Life Of Thermal Barrier Coatings Research Articles

TBC bond coat–top coat interface roughness: Influence on fatigue life and modelling aspects

Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, causing the ceramic top coat to spall off the metallic bond coat. The life prediction of TBCs often involves finite element modelling of the stress field close to the bond coat/top coat interface and thus relies on accurate modelling of the interface. The present research studies the influence of bond coat/top coat interface roughness on the thermal fatigue life of plasma sprayed TBCs. By using different spraying parameters, specimens with varying interface roughnesses were obtained. During thermal cycling it was found that higher interface roughness promoted longer thermal fatigue life. The interfaces were characterised by roughness parameters, such as Ra, Rq and R∆q, as well as by autocorrelation, material ratio curves and slope distribution. The variation of spray parameters was found to affect amplitude parameters, such as Ra, but not spacing parameters, such as RSm. Three different interface geometries were tried for finite element crack growth simulation: cosine, ellipse and triangular shapes. The cosine model was found to be an appropriate interface model and a procedure for obtaining the necessary parameters, amplitude and wavelength, was suggested. The positive effect of high roughness on life was suggested to be due to a shift from predominantly interface failure, for low roughness, to predominantly top coat failure, for high roughness.

Thermal barrier coatings technology: critical review, progress update, remaining challenges and prospects

A comprehensive and integrated review of thermal barrier coatings (TBCs) applied to turbine components is provided. Materials systems, processes, applications, durability issues, technical approaches and progress for improved TBC, and our understanding of the science and technology are discussed. Thermal barrier coating prime reliance and further advances have been hampered by TBC loss by particle impact and erosion in certain locations of the turbine blades. Accumulation of low melting eutectic containing calcia, magnesia, alumina and silica resulting in TBC spallation limits maximum surface temperature. Design methodologies to address durability and data scatter issues are discussed. Compositions, morphology, characteristics and performance data for new bonds to achieve longer TBC life are described. Further reduction in the thermal conductivity of the top layer to minimise the parasitic mass of the coating on the component is being sought via top layer composition and processing modifications as well as by alternate ceramic compositions. The progress in these areas is critically reviewed including processing, stability and durability limitations. The paper also describes effort to understand various failure mechanisms including modelling and simulation.

The influence of laser treatment on thermal shock resistance of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

Abstract The main goal of this paper was to evaluate the effects of laser glazing on the microstructure and thermal shock resistance of nanostructured thermal barrier coatings (TBCs). To this end, nanostructured yttria stabilized zirconia (YSZ) top coat and NiCrAlY bond coat were deposited on Inconel 738LC substrate by air plasma spraying (APS). The Nd:YAG pulsed laser was used for laser treatment of top coat surface. The thermal shock behavior of plasma-sprayed and laser-glazed coatings was investigated by quenching the samples in cold water from 1000 °C. The microstructure and phase composition of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Energy dispersive spectroscopy (EDS) was used to analyze the interface diffusion behavior of the bond coat elements. The results of SEM revealed that the laser glazing process reduced the surface roughness, eliminated the porosity of the surface and produced network cracks perpendicular to the surface. XRD results also indicated that both as-sprayed and laser glazed coatings consisted of non-transformable (T′) phase. Thermal shock test results showed that the lifetimes of the plasma-sprayed TBCs were almost doubled by laser glazing. Continuous network of segmented cracks perpendicular to the surface produced by laser glazing improved the strain accommodation and recognized it as the main enhancement mechanism for TBC life extension.

Effects of interface roughness on the life estimation of a thermal barrier coating layer

Methodology development for life estimation of thermal barrier coating (TBC) applied on a gas turbine engine combustor liner has been undertaken using finite element method. 3-D finite element analyses (FEA) are performed using ABAQUS followed by fatigue analysis through FESAFE for life estimation of a TBC layer. Analysis considers a typical mission cycle of a gas turbine engine while modelling the transition liner of a reverse flow combustion chamber. This paper presents the influence of interface roughness on the life of TBC layer. Three different cases of interface roughness have been studied. Roughness is found to affect TBC life adversely. In addition to effect of interface roughness, the life estimation methodology is also discussed.

Influence of Thermal Cycle Frequency on the TGO Growth and Cracking Behaviors of an APS-TBC

The durability of thermal barrier coatings (TBCs) is controlled by fracture near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the influence of thermal cycle frequency on the oxidation performance, in terms of TGO growth and cracking behavior, of an air-plasma-sprayed (APS) Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat was studied. The results show that while TGO growth exhibited an initial parabolic growth behavior followed by an accelerated growth stage, higher cycle frequency resulted in a faster TGO growth and a higher crack propagation rate. It is found that a power-law relationship exists between the maximum crack length and the TGO thickness, which is independent of the cycle frequency. This relationship may warrant a TBC life prediction methodology based on the maximum crack length criterion.

Improving stability of thermal barrier coatings on magnesium alloy with electroless plated Ni–P interlayer

Thermal barrier coatings (TBCs) of zirconia stabilized by 8wt.% yttria (8YSZ) on MB26 rare earth–magnesium alloy with MCrAlY as bond coat were prepared by air plasma spraying (APS). In order to improve the thermal shock resistance of the coatings, an interlayer of Ni–P alloy between the substrate and bond coat was prepared by electroless plating. The preparation, microstructure, bond strength and thermal shock resistance of the coatings were investigated. The results indicate that Ni–P interlayer not only has favorable effects on the protection of Mg alloy substrate from thermal oxidation during thermal spraying, but also significantly improves the bond strength of TBCs. The thermal shock life of TBCs was enhanced from 5cycles to longer than 130cycles with the application of Ni–P interlayer. The failure of TBCs in thermal shock test was mainly induced by the corrosion of Mg alloy substrate.

Improving Thermal Shock Resistance of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings by Laser Remelting

This paper deals with the microstructure and thermal shock behavior of laser remelting of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) deposited by plasma spraying. The microstructures of the coatings were analyzed by scanning electron microscopy (SEM). It was found that the as-sprayed ceramic coating had laminated structure with high porosity. However, the coating exhibited a dense lamellar-like layer with segment cracks on the remained plasma-sprayed porous layer. Thermal shock experiments for the two kinds of TBCs were performed by water quenching method. Testing result showed that the laser-remelted TBC had better thermal shock resistance than the as-sprayed one. The damage mode of the as-sprayed TBC was great-size whole spalling. In contract, the failure mechanism of the laser-remelted one was mainly local pelling. Segmented cracks of the top ceramic coatings caused by laser remelting improved the stress accommodation and were mainly attributed to the enhancement for thermal shock life of TBC.

Influence of substrate strain anisotropy in TBC system failure

The lifetimes of thermal barrier coatings (TBC) are known to show considerable scatter. Experimental original data have revealed the strong correlation between spallation patterns and local strain field. This strain field was shown to be a function of the substrate single crystal orientation. Thus, this study addresses the strain variation in the single crystal substrate and its influence on TBC system life. This task was achieved using both finite element analysis (FEA) to take into account crystal plasticity in the substrate single crystal and a recent TBC life model [1].

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

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

Effect of Spinel Growth on the Delamination of Thermal Barrier Coatings

The spinel growth induces undulation of the thermal growth oxide layer and decreases the service life of plasma-sprayed thermal barrier coatings. An analytical model is introduced to investigate the effect of spinel growth on the delamination of thermal barrier coating. The analytical results show that the number per unit area and the growth rate of spinel have significant influence on the delamination of thermal barrier coating. The stiffer and thicker thermal barrier coating is more easily to delaminate from the bond coat due to the existence of spinels. The effect of spinel on the delamination cannot be neglected. How to reduce the growth rate and the number of spinel is a key problem to prolong the service life of thermal barrier coatings.

Assessing Coating Reliability Through Pore Architecture Evaluation

Plasma-sprayed thermal barrier coatings (TBCs) exhibit many interlamellar pores, voids, and microcracks. These microstructural features are primarily responsible for the low global stiffness and the low thermal conductivity commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400 °C, for times of up to 20 h) on the pore architecture of detached YSZ top coats with different impurity levels have been characterized by mercury intrusion porosimetry and gas-sorption techniques. Stiffness and thermal conductivity were also monitored to assess the effect of change in pore architecture on properties. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (surface-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high-treatment temperatures (~1400 °C). These changes are thought to be associated with intrasplat microcrack healing, improved intersplat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Increase in thermal conductivity and stiffness were found to be more pronounced for coatings with higher impurity content (particularly alumina and silica). Reliability issues surrounding such increase in thermal conductivity and stiffness are discussed along with a brief note on the effect of impurities on TBC life.

Improvement of EB-PVD thermal barrier coatings by treatments of a vacuum plasma-sprayed bond coat

The lifetime of electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC) systems with conventional 7 YSZ ceramic top layers was investigated in 1 h thermal-cyclic testing at 1100 °C. The single crystal alloy CMSX-4 and the polycrystalline IN 100 alloy that had been coated with a vacuum plasma-sprayed MCrAlY bond coat were chosen as substrate materials. A fully plasma-sprayed TBC system was studied as well. Several bond coat treatments such as smoothing, annealing in vacuum or air, and grit blasting were employed in order to study their effects on TBC life, and particularly the underlying mechanisms of thermally grown oxide (TGO) formation. Variations in TGO microstructure and especially occurrence or absence of the mixed zone have been correlated to the bond coat treatments. Spallation of the TBCs is mainly related to TGO formation that is influenced by the bond coat pre-treatment. The longest lifetime has been achieved by smoothing the bond coat prior to any heat treatment, followed by vacuum annealing prior to EB-PVD top coat deposition. TBC lifetimes were longer on CMSX-4 than on IN100 substrates. The surface roughness of the bond coat did not influence TBC life much.

Cyclic behavior of EB-PVD thermal barrier coating systems with modified bond coats

The lifetime of thermal barrier coating (TBC) systems depends on substrate, bond coat, thermally grown oxide (TGO), and ceramic top coat. In the present paper NiPtAl bond coats as well as NiCoCrAlY(X) deposited by LPPS and EB-PVD (electron-beam physical vapour deposition) underneath conventional EB-PVD yttria stabilized zirconia top coats were investigated on three different substrate alloys. Several bond coat treatments such as polishing, annealing in vacuum, and grit blasting were employed in order to study effects on TBC life, and particularly the underlying mechanisms of TGO formation. Samples were thermally cycled at 1100 °C and partly at 1150 °C. Spallation of the TBCs is mainly correlated with TGO formation that is influenced by bond coat type and pre-treatment. The longest lifetimes were achieved on a novel Hf-doped EB-PVD NiCoCrAlY-X bond coat owing to a differing TGO formation and failure mechanism. Activation energies derived from lifetimes and test temperatures were calculated to identify key failure mechanisms within these complex coating systems.

Effect of platinum on the durability of thermal barrier systems with a γ + γ′ bond coat

The effect of the Pt content on the durability of thermal barrier coatings (TBCs) with a Pt-enriched γ + γ′ bond coat was investigated. It was found that the TBCs with a higher Pt content exhibit a significantly longer isothermal life than those with a lower Pt content. The subsequent analysis indicated that the Pt content has only a negligible effect both on the stresses in the thermally-grown oxide (TGO) and the elastic properties of the bond coat, but has a significant effect on the interfacial chemistry. During oxidation, impurities such as S, C and refractory elements segregating to the interface would degrade the TGO adherence to the bond coat. However, a higher content of Pt can inhibit this segregation, and thus improve the TBCs life.

Thermal Fatigue of Thermal Barrier Coatings by Atmospheric Plasma Spraying

Turbine vanes and blades are the most intensively loaded elements in that they are subjected to a large variety of mechanical and high temperature loads. The thermal barrier coatings (TBCs) are widely used on different hot components of gas turbines, as blades and vanes, for both, power engineering as well as aeronautical applications. Currently, two methods are used for depositing TBCs on substrate, which are plasma spray (PS) and electron beam-physical vapor deposition (EB-PVD). A typical TBCs system consists of two thin coatings, including a ceramic coating and a metallic bond coat. Despite considerable efforts, the highly desirable prediction of their life time is still a demanding task. The PS coating was focused on in this work. Firstly, the TBCs systems are multiplayer material systems. The material properties are not easily determined, such as Young’s modulus of the top-coating of TBCs. Using the resonant frequency and the composite beam theory, the Young’s modulus of APS TBCs was gotten under from room temperature to 1150°C. Then using a commercial finite-element program, the model geometry is that of a cylinder specimen. The interface region between bond coat and top coating is modeled and meshed with a sinusoidal geometry. The temperature was designed and cycled over a range from room temperature to 1050°C. The force-air-cooling was designed to form temperature gradient across the thickness of TBCs. Finally, the fatigue life of TBCs was predicated. The maximum relative error is 20.1%.

Microstructure and thermal cyclic performance of laser-glazed plasma-sprayed ceria–yttria-stabilized zirconia thermal barrier coatings

In this study, substrates of Hastelloy-X superalloy coupons were first sprayed with a Ni–22Cr–10Al–1Y bond coat and then with a ceria–yttria-stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma-sprayed ceria–yttria-stabilized zirconia thermal barrier coatings (TBCs) were glazed using a pulsed CO2 laser. The effects of laser-glazing on the microstructure and cyclic oxidation behavior of the coatings were evaluated. The microstructures of both the as-processed and the tested TBCs were investigated by using optic microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The phases of the coatings were measured with X-ray diffractometry (XRD). An investigation of microstructure revealed that both plasma-sprayed and laser-glazed top coatings consisted of nonequibrium tetragonal (T) phase, cubic (C) phase and minor amount of Ce0.75Zr0.25O2 phase. Minor amount of monoclinic (M) phase found in plasma-sprayed coatings disappeared after laser-glazing. The phase analysis of tested top coat indicated that the mole% of tetragonal (T) phase decreased and that of Ce0.75Zr0.25O2 phase increased after thermal cycling for both sprayed and glazed samples. It was also found that the monoclinic phase in the plasma-sprayed sample disappeared after thermal cycling. Results showed that laser-glazing did not have a substantial effect on reducing the oxidation rate of the bond coat. Cyclic test results showed that the life times of the plasma-sprayed TBCs were enhanced about twofold by laser-glazing for initial spallation life. With regard to 50% spallation life, the laser-glazed samples exhibited about a half lifetime improvement relative to the plasma-sprayed samples. Failure of the TBCs was initiated within the top coat near the bond coat valley region and propagated mainly within the top coat and the out-growth oxide near the top coat–bond coat interface. Segmented cracks produced by laser-glazing improved the strain accommodation ability of top coat and could be identified as the major enhancement mechanism for TBC life extension.

Manipulation of air plasma spraying parameters for the production of ceramic coatings

Thermal barrier coatings (TBCs) were sprayed on stainless steel coupons by an air plasma thermal spray (APS) technique. The porosity of the topcoat was varied by controlling the different spraying parameters. Three types of thermal shock tests were designed to determine the TBCs life. Effect of different spraying parameters on the thermal shock life was observed. The spraying distance was found to be an important parameter that controls the thermal shock life. It was observed that the thermal shock properties could be related with an empirical parameter called the critical thermal shock parameter (CTSP). Fracture toughness ( K IC) was determined by Vicker's indentation technique and it was observed that for a certain range of the porosity the toughness increased with increase of porosity. The possible increase in thermal shock life for a certain range of CTSP may be attributed to residual stresses and increase in fracture toughness of the topcoat.

Regularities of degradation of thermal barrier coatings of the MCrAlY/ZrO2 system under sulfide-oxide corrosion conditions

The results from investigation of the service life of thermal barrier coatings under the conditions of sulfide-oxide corrosion at 800, 850, and 900°C are considered. The effect of the ceramic layer thickness, the aluminum and chromium concentration in the metallic layer, and the formation of the oxide film in the boundary region on degradation and corrosion longevity of coatings is determined.

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

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

Effects of substrate rotation speed during deposition on the thermal cycle life of thermal barrier coatings fabricated by electron beam physical vapor deposition

The effects of substrate rotation speed during deposition of an Y 2O 3 stabilized ZrO 2 (YSZ) layer fabricated by electron beam physical vapor deposition (EB-PVD) on the microstructure, elastic modulus and lifetime were investigated. The microstructure and elastic modulus of EB-PVD YSZ coatings were highly influenced by the rotation speed of the substrates. The elastic modulus of the coatings was found to decrease as the rotation speed was increased, which led to a longer thermal cycle life.