In this study, the failure mechanism of a double-ceramic-layer TBC (DCL-TBC) system was investigated experimentally and analytically. First, a series of in-situ four-point bending tests were conducted on DCL-TBC system with the two ceramic layers having different thickness ratios. A cracking phenomenon was observed, i.e. vertical cracks (visible on the surface) and interfacial cracks (invisible unless sectioned) simultaneously evolved, from which a concept of critical vertical crack density associated with interfacial crack propagation was proposed to evaluate the bending-driven failure of DCL-TBC system. Second, an analytical model was developed to predict the critical crack density, which was based on a modified shear-lag model and interfacial fracture mechanics. The analytical prediction agreed well with the experimental results. Third, the analytical model was used to establish the failure maps of DCL-TBC system with respect to a series of physical and geometrical parameters in dimensionless form, considering both multiple surface cracking (i.e., vertical cracks) and interfacial cracking. The mechanism-based analytical approach, which has been validated by experimental results, may be used to provide a better evaluation of the damage of DCL-TBC system under bending.

Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La2Zr2O7; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8wt% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the top coat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60MPa were performed for 1000cycles, at a surface temperature of 1100°C and a dwell time of 10min, and then the samples were cooled at room temperature for 10min in each cycle. For the single-layer TBCs, the composite top coat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and top coats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the top coat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems.

Comparison of damage evolution during thermal cycling in a high purity nano and a conventional thermal barrier coating

Thermal barrier coatings (TBCs), consisting of a ceramic top coat and a metallic bond coat, offer resistance against high temperature degradation of turbine components. Cyclic oxidation of the bond coat, thermal stresses due to their thermal mismatches during cyclic operations, and sintering of the top coat are considered to be the common ways by which thermal barrier coatings fail. To reduce sintering, a nano structured high purity yttria stabilized zirconia (YSZ) was developed. The focus of this work is to compare the damage development of such high purity nano YSZ TBC during thermal cycling with a conventional YSZ TBC. Thermal cyclic fatigue (TCF) tests were conducted on both the TBC systems between 100°C and 1100°C with a 1h hold time at 1100°C. TCF test results showed that conventional YSZ TBC exhibited much higher life compared to the high purity nano YSZ TBC. The difference in the lifetime is explained by the use of microstructural investigations, crack length measurements along the cross-section and the difference in the elastic modulus. Furthermore, stress intensity factors were calculated in order to understand the difference(s) in the damage development between the two TBC systems.

Bending Fatigue of Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are ceramic coatings used in gas turbines to lower the base metal temperature. During operation, the TBC may fail through, for example, fatigue. In this study, a TBC system deposited on a Ni-base alloy was tested in tensile bending fatigue. The TBC system was tested as-sprayed and oxidized, and two load levels were used. After interrupting the tests, at 10,000–50,000 cycles, the TBC tested at the lower load had extensive delamination damage, whereas the TBC tested at the higher load was relatively undamaged. At the higher load, the TBC formed vertical cracks which relieved the stresses in the TBC and retarded delamination damage. A finite element (FE) analysis was used to establish a likely vertical crack configuration (spacing and depth), and it could be confirmed that the corresponding stress drop in the TBC should prohibit delamination damage at the higher load.

Hot corrosion behavior of YSZ, Gd2Zr2O7 and YSZ/Gd2Zr2O7 thermal barrier coatings exposed to molten sulfate and vanadate salt

Thermal barrier coatings (TBCs) are mostly used in critical components of aircraft gas turbine engines. Hot corrosion is among the main deteriorating factors in TBCs which results from the effect of molten salt on the coating–gas interface. This type of corrosion is observed as a result of contamination accumulated during combustion processes. Fuels used in aviation industry generally contain impurities such as vanadium oxide (V2O5) and sodium sulfate (Na2SO4). These impurities damage turbines’ inlet at elevated temperatures because of chemical reaction. Yttria stabilized zirconia (YSZ) is a conventional top coating material for TBCs while Gd2Zr2O7 is a new promising top coating material for TBCs. In this study, CoNiCrAlY metallic bond coat was deposited on Inconel 718 nickel based superalloy substrate material with a thickness about 100μm using cold gas dynamic spray (CGDS) method. Production of TBCs were done with deposition of YSZ, Gd2Zr2O7, YSZ/Gd2Zr2O7 ceramic top coating materials using EB-PVD method, having a total thickness of 300μm. Hot corrosion behavior of YSZ, Gd2Zr2O7, YSZ/Gd2Zr2O7 TBC systems were exposed to 45wt.% Na2SO4 and 55wt.% V2O5 molten salt mixtures at 1000°C temperature. TBC samples were investigated and compared using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis and X-ray diffractometer (XRD). The hot corrosion failure mechanisms of YSZ, Gd2Zr2O7 and YSZ/Gd2Zr2O7 TBCs in the molten salts were evaluated.

Microstructure, Tensile Adhesion Strength and Thermal Shock Resistance of TBCs with Different Flame-Sprayed Bond Coat Materials Onto BMI Polyimide Matrix Composite

In this study, thermal barrier coatings (TBCs) composed of different bond coats (Zn, Al, Cu-8Al and Cu-6Sn) with mullite top coats were flame-sprayed and air-plasma-sprayed, respectively, onto bismaleimide matrix composites. These polyimide matrix composites are of interest to replace PMR-15, due to concerns about the toxicity of the MDA monomer from which PMR-15 is made. The results showed that pores and cracks appeared at the bond coat/substrate interface for the Al-bonded TBC because of its high thermal conductivity and diffusivity resulting in transferring of high heat flux and temperature to the polymeric substrate during top coat deposition. The other TBC systems due to the lower conductivity and diffusivity of bonding layers could decrease the adverse thermal effect on the polymer substrate during top coat deposition and exhibited adhesive bond coat/substrate interfaces. The tensile adhesion test showed that the adhesion strength of the coatings to the substrate is inversely proportional to the level of residual stress in the coatings. However, the adhesion strength of Al bond-coated sample decreased strongly after mullite top coat deposition due to thermal damage at the bond coat/substrate interface. TBC system with the Cu-6Sn bond coat exhibited the best thermal shock resistance, while Al-bonded TBC showed the lowest. It was inferred that thermal mismatch stresses and oxidation of the bond coats were the main factors causing failure in the thermal shock test.

Tensile properties of a non-line-of-sight processed β-γ-γ′ MCrAlY coating at high temperature

The tensile properties of a NiCoCrAlYTa coating manufactured by the Tribomet® process were examined in a wide range of temperatures representative to turbine in-service conditions. Thin and bulky freestanding coating specimens (FSCS) were tested and compared in order to evaluate the mechanical strength and ductility of this material. Comparable results were found for both the specimen geometries, i.e. a brittle-to-ductile transition temperature (BDTT) between 650 and 700°C, a significant increase of the ductility (up to 5.5% at 800°C) and a drop of the tensile strength above the BDTT. Thin FSCS demonstrated slightly lower yield strength and ultimate tensile stress compared to bulky FSCS. Fractographic examinations aimed to suggest that this difference in mechanical properties was partly attributed to a lesser cohesion and brittle behavior of a β-rich layer found in the extreme surface of the overlay coating due to the deposition technique. This poor local mechanical strength is believed to be detrimental for the integrity of thermal barrier coatings (TBC) systems and attentions in the design of such functionally graded materials have to be paid to prevent early in-service damages.

A Guide to Finite Element Simulations of Thermal Barrier Coatings

To understand the stress evolution and failure mechanisms of thermal barrier coatings (TBCs), finite element simulations are an invaluable tool. Simulations are especially useful to unwrap complex interactions of different phenomena at high temperature, including creep, sintering, diffusion, and oxidation. However, the correct setup and evaluation of a finite element model for this problem are difficult. This article reviews critical issues in modelling TBC systems. Some of the most important aspects are as follows: (a) stresses in 3D simulations may differ considerably from 2D models; (b) the interface shape strongly affects the stresses and using an idealized geometry may underestimate stresses; (c) crack propagation requires simulating sufficiently large regions to correctly capture stress redistribution; (d) a correct description of the material behaviour (visco-plasticity, TGO growth, sintering) is crucial in determining the stress state. The article discusses these and other issues in detail and provides guidelines on the choice of model parameters, boundary conditions, etc. The paper also points out open questions in modelling TBC systems and discusses aspects of verification and validation.

Performance of Bond Coats Modified by Platinum Group Metals for Applications in Thermal Barrier Coatings

We have investigated the partial replacement of Pt with other less expensive Pt group metals on the properties of γ′ + γ bond coats used in thermal barrier coatings (TBCs) deposited on a nickel-base superalloy. The microstructure, thermal stability, oxidation behavior and performance in TBC systems of bond coats synthesized with Pt + Ru, Pt + Ir and Pt + Rh are compared with those of a reference bond coat synthesized with Pt. Yttria-stabilized zirconia has been employed as top coat in all coating systems. It is shown that at high temperatures all bond coats are degraded by interdiffusion and oxidation, however, with different kinetics. The lifetime of each TBC system is found to be limited by the cohesion between the thermally grown oxide and underlying bond coat. Differences in the behavior of various bond coats are correlated with their properties. Among the three Pt group metals investigated, the properties of the Pt + Ru bond coat are shown to closely approach those of the Pt bond coat. It is concluded that Ru with much lower cost presents a potential candidate for reducing the consumption of Pt.

Mechanistic Study on the Degradation of Thermal Barrier Coatings Induced by Volcanic Ash Deposition

Thermal stress generated on thermal barrier coatings (TBCs) by volcanic ash (VA) deposition was assessed measuring the tip deflection of a multilayered beam structure as a function of temperature. The TBC in this study was deposited onto the surface of a blade utilized in a land-based gas turbine which is composed of 8 wt.%Y2O3-ZrO2/CoNiCrAlY on a Ni-based superalloy. The VA-deposited TBC sample was heated at 1453 K, and the effect of VA deposition on TBC delamination was examined in comparison with a TBC sample without VA deposition as a reference. On the basis of the VA attack damage mechanism which was investigated via the tip deflection measurement and a comprehensive microstructure examination, a damage-coupled constitutive model was proposed. The proposed model was based on the infiltration of the molten VA inside pores and phase transformations of yttria -tabilized zirconia in the TBC system. The numerical analysis results, which were simulated utilizing the finite element code installing the developed constitutive model, showed us that VA attack on the TBC sample induced near-interfacial cracks because of a significant increasing in the coating stress.

Effects of three-dimensional coating interfaces on thermo-mechanical stresses within plasma spray thermal barrier coatings

It has been acknowledged that stresses within a thermal barrier coating (TBC) and its durability are significantly affected by the coating interfaces. This paper presents a finite element approach for stress analysis of the plasma sprayed TBC system, using three-dimensional (3D) coating interfaces. 3D co-ordinates of the coating surfaces were measured through 3D reconstruction of scanning electron microscope (SEM) images. These co-ordinates were post processed to reconstruct finite element models for use in stress analyses. A surface profile unit cell approach with appropriate boundary conditions was applied to reduce the problem size and hence computation time. It has been shown that for an identical aspect ratio of the coating interface, interfacial out-of-plane stresses for 3D models are around twice the values predicted using 2D models. Based on predicted stress development within the systems, possible crack development and failure mechanisms of the TBC systems can be predicted.

Parametric study for optimal design of an air plasma sprayed thermal barrier coating system with respect to thermal stress

The use of thermal barrier coatings (TBCs) is expected to become more popular in various gas turbines because these coatings provide excellent thermal insulation and damage protection. However, unexpectedly early failure has often discouraged the full use of TBC, resulting in a shortening of the life span of gas turbines because these substrates are directly exposed to harsh operation conditions. The general mechanics of TBC and its complex inner phenomena, mainly related to failure, have been investigated to prolong the lifetime of TBC. However, our understanding is limited because TBC has various specifications and operating conditions, and complex interplays between many factors. The primary goal of this study is to construct an extensive finite element method (FEM) model to evaluate thermal stress of an air plasma sprayed TBC under its operating conditions by considering various inner phenomena, including thermal grown oxide, undulating topology of coating interface, aluminum depletion, creep effect, and elastoplastic deformation. With the proposed FEM model, a parametric study has been conducted with respect to a variety of material properties, as well as the thickness of bond coat and of top coat. The influence of each design factor on thermal stress and the other parameters is fully discussed. Finally, this work leads to optimal design criteria for minimizing thermal stress across a wide range of TBC systems.

A Theoretical Model for Predicting Residual Stress Generation in Fabrication Process of Double-Ceramic-Layer Thermal Barrier Coating System.

Residual stress arisen in fabrication process of Double-Ceramic-Layer Thermal Barrier Coating System (DCL-TBCs) has a significant effect on its quality and reliability. In this work, based on the practical fabrication process of DCL-TBCs and the force and moment equilibrium, a theoretical model was proposed at first to predict residual stress generation in its fabrication process, in which the temperature dependent material properties of DCL-TBCs were incorporated. Then, a Finite Element method (FEM) has been carried out to verify our theoretical model. Afterwards, some important geometric parameters for DCL-TBCs, such as the thickness ratio of stabilized Zirconia (YSZ, ZrO2-8%Y2O3) layer to Lanthanum Zirconate (LZ, La2Zr2O7) layer, which is adjustable in a wide range in the fabrication process, have a remarkable effect on its performance, therefore, the effect of this thickness ratio on residual stress generation in the fabrication process of DCL-TBCs has been systematically studied. In addition, some thermal spray treatment, such as the pre-heating treatment, its effect on residual stress generation has also been studied in this work. It is found that, the final residual stress mainly comes from the cooling down process in the fabrication of DCL-TBCs. Increasing the pre-heating temperature can obviously decrease the magnitude of residual stresses in LZ layer, YSZ layer and substrate. With the increase of the thickness ratio of YSZ layer to LZ layer, magnitudes of residual stresses arisen in LZ layer and YSZ layer will increase while residual stress in substrate will decrease.

三層試験片の曲率に基づく遮熱コーティングシステムの残留応力評価法

A thermal barrier coating (TBC) system consists of a TBC topcoat (TC), a bond coat (BC), and a substrate. Residual stress acting on a coating is a important parameter for materials mechanics and hence need to be evaluated. We propose a simple method for evaluating the residual stresses acting on each layer of a TBC system by using a curvature method. First, a three-layered model was constructed based on the misfit strain between the coatings and the substrate. This method uses the curvatures of the substrate with a blast treatment, the BC system specimen with the blast and BC, and the TBC system specimen with the blast, BC, and TC instead of a change in curvature during thermal spraying. Next, the residual stresses acting on the TBC system specimen with varying thicknesses were evaluated by using the proposed method. It was confirmed that the proposed method provides reasonable residual stresses in the BC and TC. Finally, the thermal stress after the coating deposition was theoretically evaluated, and the quenching stress of the coating during the deposition was evaluated by using the difference between the experimentally determined residual stress and the theoretical thermal stress. The mechanism for generating the residual stress in the TBC system during thermal spraying was investigated using the quenching stress and thermal stress.

Advanced study of thermal behaviour of CSZ comparing with the classic YSZ coating

Thermal barrier coatings (TBC) are advanced materials typically applied to metal surfaces subjected to extreme temperatures to protect them and increase their lifetime. Ceria stabilized zirconia ceramic layer (CSZ) is increasingly used as an alternative improved as replace for classical TBC system - yttria stabilized zirconia - thanks to superior properties, including mechanical and high resistance to thermal corrosion. The paper describes the thermal shock testing of two types of thermal barrier coatings used to protect a nickel super alloy. For the experimental procedure, it was used plate samples from nickel super alloy with a bond coat and a ceramic top coat. The top coat was different: on some samples, it was used YSZ and on others CSZ. Ni based super alloys have good corrosion resistance in reducing environments action, but poor in oxidizing conditions. Extreme environments can lead to loss of material by oxidation / corrosion, along with decreased mechanical properties of the substrate due to damaging elements which diffuses into the substrate at high temperatures. Using laboratory equipment, the TBC systems were exposed repeatedly to extreme high temperatures for a short time and then cooled. After the thermal shock tests, the samples were morph-structured characterized using electronic microscopy to analyze the changes. The experimental results were compared to rank the TBC systems in order of performance.

Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs)

Yttria Stabilized Zirconia (YSZ) is widely used as a traditional ceramic top coat material in gas turbine engine components. Nowadays, rare earth zirconates, as alternative materials to YSZ, are enhanced for use as top coat layer of TBC owing to their better thermal isolation properties. In the present study, metallic CoNiCrAlY bond coat powder was sprayed on Inconel 718 superalloy substrate by cold gas dynamic spray (CGDS) technique. After the deposition of bond coats, YSZ and Gd2Zr2O7 top coats were produced using EB-PVD process. TBCs were exposed to isothermal oxidation tests at 1100°C for 8, 24, 50 and 100h. Oxidation and growth behaviors of thermally grown oxide (TGO) layer were observed. TBC samples were investigated using scanning electron microscope (SEM), EDS elemental mapping and X-ray diffractometer (XRD) analysis before and after the oxidation tests. Oxidation performances of two different TBC systems were compared to each other according to the analysis results.

Thermal fatigue testing and simulation of an APS TBC system in presence of a constant bending load

Thermal Barrier Coatings (TBCs) are used as thermal insulation to protect components operating at high temperature. A multiple failure mechanism may exist in a TBC system associated with diverse in-service conditions. In this paper, Inconel 617 superalloy substrates were coated with 8% Y2O3 stabilized tetragonal ZrO2 powder, deposited on top of a Ni22Cr10Al1Y bond coat by atmospheric plasma spraying (APS). In order to investigate the damage mechanism of the in-service components, experiments and numerical FE analysis were conducted to simulate the loading conditions of the TBC system in a land-based gas turbine combustor component. A specific test rig was designed and manufactured such that coated specimens were simultaneously subjected to constant bending load and thermal cycles. The temperature history was measured on both sides of the specimens and a maximum surface temperature of 1170°C on the coating surface and a thermal gradient of 500°C was obtained across the TBC system. The TBC system - failed after 82 thermal cycles. The microstructural observation showed multiple vertical and horizontal cracks confined within the top coat. By increasing the mechanical load, more extensive cracks were observed in a similar pattern. The final failure was governed by delamination at the top coat and bond coat interface. A subsequent finite element analysis confirmed that the application of external mechanical load can change the failure mechanism of the TBC. It also demonstrated that the TBC life is limited by crack propagation instead of crack initiation.