Thermal Barrier Coatings Samples Research Articles

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.

Topcoat Thickness Measurement of Thermal Barrier Coating of Gas Turbine Blade Using Terahertz Wave

SUMMARYThe topcoat thickness of thermal barrier coating (TBC) applied to a gas turbine blade was measured using terahertz waves. The refractive index of the topcoat was obtained by frequency analysis of the waves reflected from the topcoat surface and the interface between the topcoat and bondcoat. The surface roughness of the topcoat surface was considered for improving the accuracy of the refractive index calculation. The topcoat thickness was obtained from the refractive index and the time separation between the reflected waves. The validity of the method was confirmed using a TBC sample with variable topcoat thickness, and the obtained topcoat thickness was in agreement with measurement results using a contact thickness gauge. Error analysis showed that the measurement error of the topcoat thickness was about 3%. The method was applied to a gas turbine blade with TBC, and the measured topcoat thickness agreed with microscope observation results of the cross section to within 6%, which was within the range of measurement error. The effect of curvature of the gas turbine blade surface did not result in significant measurement error. The results showed that terahertz waves are effective for nondestructive measurement of the topcoat thickness of TBC applied to gas turbine blades.

An Investigation of Phase Crystallinity in Laser Modified Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coating<sup></sup>

This paper presents laser surface modification process of plasma sprayed yttria stabilized zirconia (YSZ) thermal barrier coating (TBC) for enhanced hardness properties and low surface roughness. A 300W JK300HPS Nd: YAG laser was used to process YSZ TBC sample surface. The parameters selected for examination were laser power, pulse repetition frequency (PRF) and residence time. Micrographs of the TBC system were captured using EVO 15 Scanning Electron Microscope (SEM). Surface roughness was measured using 2-dimensional stylus profilometer. X-ray diffraction analysis (XRD) was conducted to measure phase crystallinity of the laser-modified coating surface. X-ray diffraction patterns were recorded in the 2θ range of 10 to 80° using Bruker D8 Advance system with 0.7Å wavelength from a copper source (~1.5Å). The laser modified surface exhibited higher crystallinity compared to the as-sprayed samples. The presence of tetragonal phase was detected in the as-sprayed and laser processed samples. The hardness properties of laser modified TBC increased 15% of the as-sprayed sample. These finding are significant to development of thermal barrier coating design optimization for enhanced surface properties of semi-solid forming die.

Pulsed Thermal Imaging Measurement of Thermal Properties for Thermal Barrier Coatings Based on a Multilayer Heat Transfer Model

Thermal properties of thermal barrier coatings (TBCs) are important parameters for the safe and efficient operation of advanced turbine engines. This paper presents a new method, the pulsed thermal imaging–multilayer analysis (PTI–MLA) method, which can measure the coating thermal conductivity and heat capacity distributions over an entire engine component surface. This method utilizes a multilayer heat transfer model to analyze the surface temperature response acquired from a one-sided pulsed thermal imaging experiment. It was identified that several experimental system parameters and TBC material parameters may affect the coating surface temperature response. All of these parameters were evaluated and incorporated as necessary into the formulations. The PTI–MLA method was demonstrated by analyzing three TBC samples, and the experimental results were compared with those obtained from other methods.

In situ synthesis of α-alumina layer at top yttrium-stabilized zirconia thermal barrier coatings for oxygen barrier

The thermally grown oxide (TGO) at the interface of a ceramic-bond coating plays an important role in the failure of the thermal barrier coating (TBC). Therefore, it is important to control the growth of TGO to improve its service lifetime. In this paper an aluminum film was first deposited on the surface of atmospheric plasma sprayed TBC by direct current circular magnetron sputtering. Then the TBC samples with the aluminum film received a vacuum heat treatment. In situ synthesis of α-Al2O3 layer was observed at the top of the ceramic coating through the reaction of Al and ZrO2. The schematic diagram and thermodynamic mechanism of in situ synthesis of α-Al2O3 layer were presented. The cross sections of as-sprayed and treated TBCs were characterized by field emission-scanning electron microscopy (FE-SEM). Both phase structures were analyzed by X-ray diffraction (XRD) and transmission electron microscope (TEM) assisted with energy dispersive spectroscopy (EDS) and focused ion beam (FIB). Finally, oxidation resistance was compared between as-sprayed TBCs and vacuum treated TBCs and the average thickness of TGO was 4.89±0.53 and 2.52±0.47μm correspondingly.

Microstructure Evolution and Thermal Durability with Coating Thickness in APS Thermal Barrier Coatings

The effects of the coating thickness on the delamination or fracture behavior of thermal barrier coatings (TBCs) were investigated through the cyclic furnace thermal fatigue (CFTF) and thermal shock (TS) tests. The TBCs were prepared using a NiCrAlY bond coat and an yttria-stabilized zirconia top coat, which were formed using the air plasma spray (APS) process. The thicknesses of the top coat were 200 and 400μm, and those of the bond coat were 100 and 200μm. TBC samples with a thickness ratio of 2:1 in the top and bond coats were employed in the CFTF and TS tests. After CFTF for 1429 cycles, the interface microstructure of the relatively thick TBC was in a sound condition without any cracking or delamination; however, the relatively thin TBC was delaminated near the interface between the top and bond coats after 721 cycles. In the TS, the TBCs were fully delaminated (> 50%) after 140 and 194 cycles for thicknesses of 200 and 400μm in the top coat, respectively. These observations allow us to control the thickness of TBC prepared using the APS process, and the thicker TBC is more efficient in improving thermal durability in the cyclic thermal exposure and thermal shock environments.

Evolution of Lamellar Interface Cracks During Isothermal Cyclic Test of Plasma-Sprayed 8YSZ Coating with a Columnar-Structured YSZ Interlayer

The failure of plasma-sprayed thermal barrier coatings (TBC) usually occurs through spalling of ceramic coating. The crack evolution during thermal cycling of TBC is directly associated with its spalling. In this paper, the cracks in TBC along the direction of the interface between ceramic coating and bond coat were examined from cross-section of TBC experienced different numbers of thermal cycle, and crack number and the total length of cracks were measured to aim at understanding the failure mechanism. TBC consists of cold-sprayed NiCoCrAlTaY bond coat on IN738 superalloy and double layered plasma-sprayed 8YSZ with a columnar grain structured YSZ interlayer of about 20 μm thick and about 230 μm lamellar YSZ. With each isothermal cyclic test, the TBC samples were kept at 1150 °C for 26 min hold and then cooled down to a temperature less than 80 °C in 4 min by air forced cooling. Results showed that cracks propagated primarily within lamellar-structured YSZ over the columnar YSZ along lamellar interface. The measurement from the cross-section revealed that crack number and total crack length apparently increased with the increase of the number of thermal cycle. It was found that cracks with a length less than a typical size of 200 μm accounted for the majority of cracks despite the number of thermal cycle during the test. A crack initiation and propagation model for plasma-sprayed TBC is proposed with a uniform distribution of circular cracks. The propagatable cracks form homogeneously within plasma-sprayed porous YSZ coating at the early stage of thermal cycling and propagate at an identical rate during thermal cycling. Only a few of large cracks are formed before most cracks reach to the critical size for multi-cracks linking-up. The propagation of most cracks to the critical size will leads to the rapid crack bridging and subsequent spalling of top ceramic TBC.

Laser Shock Adhesion Test (LASAT) of EB-PVD TBCs: Towards an industrial application

The assessment of the interface strength of EB-PVD thermal barrier coating (TBC) is a key issue to control the production and better understand the ceramic spallation that will occur during life duration of coated turbine blades. The Laser Shock Adhesion Test (LASAT) involving bi-dimensional shock wave propagation, namely the LASAT-2D, consists in measuring the interfacial crack diameter when implementing a set of laser shocks with increased laser power densities. From the resulting “LASAT-2D plots”, adhesion strength could be obtained from tensile stress calculated at the interface with a finite element model of shockwave propagation combined to interface cracking criterion. This work is focusing on the qualitative approach to compare adhesion on TBC systems directly through LASAT-2D plots. Two different testing modes were investigated: the conventional MS mode when the laser irradiation is carried out on the metallic side of a TBC coupon and the newly developed CS mode for which the laser shock is implemented onto the ceramic side. The interfacial crack is revealed by the presence of a spot that could be measured on a top-view optical image of the ceramic. In this work, the actual relationship between spot diameter and interfacial inner crack size is clearly established for both MS and CS modes. It was achieved experimentally by examinations of SEM cross-sections or non-destructive piezospectroscopic measurements involving lifetime decay map of the photoluminescence signal. From LASAT-2D curves, the role of the laser conditions like laser shock diameter and substrate thickness is discussed. This discussion is illustrated through the results of the qualitative LASAT-2D method applied on two different TBC samples (as-coated and thermally-cycled). The final objective of this work is to enable a sound and relevant application of the LASAT on TBC systems for both modes. With this goal, the LASAT-2D CS was successfully applied directly on a TBC coated turbine blade.

Measurement of topcoat thickness of thermal barrier coating for gas turbines using terahertz waves

AbstractThe topcoat thickness of thermal barrier coating (TBC) samples was measured using terahertz (THz) waves. The index of refraction of yttria‐stabilized zirconia (YSZ), which is the topcoat material, is necessary to obtain the topcoat thickness from time domain reflectometry. Time domain THz spectroscopy was applied to YSZ samples, and the index of refraction was measured to be 4.8 in the frequency region 0.1 to 1.2 THz. The topcoat thickness of six different TBC samples, which varied from 300 to 600 µm, was measured using THz waves. The results were in agreement with microscope observation results to within measurement error. In addition, the topcoat thickness profile of a turbine blade sample was measured with a resolution of 2.4 µm. The profile showed a standard deviation of 4 µm, which reflects the actual variation in the topcoat thickness. The results showed that THz waves are effective for high‐resolution measurement of the topcoat thickness. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(4): 1–9, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22385

Microstructural characterization of porous thermal barrier coatings by IR gas porosimetry and sintering forecasts

It is known that the thermal diffusivity of plasma sprayed coatings is quite sensitive to the operating conditions, namely the composition and pressure of the operating atmosphere. This makes it possible, in principle, to obtain information, in a non-destructive way, on the microstructure of thermal barrier coatings (TBCs) from thermal diffusivity measurements. An experimental assessment of this idea is presented in this paper. The microstructure of as-sprayed and sintered free-standing atmospheric plasma sprayed YPSZ TBC samples have been characterized by a new technique, named infrared (IR) gas porosimetry, as well as by mercury intrusion porosimetry and image analysis. The results from these different techniques have been compared. Furthermore, the microstructure and thermal diffusivity of sintered samples have been compared with the sintering forecasts produced by Cipitria’s sintering code coupled with the Bruggeman model for thermal diffusivity estimation. Sample-to-sample variations have been discussed and uncertainties in experimental techniques have been analysed. Conditions for accurate microstructural estimations have been obtained and suggestions on the reliability of the statistical evaluation are provided.

Measurement of Topcoat Thickness of Thermal Barrier Coating for Gas Turbines using Terahertz Waves

The topcoat thickness of thermal barrier coating (TBC) samples was measured using terahertz (THz) waves. The index of refraction of yttria stabilized zirconia (YSZ), which is the topcoat material, is necessary to obtain the topcoat thickness from time domain reflectometry. Time domain THz spectroscopy was applied to YSZ samples, and the index of refraction was measured to be 4.8 in the frequency region 0.1-1.2 THz. The topcoat thickness of 6 different TBC samples, which varied from 300 to 600 μm, was measured using THz waves. The results were in agreement with microscope observation results to within measurement error. In addition, the topcoat thickness profile of a turbine blade sample was measured with 2.4 μm resolution. The profile showed a standard deviation of 4 μm, which reflects the actual variation in the topcoat thickness. The results showed that THz waves are effective for high resolution measurement of the topcoat thickness.

Non-destructive thermographic monitoring of crack evolution of thermal barrier coating coupons during cyclic oxidation aging

The diffused microscopic cracking of the interface between a thick (>300μm) air plasma spray (APS) thermal barrier coating (TBC) and a bond coat cannot be clearly identified by non-destructive evaluation and testing techniques. In this work, a semi-quantitative estimation of cracks at the TBC interface is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling using a single-side thermographic technique. In fact, during thermal cycling, two phenomena occur: sintering, which promotes a significant increase in thermal diffusivity, and cracking, representing an additional thermal resistance which causes an apparent decrease in thermal diffusivity. The results refer to an experimental activity carried out on 28 APS TBC samples cyclic aged at six different fractions of their lifetime. For each sample, the thermal diffusivity was measured at fixed lifetimes, and the evolution of the cracked fraction of the interface was estimated by adopting a two-dimensional inversion model. Furthermore, at each of the six lifetime fractions, some samples were destructively characterized by image analysis, and the results were compared with the estimations given by the inversion model. Good agreement between the non-destructive estimations and image analysis results was obtained. Moreover, a figure of merit incorporating both the cracked fraction and the crack thickness is also proposed for ranking the damage.

Miniaturized bend tests on partially stabilized EB-PVD ZrO 2 thermal barrier coatings

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests. Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined. In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.

MICROSTRUCTURAL EVOLUTION AND RESIDUAL STRESSES OF AIR-PLASMA SPRAYED THERMAL BARRIER COATINGS UNDER THERMAL EXPOSURE

Microstructural evolution and fracture behavior of zirconia ( ZrO2 )-based thermal barrier coatings (TBCs) were investigated under thermal exposure. New ZrO 2 granule with 8 wt.% yttria ( Y2O3 ) with a deformed hollow morphology was developed through a spray drying process and employed to prepare TBCs. The thermal exposure tests were conducted at 1210°C with a dwell time of 100 h till 800 h. The residual stress at the interface between top coat and thermally grown oxide (TGO) layer was measured using a nanoindentation technique before and after thermal exposure. Vertical cracks on the top coat were newly formed and interlamellar cracks at the interface were enhanced after the thermal exposure of 800 h. Especially, partial delamination was observed at the interface after the thermal exposure of 800 h in TBC samples tested. The microstructural evolution in the top coat could be defined through load–displacement curves, showing a higher load or a less displacement after the thermal exposure of 800 h. The stress state was strongly dependent on the TGO geometry, resulting in the compressive stresses at the "valleys" or the "troughs," and the tensile stresses at the "crests" or peak areas, in the ranges of -500 to -75 MPa and of +168 to + 24 MPa, respectively. These stress terms incorporated with resintering during thermal exposure affected the mechanical properties such as hardness and elastic modulus of the top coat.

Effect of Water Vapor on the Spallation of Thermal Barrier Coating Systems During Laboratory Cyclic Oxidation Testing

The effect of water and water vapor on the lifetime of Ni-based superalloy samples coated with a typical thermal barrier coating system—β-(Ni,Pt)Al bond coat and yttria stabilized zirconia (YSZ) top coat deposited by electron beam physical vapor deposition (EB-PVD) was studied. Samples were thermally cycled to 1,150 °C and subjected to a water-drop test in order to elucidate the effect of water vapor on thermal barrier coating (TBC) spallation. It was shown that the addition of water promotes spallation of TBC samples after a given number of cycles at 1,150 °C. This threshold was found to be equal to 170 cycles for the present system. Systems based on β-NiAl bond coat or on Pt-rich γ/γ′ bond coat were also sensitive to the water-drop test. Moreover, it was shown that water vapor in ambient air after minutes or hours at room temperature, promotes also TBC spallation once the critical number of cycles has been reached. This desktop spalling (DTS) can be prevented by locking up the cycled samples in a dry atmosphere box. These results for TBC systems confirm and document Smialek’s theory about DTS and moisture induced delayed spalling (MIDS) being the same phenomenon. Finally, the mechanisms implying hydrogen embrittlement or surface tension modifications are discussed.

Surface sealing of electron beam physical vapor deposited thermal barrier coatings by high-intensity pulsed ion beam

A surface sealing technology by employing high-intensity pulsed ion beam (HIPIB) technique is developed for electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) to further improve the high temperature performance. The HIPIB irradiation can deliver a high-density energy up to 100J∕cm2 into the shallow ion range of about 0.1–10μm within a short pulse width of less than 1μs, by which a micrometer-thick bridging layer was achieved on the gap between columnar grains of EB-PVD TBCs under a single pulse of 4.5J∕cm2. The thin bridging layer provided atmosphere shielding for the TBCs along with matching strain tolerance of the columnar top coat during thermal cycling. Cyclic oxidation tests at 1050°C demonstrated that weight gain of the sealed TBC samples is decreased by about 60% from 0.8mg∕cm2 of the as-deposited ones.

Formation of diffusion barrier on the Ni-based superalloy by low-pressure pre-oxidation

The formation of diffusion barrier on the Ni-based superalloy substrate by low-pressure pre-oxidation is studied in this paper. The pre-oxidation was carried out by heating the substrate under a low-oxygen partial pressure with electron beam of the electron beam physical vapor deposition (EB-PVD) facility. The effect of diffusion barrier was investigated by both isothermal and cyclic oxidations at 1373 K, and the oxidation-kinetic curves were then obtained. The diffusion barrier mainly contains NiO, α,θ-Al 2O 3 and Cr 2O 3, and has a thickness of 185–225 nm. The investigation indicates that the barrier could partially inhibit the interdiffusion of elements of the superalloy substrate and then improve the oxidation-resistance performance of thermal barrier coatings (TBCs) to some extent. However, the difference of thermal cycling lifetime between TBCs samples with and without diffusion barrier is not extremely visible.

An Experimental Investigation of Interface Fracture Characteristic of APS Thermal Barrier Coating System under Bending at 1000 °C

This paper investigated the thermo-mechanical bending failure characteristic of air plasma-sprayed (APS) thermal barrier coating (TBC) system at 1000 oC by three-point bending test. A through-width surface pre-crack on the centre surface of the top ceramic coating (TC) in TBC sample was firstly designed and made by air plasma spraying technology. The fracture characteristic and spallation phenomena of the TBC were investigated under bending test in air at 1000 oC. The effects of constant displacement rate, span width of bending equipment and thermal aging time on the thermo-mechanical bending failure of the APS TBC system were slight at 1000 oC. Using SEM observations and EDX analysis, the interface cracks initiation, propagation and coalescence were discussed in detail. Furthermore, the fracture surface located within the TBC close to the TBC/TGO interface for all APS TBC samples under bending at 1000 oC.

Metallographic techniques for evaluation of Thermal Barrier Coatings produced by Electron Beam Physical Vapor Deposition

Thermal Barrier Coatings (TBC) produced by Electron Beam Physical Vapor Deposition (EB-PVD) are primarily applied to critical hot section turbine components. EB-PVD TBC for turbine applications exhibit a complicated structure of porous ceramic columns separated by voids that offers mechanical compliance. Currently there are no standard evaluation methods for evaluating EB-PVD TBC structure quantitatively. This paper proposes a metallographic method for preparing samples and evaluating techniques to quantitatively measure structure. TBC samples were produced and evaluated with the proposed metallographic technique and digital image analysis for columnar grain size and relative intercolumnar porosity. Incorporation of the proposed evaluation technique will increase knowledge of the relation between processing parameters and material properties by incorporating a structural link. Application of this evaluation method will directly benefit areas of quality control, microstructural model development, and reduced development time for process scaling.

An experimental investigation on thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings

The primary intention of this work is to investigate the thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings (TBCs) under compression tests at high temperature. The TBCs samples with a pre-delamination were firstly designed and they had been successfully prepared by air plasma sprayed technique. The main novelty of this paper is that the first work to validate and obtain three kinds of the interface failure forms in TBCs system during compression tests, i.e. buckling delamination, edge delamination and global buckling failure. The effects of the initial delamination length, temperature gradient and applied mechanical load on the delamination resistance of the TBCs system were discussed in detail. It is difficult to observe buckling delamination or edge delamination failure phenomena until the initial delamination length in TBCs reaches or exceeds 4 mm or more. For edge delamination failure, the interface fracture toughness ( Γ i II), energy release rate ( G ss edge) and stress intensity factor ( K II) between the TBC/TGO interface were 35 J m − 2 , 38.8 J m − 2 and 0.97 MPa m at high temperature gradient, respectively. Using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX), it was inferred that the delamination fracture located within the ceramic coating close to the TBC/TGO interface. The results agree well with other experimental and theoretical results.