Metallic Bond Coat Research Articles

Degradation of a TBC with HVOF-CoNiCrAlY Bond Coat

Thermal barrier coatings (TBCs) provide both thermal insulation and oxidation and corrosion protection to the substrate metal, and their durability is influenced by delamination 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 degradation process of a TBC with an air-plasma-spray ZrO2-8 wt.%Y2O3 topcoat and a high-velocity oxy-fuel CoNiCrAlY bond coat was studied, in terms of TGO growth kinetics and aluminum depletion in the bond coat, as well as cracking behavior. The results show that the TGO growth kinetics can be described by a transient oxidation stage with δ3 = k1t followed by a steady-state oxidation stage with δ2 = c + k2t. Significant aluminum depletion was observed in the bond coat after extended thermal exposure; however, chemical failure of the bond coat did not occur even after the aluminum content near the TGO/CoNiCrAlY interface decreased to 4.5 at.%. A power-law relationship between the maximum crack length in the TBC and the TGO thickness was observed, which may serve as the basis for TBC life prediction.

Effects of Microstructural Transformation in TBCs Consisting of NiCrAlY Metallic Bond Coat and YSZ Ceramic Top Coat after Oxidation at 900°C

Effects of Microstructural Transformation in TBCs Consisting of NiCrAlY Metallic Bond Coat and YSZ Ceramic Top Coat after Oxidation at 900°C

Mechanical properties of stabilized zirconia nanocrystalline EB-PVD coating evaluated by micro and nano indentation

Yttria-stabilized zirconia (YSZ) thin nanocrystalline coatings at different substrate preheating temperatures were deposited via electron beam-physical vapour deposition (EB-PVD). Nanocrystalline ZrO2-Y2O3 was deposited on the bond coat in order to compensate for the coefficient of thermal expansion (CTE), which can be functionalized as a thermal barrier coating (TBC). The aim of this study was to evaluate mechanical properties with respect to adhesion of zirconia nanocrystalline’s top ceramic layer to the interfacial bond coat by utilizing micro and nano indentation tests. In the present paper, the structural studies were carried out using X-ray diffraction (XRD) analysis of coating content (8 mol% of Y2O3). The tetragonal phase of stabilized zirconia was observed. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were employed to characterize the coatings’ morphology and microstructure. The mechanical behavior of ZrO2-Y2O3 thin films under point loading conditions was studied by nanoindentation using a Berkovich indenter with 130 nm tip radius. Therefore, adhesion of top coat to the interfacial underlying metallic bond coat known as MCrAlY (M = Ni, Co) was estimated according to the highest peak load tests; for a 120 mN peak load, the film manifested tolerable adhesion properties. Moreover, nanoindentation of ZrO2-Y2O3 nanostructure deposited at 1050 °C substrate preheating temperature produced the highest hardness value of about 21.7 GPa. Vickers micro hardness was utilized with the aid of the Tabor equation in order to achieve deeper insight into the correlation between adhesion and deposition process parameters.

Deposition and Oxidation of Oxide-Dispersed CoNiCrAlY Bondcoats

CoNiCrAlY powder and nano-size alumina powder were milled by a high-energy-attrition ball-mill, and an oxide-dispersed powder was produced with a mixed structure of metal and alumina in each particle. The oxide-dispersed bond coat powder was deposited by HVOF. Pores, however, were observed in the coating since the alumina was deposited without sufficient melting. Isothermal oxidation tests were carried out for the bond coat specimens at a temperature of 1373 K up to 1000 h in air. As a result, oxidation proceeded inside the coating, since oxygen penetrated through pores formed in the dispersed alumina. However, the authors find that another deposition using higher power levels led to a bond coat without pores. A commercially available oxide-dispersed CoNiCrAlY powder was also deposited by HVOF and VPS, and isothermal oxidation tests were performed. The analysis clarifies that the HVOF bond coat exhibited the thinnest thermally grown oxide than those of the VPS bond coat and conventional metallic bond coat. Furnace cycling tests were conducted using the specimens with an additional ceramic thermal-barrier coating. The specimen with the bond coat sprayed by VPS using commercial oxide-dispersed powder showed almost same number of cycles to delamination compared with the specimen with the conventional metal bond coat.

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.

Improved Thermal Cycling Durability of Thermal Barrier Coatings Manufactured by PS-PVD

The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 °C/1050 °C surface/substrate exposure temperatures.

Investigation of failure mechanism of thermal barrier coatings (TBCs) deposited by EB-PVD technique

Failure mechanism of thermal barrier coatings (TBCs) prepared by electron beam physical vapor deposition (EB-PVD) technique owing to formation of micro cracks was investigated. The TBCs were deposited on the Ni-based super alloy IN-100 and the micro cracks were observed within the top ceramic coat of thermally cycled TBCs at 1050°C. It was observed that these cracks propagate in the ceramic coat in the direction normal to interface while no cracks were observed in the bond coat. SEM/EDS studies revealed that some non-uniform oxides were formed on the interface between ceramic top and metallic bond coat just below the cracks. Study proposed that the cracks were initiated due to stress owing to big difference in Pilling-Bed worth ratio of non-uniform oxides as well as thermal stress, which caused the formation of cracks in top ceramic coat leading to failure of TBCs

Laser glazing of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

Nanostructured thermal barrier coatings (TBCs) consisting of yttria stabilized zirconia (YSZ) ceramic top coat and NiCrAlY metallic bond coat were prepared by atmospheric plasma spraying (APS). The Nd:YAG pulsed laser was used for laser treatment of top coat surface. The microstructure and thermal insulation capability of plasma-sprayed and laser-glazed thermal barrier coatings were investigated. The microstructure and phase composition of the coating were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD). The nanostructured plasma-sprayed coating contained non-melted or partially melted nanosized particles and columnar grains, whereas the microstructure of laser-glazed coating consisted of columnar grains in the fracture surface and equiaxed grains on the surface. Laser glazing, which helps to eliminate the surface porosities and other structural defects of coatings deposited by the plasma spraying method, should contribute to the improvement of their surface roughness. XRD results revealed that both as-sprayed and laser-glazed coatings consisted of non-transformable tetragonal (T′) phase. The results of the thermal insulation capability test indicated that the thermal insulation capability of the laser-glazed coating, as compared to the as-sprayed coating, was slightly lower due to the microstructural change in the glazed layer of top coat.

Experimental fatigue lifetime of coated and uncoated aluminum alloy under isothermal and thermo-mechanical loadings

This paper presents the fatigue lifetime of an aluminum–silicon–magnesium alloy, widely used in diesel engine cylinder heads, both with and without a thermal barrier coating (TBC) system. The coating system in this study consists of two layers including a 150 μm thick metallic bond coat and a zirconium oxide top coat 350 μm thick. These coating layers were applied on the substrate of A356.0 alloy by air plasma thermal spraying. The isothermal fatigue tests were conducted in low cycle fatigue (LCF) regime at various temperatures. Out-of-phase thermo-mechanical fatigue (OP-TMF) tests were also performed at different maximum temperatures and constraint factors. Experimental results demonstrate that at high temperature the coating increases the LCF lifetime of A356.0 alloy at higher strain amplitude and decreases it at lower strain amplitude. At LCF situation, thicker coating has more detrimental effect. However, the coating increases significantly the OP-TMF lifetime. To analyze the failure mechanisms, the fracture surfaces of specimens were investigated by scanning electron microscopy. The fractography of surfaces showed that the failure mechanism of TBC system in coated A356.0 alloy was separations of coating layers in the interface of the substrate and the bond coat layer. This was observed on both high temperature LCF and OP-TMF conditions.

Surface Modification of Thermal Barrier Coatings by Single-Shot Defocused Laser Treatments

Thermal barrier coatings (TBC) consisting of atmospheric plasma-sprayed ZrO2-8 wt.% Y2O3 and a high velocity oxygen fuel-sprayed metallic bond coat were subjected to CO2 continuous wave laser treatments. The effects of laser power on TBCs were investigated as was the thermally grown oxide (TGO) layer development of all as-sprayed and laser-treated coatings after thermal oxidation tests in air environment for 50, 100, and 200 h at 1100 °C. The effects of laser power on TBCs were investigated. TGO layer development was examined on all as-sprayed and laser-treated coatings after thermal oxidation tests in air environment for 50, 100, and 200 h at 1100 °C. Melted and heat-affected zone regions were observed in all the laser-treated samples. Oxidation tests showed a stable alumina layer and mixed spinel oxides in the TGO layers of the as-sprayed and laser-treated TBCs.

Lifetime prediction of plasma-sprayed thermal barrier coating systems

Relying on the statistical treatment of the morphological characteristics of the interface between yttria stabilized zicronia (YSZ) top coat (TC) and metallic bond coat (BC), finite element model of thermal barrier coating (TBC) is generated by a sinusoidal function. Meanwhile, considering the thermally grown oxide (TGO) growth, creep effects and top coating sintering, lifetime prediction methodology is proposed. Furthermore, stress development during thermal cycling is calculated by finite element method (FEM). Comparing the numerically predicted TBC stresses with the failure stress of top coating, the lifetime of plasma-sprayed (PS) TBC is predicted between 810 and 900cycles, in agreement with experimental result of about 860cycles, the average data of 8 specimen lifetimes. Different factors to the failure of TBC are compared.

Optimizing an SEM-based 3D surface imaging technique for recording bond coat surface geometry in thermal barrier coatings

Creation of three-dimensional representations of surfaces from images taken at two or more view angles is a well-established technique applied to optical images and is frequently used in combination with scanning electron microscopy (SEM). The present work describes specific steps taken to optimize and enhance the repeatability of three-dimensional surfaces reconstructed from SEM images. The presented steps result in an approximately tenfold improvement in the repeatability of the surface reconstruction compared to more standard techniques. The enhanced techniques presented can be used with any SEM friendly samples. In this work the modified technique was developed in order to accurately quantify surface geometry changes in metallic bond coats used with thermal barrier coatings (TBCs) to provide improved turbine hot part durability. Bond coat surfaces are quite rough, and accurate determination of surface geometry change (rumpling) requires excellent repeatability. Rumpling is an important contributor to TBC failure, and accurate quantification of rumpling is important to better understanding of the failure behavior of TBCs.

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.

Effect of physical vapor deposited Al2O3 film on TGO growth in YSZ/CoNiCrAlY coatings

Thermal barrier coatings (TBCs) comprising of yttria stabilized zirconia (YSZ) ceramic top coat and CoNiCrAlY metallic bond coat have been widely used in gas turbines. However, the developed oxides layer in the interface of the top and bond coats during thermal exposure of the TBCs always results in the destruction of the system. In order to restrain the growth of oxides layer and improve the thermal shock resistance of TBCs, a thin Al2O3 film was pre-deposited on CoNiCrAlY bond coat by physical vapor deposition (PVD) technology. After thermal exposure, morphologies and phase compositions of the thermal growth oxides (TGO) layer in the conventional and pre-deposited Al2O3 film TBCs were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The residual stresses in the coatings were analyzed using micro-Raman spectroscopy (LabRam-1B). It was found that TGO layer formed in the conventional TBCs was mainly composed of Al2O3, (Cr,Al)2O3+(Co,Ni)(Cr,Al)2O4+NiO (CSN), and (Cr,Al)2O3+(Co,Ni)(Cr,Al)2O4 (CS), while in the treated TBCs, the formed TGO layer appeared more uniform and compact. The CSN and CS clusters, which are normally considered as a weakness for TBCs, were greatly limited. The residual stresses in the TBCs after thermal shock were also reduced by the deposition of Al2O3 film.

Analysis of the Residual Stress in Multi-Layer Materials

The residual stresses due to the mismatch between metallic bond coat and ceramic top coat lead to a collapse of all thermal barrier system. For measuring residual strain in elastic multilayer materials, the incremental hole-drilling strain-gauge method was studied. The main stresses, which is interrelated with the residual strain, is resolved on planes that are normal to a deep hole. In order to link the surface strain to the residual stress, calibration coefficients were obtained by the finite element method. The result shows that the coefficients depend on the substrate and the type of coating, the strain gauge used and the size of the step drilled.

Microstructure of Thermal Barrier Coatings (TBC's) Obtained by Using Plasma Spraying and VPA Methods

Thermal Barrier Coatings are the main type of coatings used for protecting turbine blade surfaces and the surface of modern jet engines combustion chamber parts. Depending on the type of engine element, the coatings are produced as plasma-sprayed MeCrAlY bond-coats with a ceramic outer layer or as Pt-modified aluminide coatings with a ceramic EB-PVD-deposited layer. Currently, research is being conducted on the deposition of a new type of coatings consisting of bond-coats with mulitlayer structure. In the article, the results of the study on the obtainment of TBC's with multilayer structure are presented. To obtain the metallic bond-coat, the process of atmospheric plasma spraying and the out of pack aluminizing (VPA) method were combined. The coatings were deposited on the surface of Rene 80 nickel superalloy. The first layer of the coating was a plasma-sprayed MCrAlY bond-coat, on which a diffusion aluminide layer was deposited with out of pack method. On the bond-coat, a standard ceramic zirconium oxide (ZrO2*20Y2O3) layer was deposited. The microstructure analysis, was conducted, using light and SEM microscopy. The phase and chemical composition analyses were done using EDS and XRD methods.

Stress measurements via photoluminescence piezospectroscopy on engine run thermal barrier coatings

Optical measurements of stress in a thermally grown oxide (TGO) layer that has formed between a metallic bond coat and thermal barrier coating (TBC) have been previously demonstrated and shown useful in understanding aspects of TBC failure. These measurements have promise for nondestructive evaluation of coated turbine components. However, engine-run turbine parts collect significant surface contamination consisting of calcium-magnesium-alumina-silicate (CMAS). The deposited CMAS both blocks optical measurement of stress in the TGO and produces false stress signals. A recently developed laser ablation technique has enabled contaminant removal with minimal TBC damage. The locally cleaned engine run parts can then have their TGO stresses optically determined. In the present paper, it is shown that sporadic signals from contamination are still present due to CMAS infiltration into the TBC and due to local regions that cannot be cleaned without damaging the TBC. Methods for separating the contamination signal from the TBC signals are presented as needed for successful use of optical stress measurement on engine run turbine components. The stress measurements censored for false signals are compared to other destructive approaches and the methods are shown to provide robust nondestructive evaluation of the coating stress.

Microstructure and Thermal Behaviour of Plasma Sprayed Zirconia/Alumina Composite Coating

In thermal barrier coatings (TBC), failure occurs near or at the interface between the metallic bondcoat and topcoat. On high temperature conditions, an oxide scale which is named thermally grown oxide (TGO) occurs along the bond/topcoat interface. For diminishing the creation of TGO, a dense coating with low residual stress and thermal stress buffer layer was preferable. High hardness ceramic coatings could be obtained by gas tunnel type plasma spraying, and the deposited coating had superior property in comparison with those deposited by conventional type plasma spray method. In this study, the gas tunnel type plasma spraying system was utilized to produce a zirconia/alumina functionally graded thermal barrier coating and discussed its physical and mechanical properties, thermal behavior and high temperature oxidation resistance of the coating are discussed. Consequently, the proposed system exhibited superior mechanical properties and oxidation resistance at the expenses of a slightly lower thermal insulating effect. This interlayer is preferred in order to minimize the detrimental effect of the phase transformation of gamma-Al2O3 to alpha-Al2O3.

Processing and microstructure of NiRuAl diffusion barrier coating on Ni-based superalloy

Thermal barrier coatings (TBCs), consisting of ceramic topcoat and metallic bond coat, are applied onto Ni-based superalloy hot components in turbine engines to protect the components from high temperature gas. Detrimental phases in the superalloy substrate such as topologically close-packed phase (TCP) and secondary reaction zone (SRZ), resulting from interdiffusion between the bond coat and the substrate, significantly deteriorate the desired mechanical properties of the superalloy substrate. In this paper, a NiRuAl coating was produced onto Ni-based superalloy to inhibit the interdiffusion between the coating and substrate. During the processing, a dense and continuous Ru film of around 4 μm thickness was first electrodeposited onto a K3 superalloy at 70°C using a current density of 1·2 A dm−2. The NiRuAl coating was formed by pack aluminisation of the Ru coated specimen in argon atmosphere at 900°C. The coating shows a two-layered structure: the top layer NiAl and the bottom layer NiRuAl, with the potential functions of diffusion barrier and high temperature oxidation resistance.

Transient Finite-Volume Analysis of a Graded Cylindrical Shell Under Thermal Shock Loading

A major failure mechanism in thermal barrier coatings involves delamination of the ceramic top coat from the substrate, which is accompanied and/or aided by transverse cracks that initiate at the outer surface. Techniques to mitigate these failure modes include grading the transition region between the ceramic top coat and the metallic bond coat by gradually varying the content of the two phases. This concept is explored herein for a graded cylinder subjected to transient thermal cyclic loading, that simulates a thermal shock durability test, using the parametric finite-volume theory for functionally graded materials. Previous investigation into the transient response of a three-layer cylinder under the considered thermal shock loading revealed two potential failure modes, one of which was a direct result of transient effects. Herein, examination of average phase-level stress fields indicates that grading influences the initiation of delamination and potential radial cracking through redistribution of the radial and hoop stress components in the thermal coating's graded region upon rapid heating. The large hoop stress reversal responsible for radial crack initiation at the outer surface, however, is not reduced upon rapid cooling, pointing to the importance of accurately modeling transient effects in thermal shock testing as well as crack-growth management through grading.