Thermal Barrier Coatings Samples Research Articles

Experimental and Modeling Studies of Bond Coat Species Effect on Microstructure Evolution in EB-PVD Thermal Barrier Coatings in Cyclic Thermal Environments

In this work, the effects of bond coat species on the thermal barrier coating (TBC) microstructure are investigated under thermal cyclic conditions. The TBC samples are prepared by electron beam-physical vapor deposition with two species of bond coats prepared by either air-plasma spray (APS) or high-velocity oxygen fuel (HVOF) methods. The TBC samples are evaluated in a variety of thermal cyclic conditions, including flame thermal fatigue (FTF), cyclic furnace thermal fatigue (CFTF), and thermal shock (TS) tests. In FTF test, the interface microstructures of TBC samples show a sound condition without any delamination or cracking. In CFTF and TS tests, the TBCs with the HVOF bond coat demonstrate better thermal durability than that by APS. In parallel with the experiments, a finite element (FE) model is developed. Using a transient thermal analysis, the high-temperature creep-fatigue behavior of the TBC samples is simulated similar to the conditions used in CFTF test. The FE simulation predicts a lower equivalent stress at the interface between the top coat and bond coat in bond coat prepared using HVOF compared with APS, suggesting a longer cyclic life of the coating with the HVOF bond coat, which is consistent with the experimental observation.

Cracking behaviors of EB-PVD thermal barrier coating under temperature gradient

In this paper, we study the cracking behaviors of single-crystal nickel-based superalloy samples coated with electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) under a thermal gradient experimentally and via the finite element method (FEM). Our results indicate that the stress distribution and failure mode of the TBC samples under the thermal gradient are different from those of samples under a uniform temperature field. The failure of the TBCs under uniform temperature is initiated by interfacial and horizontal cracks, which can result in the separation and buckling of the top coat (TC) layer. However, for the TBCs under a thermal gradient, failure is mainly caused by both vertical TC cracks and interfacial cracks because of the increased transversal stress in the TC layer. Moreover, the initiation and propagation of vertical and horizontal cracks change the failure mode to local spallation of the TC layer. We believe that our findings can contribute to further developments in TBC technology.

Effect of bond-coat surface roughness on failure mechanism and lifetime of air plasma spraying thermal barrier coatings

This work investigates the failure mechanism of an air plasma spraying (APS) thermal barrier coatings (TBCs) with various bond coat (BC) surface toughness under constant temperature loading condition. Results show that the increased BC surface roughness decreased the lifetime. Moreover, the relationship between the TGO depth and the lifetime can be well described by linear law. The failure mechanism of the TBCs samples was studied by cross-sectional morphologies observation and using the finite element method (FEM). Cracks in the TBCs samples with higher BC surface roughness are initiated from the defects on the BC caused by Al depletion in the BC layer. However, cracks in the samples of TBCs with lower BC surface roughness condition preferentially occur at the TC-TGO interface. This finding could be attributed to the high mismatch stresses in the TC-TGO interface.

Thermal radiation and conduction in functionally graded thermal barrier coatings. Part I: Experimental study on radiative properties

Functionally graded thermal barrier coatings (FGTBCs) are functional composites with multilayered structures whose mechanical properties are better than those of conventional ceramic thermal barrier coatings (TBCs). Although many theoretical and experimental studies concerning both the mechanical properties and heat transfer including heat conduction and thermal radiation of TBCs have been conducted, little attention was paid to heat transfer process, especially the coupled conduction-radiation heat transfer, inside FGTBCs. This two-part paper systematically studied the heat transfer inside YSZ/NiCoCrAlY FGTBCs. In Part I, we investigated their radiative properties through experimental measurements. In Part II, we presented a study on the thermal conductivity of FGTBCs and established a coupled heat transfer model. In this paper, more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY duplex TBC and multilayered FGTBC samples with different multilayer structures, porosities and thicknesses were fabricated via air plasma spraying. The transmittance and reflectance spectra within the 0.3–15μm wavelength range were experimentally obtained by UV–visible spectrometry and Fourier transform infrared spectroscopy. A scanning electron microscope analysis was carried out to characterize the microstructures of FGTBCs. Additionally, we studied each layer of the FGTBCs individually; the four-flux method was employed to obtain the radiative properties based on our experimental measurements. Functionally graded materials (FGM) show larger absorption coefficients and smaller scattering coefficients compared to pure YSZ. The FGTBCs show lower reflectance than YSZ TBCs. We also study the influence of the number of layers on reflectance. The results indicate that FGTBCs have worse thermal insulation ability against radiative flux tconventional ceramic TBCs of the same thickness. This paper provides practical guidance to improve the design of YSZ/NiCoCrAlY FGTBCs and other FGM coatings to reach a balance between good thermal insulation and mechanical strength.

Numerical investigation on the cracking behaviors of thermal barrier coating system under different thermal cycle loading waveforms

The influence of a thermal cycle fatigue (TCF) loading waveform on cracking behaviors of an atmospheric plasma-sprayed thermal barrier coating (TBC) system was numerically studied using the commercial software. Results showed that the TCF loading waveform considerably affects stress distributions in the TBCs and is the leading factor for the initiation and propagation of cracks in TBCs samples. Under triangular load waveform, the cracks initiate and grow in top-coat (TC) layers. However, under trapezoidal load waveform, the cracks are likely to initiate at an off-valley location and propagate along the TC/thermally-grown-oxide (TGO) interface, causing the complete spallation of the TC layer. Furthermore, the influences of the prior time and holding time on the crack initiation and propagation under the trapezoidal loading conditions were discussed, respectively. Interfacial cracks are likely to initiate when the prior time is prolonged. Under the trapezoidal loading conditions, the location of crack initiation moves from the peak to the off-valley location of a TC/TGO interface as holding time increases. Under a long holding time of the trapezoidal loading conditions, an improved spray quality of TC/TGO interface can increase the service life of TBCs.

Investigation on Topcoat Uniformity of Thermal Barrier Coating Using Pulsed Infrared Thermography

In this study, the uniformity of the topcoat of a thermal barrier coating (TBC) was investigated using pulsed infrared thermography. Two plasma-sprayed TBC samples on different substrate materials, with non-uniform topcoats varying from 0.1 mm to 0.6 ㎜ were considered. A short high-energy light pulse was applied to the surface of the samples, and an infrared camera was used to record surface temperature evolution during the entire thermal transient period. The relationship between the measured temperatures and their spatial distributions in the non-uniform topcoat was studied, and it was observed that temperatures in the thinner topcoat region were lower than those in the thicker topcoat region. Furthermore, the recorded thermal image sequences were processed using Fourier transforms to obtain phase angle images, and the phase angle distribution was examined as a function of the non-uniform topcoat. It was found that phase angle decreased with topcoat thickness. Our results showed that variations in temperature and phase angle were significantly correlated with non-uniformity in our investigation of TBC topcoat uniformity.

Eddy Current Thermography with Adaptive Carrier Algorithm for Non-destructive Testing of Debonding Defects in Thermal Barrier Coatings

Interfacial defects can cause the premature failure of thermal barrier coatings (TBCs). An Eddy current thermography (ECT) method under the transmission mode in the heating phase is developed in this study to detect the artificial debonding defects in TBCs samples. When ECT is used, the temperature distribution of specimen surface is uneven on account of geometric heating effect, skin effect, edge effect and abnormal emissivity. Among them, the influence of the surface emissivity is the smallest, because the background noise is subtracted before the thermal images are processed. The uneven temperature distribution shields the weak thermal response characteristics of the defects and interferes with the identification of the defects. Adaptive carrier algorithms are established as post-processing algorithms to resolve this problem. The feasibility and validity of the developed methods are verified by simulation and validation tests using TBCs samples with a 2 mm artificial debonding defect and a 0.5 mm blind-hole defect.

The effect of thermal loading waveform on the failure mechanism of atmospheric-plasma-sprayed thermal barrier coating system

The hot-end components of gas turbine engines in the serviced condition usually suffer from the thermal cycle loading effect. Thermal barrier coating (TBCs) have been used as a protective coating to prevent degradation during practical applications. In the current work, the influence of a loading waveform on the failure mechanism of an atmospheric plasma-sprayed TBCs is investigated at an elevated temperature. The samples under the trapezoidal loading condition present a lower thermal cycling fatigue lifetime than that under the triangular loading condition. Moreover, cracks in the TBCs samples under the triangular loading condition are initiated from the defects on the top coat formed during preparation, and cracks under the trapezoidal loading condition preferentially occur at the thermally grown oxide-top coat interface due to higher stress at the interface. Our results demonstrate that the thermal loading waveform should be carefully designed to characterize the in-service condition of hot-end components.

Mechanical Properties of Layered La2Zr2O7 Thermal Barrier Coatings

Lanthanum zirconate (La2Zr2O7) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La2Zr2O7 and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La2Zr2O7 + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La2Zr2O7 TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La2Zr2O7. The erosion rate of La2Zr2O7 TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La2Zr2O7. DCL porous 8YSZ + La2Zr2O7 had a lower erosion rate than other SCL and DCL La2Zr2O7 coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.

Mechanisms governing the thermal shock and tensile fracture of PS-PVD 7YSZ TBC

Thermal barrier coating (TBC) system including NiCoCrAlYTa metallic bond coating and 7YSZ (7wt%Y2O3-ZrO2) ceramic top coating was deposited on nickel-based superalloy by plasma spray-physical vapor deposition (PS-PVD). Thermal shock property of 7YSZ TBC was characterized by water-quenching test at 1100℃ and its failure behaviors were investigated in detail. Besides, tensile test was performed for TBC sample and its cross-sectional fracture microstructure was studied as well. The results showed that after water-quenching test lots of pitting spallation took place in TBC surface, but no obvious microcracks were observed. Additionally, the tensile test indicated that fracture occurred in 7YSZ coating near the interface of ceramic-bond coating. After conduction of water-quenching and tensile testing, a lot of spherical particles and nano-sized agglomerated clusters were observed in the quasi-columnar structured 7YSZ coating. These lead to the formation of weak inter-column bonding and the failure of PS-PVD 7YSZ TBC. Moreover, in order to better understand the failure process, a deposition mechanism of coating was proposed.

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.

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.

Effects of Composition, Structure Design, and Coating Thickness of Thermal Barrier Coatings on Thermal Barrier Performance

The effects of composition, structure design, and coating thickness of thermal barrier coating (TBC) on thermal barrier performance were investigated by measuring the temperature differences of TBC samples. TBCs with the thin and thick top coats were used for these studies, including TBCs with rare-earth (Gd, Yb, and La) compositions. The thermal barrier performance was enhanced with increasing the thickness of top coat even for thin TBCs, indicating that the thermal barrier performance was commensurate to the thickness of top coat. On the other hand, the bi-layered TBC, which was prepared with Yb-Gd-YSZ feedstock powder, with the buffer layer of high purity 8YSZ showed a better thermal barrier performance than that of regular purity 8YSZ. The interfaces in the bi-layered TBCs had a decisive effect on the thermal barrier performance, showing the performance enhanced with increasing numbers of interfaces. However, a new structural design and an additional process should be considered to reduce stress concentrations and to ensure interface stability, respectively, for improving thermal durability in the multilayered TBCs. Key words: Thermal barrier coating, Thermal barrier performance, Thickness, Composition, Layered structure, Interface

Cyclic oxidation behavior of SiC-containing self-healing TBC systems fabricated by APS

It is well-known that thermally grown oxide (TGO) deteriorates thermal barrier coating (TBC) systems after extended operating time. In this study, SiC-self-healing coatings (called “SAZ coatings” here) were investigated as a possible solution to enhance the oxidation resistance and spallation resistance of TBC systems. The self-healing-related filling and sealing effects of the proposed coatings were investigated and the oxidation resistance of the system after SAZ coating treatment was examined. To fully understand the oxidation resistance and spallation resistance mechanisms, we also conducted high-temperature cyclic oxidation tests (1100 °C) and recorded the mass gain and mass loss of samples after each cycle. Results showed that the corrosion current of YSZ/SAZ TBC samples decreased 79.23% after oxidation at 720 °C for a period of 10 h, and that 25.77% less TGO grew in YSZ/SAZ TBC samples after 20 h at 1100 °C compared to YSZ TBC samples due to the self-healing coating layer. The lifetime of YSZ/SAZ TBC was improved 58.33% compared to YSZ TBC in the high-temperature cyclic oxidation test, owing to the higher oxidation resistance and toughening effect of the SAZ coating.

Enhancement of high temperature oxidation resistance and spallation resistance of SiC-self-healing thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) are extensively used to protect metallic blades in gas-turbine engines where the operating conditions include an aggressive environment at high temperatures. The most important factor controlling TBC durability is the growth of a thermally grown oxide (TGO) layer which is formed during high temperature oxidation. To improve the oxidation resistance of conventional TBC, the SiC-self-healing coating, named the SAZ coating, was prepared by air plasma spray (APS) technique on the top of the classic bilayer TBC (YSZ TBC), which consist of the bond coating (BC) and the YSZ coating. Cracks allow SiC particles locating on the crack outside surface to react with the oxygen in the atmosphere at the temperature above 720 °C resulting in healing. As oxidation progress, the crack surfaces are covered with the formed oxide. Finally, the space between the crack surfaces is completely or incompletely filled with the formed oxide. The sealing effect enhanced the oxygen diffusion resistance of SAZ coatings, therefore the partial pressure of oxygen at the BC-YSZ interface of SAZ TBC samples would be lower than of YSZ TBC samples. The oxidation and spallation resistance of the SAZ TBC samples will be enhanced because the growth rate of TGO is slower due to the low partial pressure of oxygen. The high temperature cyclic oxidation tests performed at 1127 °C was carried out to investigate the oxidation and spallation resistance of the SAZ TBC. The results demonstrated that the mass gain of SAZ TBC samples, which was related to oxidation resistance, was 63.29% of YSZ TBC samples, and the mass loss of SAZ TBC samples, which was related to spallation resistance, was 56.08% of YSZ TBC samples.

Evaluation of Titanium Nitride-Modified Bondcoat System Used in Thermal Barrier Coating in Corrosive Salts Environment at High Temperature

Thermal barrier coating (TBC) systems were produced by air plasma spraying system on nickel base superalloy. These coatings were composed of a Y2O3-stabilized ZrO2 topcoat and a CoNiCrAlY bondcoat and are known as standard TBC. In this paper, standard TBC samples were compared with TiN-modified bondcoat TBC samples. Titanium nitride was deposited by utilizing a physical vapor deposition technique. Both TBC systems were exposed to high temperature in the presence of corrosive salts, i.e. a mixture of V2O5 and Na2SO4 (50:50) for 50 h. It was observed that the TiN-modified samples showed better results in terms of oxidation resistance and delamination. The formation of Cr2Tin−2O2n−1 phases at the interface of the topcoat–bondcoat, in TiN-modified samples were found to enhance the thermal and oxidation properties of the TBC.

Fatigue Performance of TBCs on Hastelloy X Substrate During Cyclic Bending

Our previous experiments with low-cost steel substrates confirmed that individual steps of conventional thermal barrier coating (TBC) deposition may influence fatigue properties of the coated samples differently. In the presented study, testing was carried out for TBC samples deposited on industrially more relevant Hastelloy X substrates. Samples were tested after each step of the TBC deposition process: as-received (non-coated), grit-blasted, bond-coated (NiCoCrAlY), and bond-coated + top-coated yttria-stabilized zirconia (YSZ). Conventional atmospheric plasma spraying (APS) was used for deposition of bond coat and top coat. In addition, for one half of the samples, dual-layer bond coat was prepared by combination of high-velocity air-fuel (HVAF) and APS processes. Samples were tested in the as-sprayed condition and after 100 hours annealing at 980 °C, which simulated application-relevant in-service conditions. Obtained results showed that each stage of the TBC manufacturing process as well as the simulated in-service heat exposure may significantly influence the fatigue properties of the TBC coated part. HVAF grit-blasting substantially increased the fatigue performance of the uncoated substrates. This beneficial effect was suppressed by deposition of APS bond coat but not by deposition of dual-layer HVAF + APS bond coat. All heat-treated samples showed again enhanced fatigue performance.

Nondestructive testing of the fatigue properties of air plasma sprayed thermal barrier coatings by pulsed thermography1

The fatigue properties of air plasma sprayed (APS) thermal barrier coatings (TBC) were investigated by quantitative Pulsed Thermography (PT). This analysis principally concerned with microstructural evolution during the thermal cyclic. The microstructural features of the specimens were examined by scanning electron microscopy (SEM). The TBC fatigue life was correlated with the development of the thermally grown oxide (TGO) and defects cracks. For better understanding of the thermal response from the surface of TBC, APS TBC samples were inspected as a function of ageing time by PT. The changes in the thermal response amplitude (TRA) were found to correlate with the TGO layer growth and crack nucleation, which was verified by SEM results. The experimental results demonstrated that the thermal response decreased correspondingly along with the increase of the TGO and cracks thickness. The relationships of TRA obtained from the surface of TBC samples with the thickness of TGO and cracks were established. A simple and effective method for quantitative measuring TGO layer and cracks thicknesses using the nondestructive testing of PT was proposed. The experimental results provide a basis for the application of PT in the nondestructive testing of the fatigue properties of TBC thermal fatigue.

Can TBC porosity be estimated by non-destructive infrared techniques? A theoretical and experimental analysis

Since thermal properties of solids such as thermal conductivity, thermal diffusivity and specific heat are related by equations containing the density of the material, by measuring two of the aforementioned parameters it would be possible to estimate the density and thus the porosity, if the theoretical density of the material it is known.Most of the techniques used for measuring thermal properties of TBCs are the contactless photothermal techniques. Among all these techniques, those able to measure thermal properties of TBCs deposited onto a metallic substrates offer the great advantage to be potentially applicable to real coated components without the need of any machining. This occurrence is typically related to the possibility to adopt a reflection experimental configuration, where the thermal stimulation and the detection of the sample response are measured on the same face of the component.A theoretical and experimental analysis of real capabilities of infrared techniques to estimate the porosity content of porous ceramic materials such as thermal barrier coatings (TBCs) is carried out.Experimental and theoretical analysis confirmed that, independently from the specific microstructural features of the TBC samples, the contactless photothermal techniques allow to non-destructively estimate the porosity content with an overall uncertainty estimated theoretically as small as ±5%. This uncertainty could result compatible with the requirements, especially for large porosity TBCs. Besides the uncertainty directly connected with the measurement, the uncertainty of the substrate effusivity, TBC specific heat and thickness contribute to the porosity estimation accuracy as well.

高温引張負荷下でのセラミック遮熱コーティングの損傷過程について

Thermal barrier coatings (TBCs) have been deposited onto the blade surface in a land-based gas turbine according to increase of turbine inlet temperature (TIT). In service of those gas turbines, it has been known that cracking and delamination in TBC is one of the serious damages. Thus, it is very important issue to assure the reliability for TBCs or advanced protective coatings in service. The aim of this study is to clarify how micro damage progresses in TBC and porous TBC, which is called P-TBC here, under a tensile loading at a high-temperature environment. Here, P-TBC has been developed by Arai in order to reduce thermal conductivity and infiltrate coolant gas for achieving transpiration cooling technology. As a result, the surface cracks propagated in a direction perpendicular to the interface in both TBC and P-TBC samples. When a tip of those cracks reached the interface, the crack was curved along the interface between substrate and bond coat in case of TBC sample at room temperature especially. On the other hand, the surface crack propagated along the interface between top and bond coats in case of P-TBC sample. In the tensile test conducted at 1273K, the crack path was located at the interface between top and bond coats in both TBC and P-TBC samples. The quantitative evaluation revealed that critical strain up to crack initiation was almost the same at TBC and P-TBC, and number of cracks per length in TBC sample was higher than that of P-TBC. The J-integral-based adhesive strength of TBC sample was stronger than that of P-TBC.