Top Coat Of Thermal Barrier Coatings Research Articles

Temperature Monitoring of Through-Thickness Temperature Gradients in Thermal Barrier Coatings Using Ultrasonic Guided Waves

Ultrasonic guided waves offer a promising method of monitoring the online temperature of plate-like structures in extreme environments, such as aero-engine nozzle guide vanes (NGVs), and can provide the resolution, response rate, and robust operation that is required in aerospace. Previous investigations have shown the potential of such a system but the effect of the complex physical environment on wave propagation is yet to be considered. This article uses a numerical approach to investigate how thermal barrier coatings (TBCs) applied to the surface of many components designed for extreme thermal conditions will affect ultrasonic guided wave propagation, and how a system can be employed to monitor through-thickness temperature changes. The top coat/bond coat boundary in NGVs has been shown to be a temperature critical point that is difficult to monitor with traditional temperature sensors, which highlights the potential of ultrasonic guided waves. Differences in application method and layer thickness are considered, and analysis of through-thickness displacement profiles and dispersion curves are used to predict signal response and determine the most suitable mode of operation. Heat transfer simulations (COMSOL) have been used to predict temperature gradients within a TBC, and dispersion curves have been produced from the temperature dependant material properties. Time dependant simulations of wave propagation are in good agreement with dispersion curve predictions of wave velocity for the two lowest order modes in three thicknesses of TBC top coat (100, 250, and 500 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}). When wave velocity measurements from the simulations are compared to dispersion curves generated at isotropic temperatures, the corresponding temperature represents the average temperature of a gradient system well. Such a measurement system could, in principle, be used in conjunction with surface temperature measurement systems to monitor through-thickness temperature changes.

Evaluation of major factors influencing the TBC topcoat formation in axial suspension plasma spraying (SPS)

AbstractSuspension plasma spraying (SPS) is ideally suited to produce porous or dense columnar, strain‐tolerant thermal barrier coatings (TBCs) and also offers the possibility of producing other microstructures such as feathery and dense vertically cracked coatings. The specific properties of the TBC are significantly influenced by the formed microstructure, that is, affected by feedstock material and process parameters. In this work, the effects of various process parameters in the SPS process are investigated. It was found that the suspension feed rate has a significant effect on the microstructure, especially on the column density of the coating, whereas the suspension solids content mainly affects the coating porosity. Additionally, the surface roughness and topography of the bond coat are crucial for the formation of columnar coatings and were therefore investigated. Despite comparable roughness values for as‐sprayed bond coats for high velocity oxy fuel and vacuum plasma spray produced coatings, the surface structures differ significantly from each other and affect the microstructure of the deposited topcoat. Characterization of mechanical properties by means of micro‐indenter can be suitable for columnar coatings to determine Young's modulus within a column. However, due to the heterogeneity of the coating, the method is not suitable to describe the mechanical properties of the topcoat.

Solution combustion synthesized micron sized yttrium aluminum garnet (Y3Al5O12) powder: A promising feedstock source for plasma spraying

Yttrium aluminum garnet (YAG) is a well-known high-temperature ceramic material that is used in a plethora of applications such as solid-state laser materials, cathode-ray tubes, field emission displays, scintillators, phosphors, electro-luminescent devices, etc. In recent years, plasma-sprayed yttrium aluminum garnet (YAG) has been explored as an alternative ceramic topcoat for thermal barrier coatings (TBCs) as it is known to prevent the oxidation of the bond coat. Also, it possesses high resistance to calcium magnesium aluminosilicate (CMAS) attack compared to conventional yttria-stabilized zirconia (YSZ) TBC topcoat material. In the current investigation, an attempt is made to fabricate plasma-sprayed YAG coating. The flowable powders required for plasma spraying are synthesized by the versatile solution combustion method after optimizing the process with two different fuels. The powders synthesized using hexamine fuel possess blocky angular-shaped particles and very good flowability. The flowability of the powders is correlated to the heat of combustion, adiabatic flame temperature and shape of the particles. The YAG powder synthesized from hexamine fuel is used for plasma spray application as the process results in higher yield and better flowability. The influence of different plasma spray powers (20, 30 and 40 kW) on the phase purity and crystallinity of the YAG coatings is studied using X-ray diffractometry. The influence of three different plasma powers on the microhardness and microstructure of the coatings is explored. The YAG coating plasma-sprayed at 30 kW is relatively a dense coating and exhibits a higher microhardness value of 1342 ± 9 HV at a load of 100 mN. The CMAS resistance of the YAG coatings is also reported.

Degradation of thermally sprayed Al2O3 coatings in reactor-grade liquid-sodium and its mitigation by laser treatment

Laser treatment (LT) to enhance the performance of atmospheric plasma spray (APS) deposited Al2O3 thermal barrier coatings (TBCs) was attempted for applications in extreme liquid-sodium environments of a sodium-cooled fast reactor (SFR). Thermally sprayed sacrificial ceramic TBCs are being proposed and investigated for installation on the stainless steel (SS) 316LN core catcher assembly of future SFRs to combat the core disruptive accidents (CDAs), wherein the primary criterion to be satisfied by the TBC is its long-term compatibility with the liquid-sodium coolant over a whole reactor life. Sacrificial ceramic coatings are susceptible to liquid-metal induced degradation by reactor-grade sodium. This study provides new insights into the failure mechanisms of APS Al2O3 coatings in liquid-sodium environments. Subsequently, an attempt is made to improve the performance of coatings by a surface modification approach using a laser treatment of the TBC topcoat. APS coatings exposed to liquid-sodium failed by spallation and delamination of the incrementally deposited topcoat, which is attributed to the formation and growth stress of ternary Na–Al oxides between the lamellar layers of the APS coatings. Compared to the APS coatings, LT coatings successfully retarded the liquid-sodium ingression and thereby exhibited improved degradation resistance and structural stability in reactor simulated sodium environments. The enhanced performance of LT coatings is attributed to its carefully re-engineered architecture of the APS TBC that could beneficially control the kinetics of interaction with liquid-sodium.

Residual Stress Change in Thermal Barrier Coating Due to Thermal Exposure Evaluated by Curvature Method

Thermal barrier coatings (TBCs) are used to protect the hot sections of gas turbine and jet engines. A TBC system comprises of a substrate, bond coat (BC), and TBC top coat (TC). The residual stress development mechanism by high temperature exposure in TBC is important in designing a high-performance TBC. However, quantitative studies of the stress change and its modeling are few because of its difficulty. The objective of this study is to reveal the changing mechanism of coating stress under high temperature exposure. For this purpose, we applied a three-layered beam model to evaluate the TBC’s residual stress using the curvature change. Time-dependent residual stresses in the TC and BC thermally exposed at 600-1000 °C were evaluated by the curvature method. Subsequently, we investigated the stress-generating mechanism of the coatings by using a finite element analytical (FEA) model that reproduces the measured curvatures. Our experimental result revealed that the residual stress in the BC changed from tensile to compressive by thermal exposure. However, thermal exposure had an insignificant effect on the residual stress in the TC. These changes in coating stress, including temperature and time dependency, were consistently explained by stress relaxation in the BC using the FEA model.

Lifetime study of sputtered PtAl coating on γ-TiAl with and without TBC topcoat at high temperatures

In this study, a PtAl based coating was tested under high temperature conditions at 900 °C and 1000 °C. The composition of Pt-53Al was chosen because of its excellent results in previous studies at another temperature. Due to the formation of a thin continuous alumina layer on top and the phase stability of the oxidation resistant coating, a lifetime of 1500 1 h-cycles was achieved.Depending on the testing temperature and time, the coating underwent an evolution of different Ti-Pt-Al phases due to the outward diffusion of Ti and Nb from the substrate and inward diffusion of Pt. The oxidation resistance decreases with a continued interdiffusion and a decreasing Pt content in the Ti-Pt-Al phases.Additionally, the Pt-53Al coating was successfully deposited with a Thermal Barrier Coating top coat by EB-PVD. The oxidation test at 1000 °C also revealed a lifetime up to 1500 cycles without failure, particularly based on the good adhesion between the TGO and TBC.Mass gain measurements were done at each temperature and for all coating systems. Further microstructural examination methods to study the coating evolution, such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD) for phase analysis, were used.

The Impedance Spectroscopy Study of the Oxides Layer in Thermal Barrier Coatings

Air-plasma-sprayed (APS) thermal barrier coatings (TBCs) were oxidized in air at different temperatures for 1000h and sequentially investigated by impedance spectroscopy (IS) and scanning electron microscopy (SEM). After oxidation at temperatures higher than 900°C, a thermally grown oxide (TGO) layer was formed at the bond coat/topcoat interface in TBCs. The impedance spectra of oxidized TBCs typically contains two relaxation processes that stem from the yttria-stabilized zirconia (YSZ) topcoat of TBCs and the TGO layer. The TGO resistivity that obtained by simulating the impedance spectra increased with the increasing of annealing temperature, demonstrating the growth and the densification of TGO layer.

Measurement of Young’s Modulus and Poisson’s Ratio of Thermal Barrier Coating Based on Bending of Three-Layered Plate

Thermal barrier coatings (TBCs) are used to protect the hot sections of gas turbine engines and airplane engines. A TBC system comprises a substrate, bond coat, and TBC topcoat. The development of an accurate method for determining the Young’s modulus and Poisson’s ratio of TBC using a multilayered specimen is of importance. In this study, we applied the bending theory of a laminated plate to a three-layered material and proposed models to determine the Young’s modulus and Poisson’s ratio of the TBC layer using the bending strain of the TBC system specimen. Three methods were developed by utilizing (i) the coating biaxial strain, (ii) substrate biaxial strain, or (iii) coating and substrate biaxial strains. Subsequently, we determined appropriate dimensions of the specimen and span by using three-dimensional finite element analysis, and numerically verified the usefulness of the three proposed methods. However, the Young’s modulus and Poisson’s ratio determined using the multilayered specimen with a substrate are sensitive to experimental errors. Therefore, we evaluated the sensitivity of the three proposed methods to experimental error, and we determined the most insensitive method among them. Finally, we experimentally demonstrated the usefulness of this method.

Failure Analysis of Multilayered Suspension Plasma-Sprayed Thermal Barrier Coatings for Gas Turbine Applications

Improvement in the performance of thermal barrier coatings (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria-stabilized zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multilayered TBCs consisting of advanced topcoat materials fabricated by suspension plasma spraying (SPS). The investigated topcoat materials were YSZ, dysprosia-stabilized zirconia, gadolinium zirconate, and ceria–yttria-stabilized zirconia. All topcoats were deposited by TriplexPro-210TM plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high-temperature applications.

Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties

The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic–extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined.

Material and Mechanical Aspects of CMAS Damage Progression on Thermal Barrier Coatings and its Non-Destructive Detection

Thermal barrier coatings (TBCs) on turbine blades in gas turbine engines are in some cases damaged at high temperatures exceeding 1200°C by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals. In this study, material interaction between molten CMAS and yttria-stabilized zirconia (YSZ) was investigated using a single crystal YSZ material and a synthetic CMAS product. Reaction between the molten CMAS and YSZ was significant at high temperature resulting in the infiltration of CMAS into the dense bulk-YSZ. The extent of interaction between CMAS and YSZ was found to be dependent on the crystallographic plane of the YSZ. The change in elastic stiffness due to the CMAS infiltration was also found by using a vibrating reed technique. The CMAS infiltrated layer had elastic stiffness higher by approximately five times of the non-infiltrated one. An attempt to detect the CMAS damage progression was also made through an AC impedance method. The proposed AC impedance technique is expected to be a useful technique to evaluate the CMAS infiltration as well as the associated delamination of TBC top coat.

Precise strain profile measurement as a function of depth in thermal barrier coatings using high energy synchrotron X-rays

We have developed a method of directly measuring the strain gradient as a function of depth in plasma sprayed Thermal Barrier Coatings (TBCs). A 92.8keV monochromatic synchrotron X-ray beam was used to penetrate the 10×10×8mm samples in transmission geometry. The samples had been heated to 1150°C and held at that temperature for 190h. The diffraction patterns were collected using a DECTRIS pilatus3 X CdTe 300K area detector. The patterns were analyzed by partial circular integration followed by full Rietveld refinement to obtain the lattice parameters of the TBC top coat at 25μm intervals as function of depth. The coatings surviving the heat treatment process without significant damage were found to exhibit a variable compressive stress state inside the top coat. This was found to be about −600MPa at the bond coat interface decreasing in a non-linear fashion towards the surface. By refinement of the data collected from sectors of whole Debye Scherrer rings we were able to estimate both the in-plane and out-of-plane strain.

Evaluation of Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition by Laser AE Method

Vertical cracks can be generated in the top coat of thermal barrier coatings (TBCs) deposited by atmospheric plasma spraying (APS). Since they are known to improve the durability of TBCs such as in the case of dense vertically cracked TBC, clarification of the mechanism and the criteria of cracking are very important. In this study, generation of such vertical cracks was monitored during APS process by laser acoustic emission (AE) method, which is an in situ, non-contact, and non-destructive technique. Temperature was also monitored inside and on the surface of a specimen during APS process for estimation of the temperature field in the top coat. Results of the AE and temperature monitoring were combined to evaluate the relationship between cracking and thermal stress in the top coat. Most of the AE events due to the generation of vertical cracks were detected during rapid heating of the surface of the top coat by the heat flux from the torch. It showed that the vertical cracks were induced due to the tensile stress caused by the temperature difference in the top coat from the rapid heating. Furthermore, the estimated critical thermal stress for vertical cracking from the monitoring results was consistent with a previously reported strength of YSZ coating deposited by thermal spray.

A Numerical Study on Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition

Clarification of crack generation mechanism in the top coat of the thermal barrier coatings (TBCs) during atmospheric plasma spray process is important to improve the reliability of TBC. In this study, finite element analyses of stress and strain during the deposition process were conducted with layer-by-layer method to understand the cracking behaviors. Stress relaxation by generation of vertical cracks was expressed as an elasto-plastic behavior of the coating. The effects of pre-heating temperature of the substrate and plasma power on crack development were analyzed by changing of the initial and atmospheric temperatures in simulation, respectively. The simulation results of radial strain explained the experimental results of crack monitoring by non-contact laser acoustic emission method.

無機質燃焼生成物による遮熱コーティングの損傷発達挙動

The use of thermal barrier coatings (TBC) for turbine blades in gas turbine engines has allowed higher engine operating temperatures exceeding 1200℃ at the surface of the TBC top coat. More recently, it has been recognized that at these high temperatures the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from siliceous minerals (dust, sand, ash) containing the intake air and from unclean fuels such as a syngas and biomass gas. In the present study, the CMAS damage was simulated in laboratory employing a synthetic CMAS product on Air Plasma splayed (APS) and Electron Beam Physical Vapor Deposition (EB-PVD) thermal barrier coatings. The changes in microstructure and mechanical characteristics were characterized after isothermal exposure at 1200-1250℃ for various times. Microstructural observation showed the ingression of the CMAS into the TBC top coat, the dissolution of the TBC top coat into the CMAS and the phase transformation of the TBC top coat. It was found from micro-indentation testing of the TBC top coat that shorter exposure time increased the hardness, but longer exposure time decreased the toughness. Based on these results, the effect of the CMAS product on the delamination behavior of the TBC top coat was assessed.

Mechanisms and mitigation of volcanic ash attack on yttria stablized zirconia thermal barrier coatings

Calcium–magnesium–alumino-silicate (CMAS) and dust attack on thermal barrier coatings (TBCs) have been widely researched. This present study focuses on the investigation of the degradation process of TBCs by volcanic ash and the development of potential mitigation mechanisms. The results show that, similar to CMAS, volcanic ash severely penetrates the thickness and fills the columnar gaps of the TBC top coat. It is also found that the yttria content of the yttria stabilized zirconia (YSZ) top coat decreases substantially with high temperature exposure to volcanic ash, which has a detrimental implication on the phase stability of YSZ. In terms of mitigation, volcanic ash reacts with alumina around 1310°C, forming anorthite (CaAl2Si2O8), magnetite (Fe3O4), and spinel (Al1.75Mg0.889Mn0.351O4) as reactive products, which, according to the literature, have melting temperatures above that of the volcanic ash studied and the typical values reported for CMAS. Since the new melting temperatures are now above the typical surface operating temperature of turbine components, the melting-induced penetration of volcanic ash and CMAS can be considerably suppressed.

Cold sprayed MCrAlY + X coating for gas turbine blades and vanes

The feasibility of depositing an MCrAlY+X coating via cold gas dynamic spray (CGDS), also called cold spray, was considered as an alternative to traditional thermal spray solutions where there is concern regarding the relatively high costs of the standard processes. The effect of the particle size distribution on the process deposition efficiency was assessed by using two powder size ranges. The properties of the cold spray deposited material, as well as the density and the quality of the bond with the substrate, were evaluated in the as-sprayed condition and after post-deposition heat treatment. The coated samples were exposed to high temperature oxidation at various typical in-service temperatures and for different durations to analyse the high temperature behaviour of the cold sprayed coating in terms of Al-rich β phase depletion rate. After exposure, the oxidised samples were characterised by optical and scanning electron microscope. All coatings from this study were benchmarked against those deposited using standard techniques, already widely characterised. Cold spray seems a promising alternative to the standard deposition routes, although further work is necessary, especially on samples that also incorporate a TBC top coat.

521 しゃ熱コーティング材のトップコートヤング率に及ぼす高温曝露の影響

521 しゃ熱コーティング材のトップコートヤング率に及ぼす高温曝露の影響

555 しゃ熱コーティング材のトップコートヤング率に及ぼす高温曝露の影響(材料力学II,学術講演)

555 しゃ熱コーティング材のトップコートヤング率に及ぼす高温曝露の影響(材料力学II,学術講演)

Moisture-Induced TBC Spallation on Turbine Blade Samples

Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. “The weekend effect” or “DeskTop Spallation” (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, followed by failure when exposed to ambient humidity or water. Once initiated, failure can progress to completion quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schütze), and CNRS Toulouse/SNECMA (Déneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond coat are reported. Cut sections were intermittently oxidized at 1100°, 1150°, and 1200°C and monitored by weight change and visual appearance. Failures were distributed widely over a 5–100hr time range, decreasing with oxidation temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. The failure interfaces exhibited, on the TBC side, alumina scale grains (decorated with Ta-rich oxide particles) and, on the exposed bare metal surface of the blade, imprints from the scale grains (with alumina islands and streamers). The phenomenon is based on moisture-induced delamination at the alumina scale - bond coat interface. Cycling damage is a contributing factor as cracking allows access of moisture to this interface, while high strain energy provides the driving force for spallation. It has been proposed that moisture reacts with aluminum in the bond coat and releases hydrogen atoms that ‘embrittle’ the interface. A modified chemical viewpoint of scale adhesion results, including a negative synergistic effect with interfacial sulfur.