Sprayed Thermal Barrier Coatings Research Articles

Comparison of erosion rate of EB-PVD and plasma sprayed TBC

ABSTRACT A thermal barrier coating (TBC) consisting of platinum-aluminide bond coat and 7wt-% yttria stabilized zirconia (7YSZ) ceramic top coat deposited by electron beam physical vapour deposition (EB-PVD) system cast Ni-base superalloy CM-247LC. The erosion behaviour of the EB-PVD and air plasma sprayed (APS) TBCs are compared. The APS TBC consists of 7YSZ top coat and CoNiCrAlY bond coat on Inconel 738 substrate. Erosion responses of TBCs have been examined as a function of temperature. The erosion rate of EB-PVD TBC was significantly lower than that of APS TBC. Erosion rate of EB-PVD TBC decreased with test temperature. APS TBC exhibits mostly temperature independent erosion rate. The material removal mechanism of EB-PVD TBC was by the formation of parallel cracks in the ceramic layer close to its surface and their subsequent joining. The material is removed by propagation of subsurface cracks along the splat boundaries in case of APS TBC.

Durability of plasma sprayed Thermal Barrier Coatings with controlled properties part II: Effects of geometrical curvature

The present investigation elucidates the effects of substrate curvature on the durability of Air Plasma Sprayed (APS) Thermal Barrier Coatings (TBCs). Traditionally, planar disk specimens are utilized in Furnace Cycle Testing (FCT). In most cases, delamination is initiated at the disk's free edge and then propagates along the TBC-bond coat interface. However, in turbine components (e.g., blades/vanes), they lack significant free edges and the coatings are deposited on non-flat surfaces of varying radii of curvature. These geometrical discrepancies imply significant differences in the stress states as compared to planar disk specimens. Therefore, the part geometry inevitably affects the TBC failure mechanisms in the turbine system, and consequently influences TBC durability. Nevertheless, these effects have not been fully explored in the past, despite the anecdotal knowledge which suggests the leading edge of turbine blades to be one of the most common failure/spallation locations. In this study, representative TBC systems were deposited onto superalloy disks with flat substrate and rods with curved substrate. The experimental results from FCT suggest TBC durability on rods is significantly lower than when sprayed on disks. Furthermore, the durability of TBCs on rods appears to increase with higher porosity, which is contradictory to reported trends for TBCs on disks. In addition, the effects of various bond coats are not consistent with those observed in disks. To clarify the underlying effects of curved geometries, detailed stress analyses were performed. They revealed a state of thermal stresses which is unique to curved geometries. It was found that TBC on rods experience tensile radial stresses as well as tensile hoop stresses during the cooling. These stresses result in a complex interplay in failure processes in rods, which explains the different observed trends in the durability from those obtained with disks.

Effect of bond coat topography on the fracture mechanics and lifetime of air-plasma sprayed thermal barrier coatings

A modified four-point bending test was conducted on air-plasma sprayed (APS) thermal barrier coatings (TBC) to investigate the impact of bond coat (BC) topography tortuosity on the TBC fracture mechanics. The degree of BC tortuosity was quantified by the novel total thresholded summit area surface descriptor (Ssth) and was found to influence the crack path configuration, critical energy release rate (Gc) and crack propagation velocity (νprop). Specimens with higher BC interface tortuosity and more compliant TBC microstructures displayed a reduced strain energy release rates (Gc ≤ 79.90 J. m−2) and crack propagation velocities (vprop ≤ 0.90 mm. s−1). Delamination was primarily governed by the degree of inter-splat cohesion and intra-splat segmentation, although the topography-induced inter-splat tortuosity still had a relevant impact on fracture mechanics. The correlation observed between these results and the lifetime of analogous thermally cycled specimens tested herein suggests that the BC interface can affect the effectiveness of the microstructural stress relaxation mechanisms and by extension the TBC durability.

Constrained sintering and cracking of air plasma sprayed thermal barrier coatings: Experimental observation and modeling

AbstractThe development of vertical cracks in air plasma sprayed (APS) thermal barrier coatings (TBCs) during thermal cycling and constrained sintering under a temperature gradient is investigated. Microstructural analysis shows that the development of the vertical cracks is associated with multiple processes, including sintering during the hold period and cleavage during cooldown. Inspired by the experimental observations, an image‐based sintering model is used to simulate the development of vertical cracks as the coating sinters while constrained by a substrate. The computational results show that microstructural imperfections can develop into vertical cracks, which then propagate toward the interface. A simple analytical model is presented for the threshold level of in‐plane stress for the onset of propagation of a vertical crack during constrained sintering. By combining the results of these different modeling approaches, the cross‐coupling of the material and geometric parameters, and how this determines the sintering response (microstructure evolution) and vertical crack formation is evaluated. In addition, the growth of vertical cracks by a cleavage mechanism during cooldown is examined and the coupling between sintering, cleavage crack growth, and TBC lifetime is explored.

Influence of crack features on heat transfer characteristic and cracking behaviour of APS-YSZ coating: A numerical simulation study

Spallation of air plasma sprayed (APS) thermal barrier coatings (TBCs) is a decisive factor that dominates the lifetime of APS-TBCs. It is noticeable that coating spallation is correlated with the crack initiation and propagation. The cracks will influence the temperature field and other characteristics of the coating. It is of great importance to sufficiently comprehend and specify these characteristic indexes (such as temperature gradient, thermal conductivity and elastic modulus) of coating changes that the crack evolution brings. In this study, the numerical reconstruction models of TBCs with different major crack lengths and openings were obtained. By using the thermal lattice Boltzmann method (TLBM) and finite element model, the precise temperature field and heat flux distribution of coatings were obtained. The thermal and mechanical properties of coatings were also investigated. The results showed that, the effective thermal conductivity and Young’ modulus of the top coat decrease with the increase of major crack length and opening. At the same time, the energy release rate shows a faster increase for small cracks and its increase trend slows with crack growth. Crack opening further increases the growth of energy release rate. Although the horizontal crack can reduce the thermal conductivity and modulus of the coating, the crack can induce coatings failure. In addition, the extension and expansion of crack will lead to more non-uniform distributed heat flux and larger temperature gradient in the top coat. These results can provide an informed basis for reliable assessment of performance integrity of APS-TBCs. The structural reconstruction and properties analysis models can also be applicable to the field of porous media.

Elevated Temperature Erosion of Plasma Sprayed Thermal Barrier Coating

Elevated Temperature Erosion of Plasma Sprayed Thermal Barrier Coating

Elevated Temperature Erosion of Plasma Sprayed Thermal Barrier Coating

Yttria stabilized zirconia (YSZ) is used extensively as a ceramic top coat of thermal barrier coating (TBC) system in land-based gas turbines and aero engines. TBC enables modern engines to operate at significantly higher gas temperatures. TBCs are subjected to solid particle erosion at elevated temperature, especially during the operation of aero engines in sandy terrain. Thus, the objective of the present investigation is to evaluate the impact erosion response of these coatings as a function of various eroding conditions. Toward that purpose, YSZ TBC was deposited on Ni base alloy using air plasma spraying technique. The microstructural features of the coating are evaluated, and coated samples are subjected to erosion test employing a high temperature air jet erosion testing machine at various temperatures, different impact velocities, and at different impact angles. The results show a brittle erosion response of TBC at all temperatures. The erosion rate of TBC is generally independent of temperature under the normal impact, and it increases with temperature under oblique impact at high impact velocity. Intersplat adhesion governs the erosion mechanism.

Multifarious Defects Co-Determined Thermal Conductivity in Air Plasma Sprayed Thermal Barrier Coatings

Yttria-stabilized zirconia (YSZ) is the most widely applied thermal barrier coatings (TBCs) materials in gas turbines and aviation engines to prevent components from overheating and to increase the combustion temperature and turbine efficiency. Plasma sprayed coatings usually contain about 15~25% pores, mainly including globular pores, cracks and amorphous phase that to be classified as multifarious and multi-scale defects. They all significantly lower the thermal conductivity, but in drastically different ways depending on their morphologies and orientations. Establishing an accurate correlation between the total porosity and the thermal conductivity requires not only a precise separation and estimation of different kinds of defects but also a reasonable mathematic model to describe their effect on the thermal conductivity. In the present research, cross-section ion polishing and image analysis were chosen as a reliable assembly for visualizing and characterizing multifarious defects of thermal sprayed coatings, through which tiny pores and cracks can be distinctly expressed without any mechanical damage of the microstructure. The effect of different micro-scale defects such as pores and cracks on the thermal conductivity was respectively and quantitatively calculated according to a Bruggeman-based iterative model. Additionally, the nanoscale defects, especially the amorphous phase in the matrix was also taken into consideration, which is proven to be of great importance. The predicted thermal conductivity based on this amorphous-phase-concerned iterative model has achieved a successful evaluation compared to the measured results of coating specimens originated from various spraying processes. This research confirms the applicability of image-analysis-based modeling as a simple, reliable and versatile method for thermal conductivity prediction of porous coating systems.

Effect of Laser Treatment on the Surface Roughness of Multilayer Plasma Sprayed Thermal Barrier Coating System

Thermal barrier coatings (TBCs) are used in advanced engines working at higher temperatures. Higher efficiency and performance of gas turbine engines will require careful selection of TBCs. In this study, Ni22Cr10Al1.0Y (Amdry 9625) bond coat and two types of top coat including ceria stabilized zirconia (CSZ) ZrO2-24CeO2-2.5Y2O3) and yttria stabilized zirconia (YSZ) ZrO2-8Y2O3 were deposited on IN 625 by air plasma spraying (APS). The thickness of the duplex ceramic coat based on zirconia was in the range between 350 to 400 µm. The effect of high power Yb:YAG solid state laser at different laser parameters on feature, microstructure and roughness of plasma sprayed and laser sealed coating of multilayer ceria stabilized zirconia/ yttria stabilized zirconia was investigated. Surface roughness has been reduced significantly after laser sealing. The effect of laser process parameters carried out using Taguchi’s L16 orthogonal array design. Minimum roughness can be obtained at moderate power density and longer interaction time with sufficient specific energy to produce complete melting of coating. Characterization and analysis of results was achieved by employing scanning electron microscopy (SEM) , (EDS) and image J analysis. It was found from the results, there were significant improvements in the performance of plasma sprayed coatings after laser sealing due to the reduction of surface coating defects.

Hot corrosion properties of plasma sprayed La2Ce2O7/YSZ vis-à-vis La2Ce2O7/cluster paired zirconia thermal barrier coatings

Bilayered thermal barrier topcoats have lead to a remarkable breakthrough in overcoming the disadvantages of the conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The present work is devoted to studying the hot corrosion properties of two different bilayered ceramic topcoats, viz., (i) lanthanum cerium oxide with the formula La2Ce2O7 (LCO)/YSZ and LCO/cluster paired zirconia (gadolinia, dysprosia and yttria stabilized zirconia -(Gd,Dy,Y)SZ) plasma sprayed TBCs. Plasma spray grade YSZ and (Gd,Dy,Y)SZ are prepared by co-precipitation and solution combustion techniques as reported in the literature. The effect of thickness of the plasma sprayed top LCO layer in bilayered LCO/YSZ and LCO/(Gd,Dy,Y)SZ TBC systems under Type I hot corrosion condition is examined. The LCO/(Gd,Dy,Y)SZ bilayered coating with 100 μm LCO thickness exhibits better hot corrosion resistance than the LCO/YSZ system. The phase transformation of t′-ZrO2 to m-ZrO2 in LCO/(Gd,Dy,Y)SZ (100/150 μm) TBC is reduced markedly (60%) compared to LCO/YSZ (100/150 μm) TBC.

Erosion Performance of Atmospheric Plasma Sprayed Thermal Barrier Coatings with Diverse Porosity Levels

Thermal barrier coatings (TBCs) prolong the durability of gas turbine engine components and enable them to operate at high temperature. Several degradation mechanisms limit the durability of TBCs during their service. Since the atmospheric plasma spray (APS) processed 7–8 wt.% yttria stabilized zirconia (YSZ) TBCs widely utilized for gas turbine applications are susceptible to erosion damage, this work aims to evaluate the influence of their porosity levels on erosion behavior. Eight different APS TBCs were produced from 3 different spray powders with porosity ranging from 14% to 24%. The as-deposited TBCs were examined by SEM analysis. A licensed software was used to quantify the different microstructural features. Mechanical properties of the as-deposited TBCs were evaluated using micro-indentation technique. The as-deposited TBCs were subjected to erosion tests at different angles of erodent impact and their erosion performance was evaluated. Based on the results, microstructure-mechanical property-erosion performance was correlated. Findings from this work provide new insights into the microstructural features desired for improved erosion performance of APS deposited YSZ TBCs.

Effect of Solution Treatment on Oxidation in Suspension Plasma Sprayed Thermal Barrier Coating

Effect of Solution Treatment on Oxidation in Suspension Plasma Sprayed Thermal Barrier Coating

Effect of suspension characteristics on the performance of thermal barrier coatings deposited by suspension plasma spray

This paper investigates the influence of suspension characteristics on microstructure and performance of suspensions plasma sprayed (SPS) thermal barrier coatings (TBCs). Five suspensions were produced using various suspension characteristics, namely, type of solvent and solid load content, and the resultant suspensions were utilized to deposit five different TBCs under identical processing conditions. The produced TBCs were evaluated for their performance i.e. thermal conductivity, thermal cyclic fatigue (TCF) and thermal shock (TS) lifetime. This experimental study revealed that the differences in the microstructure of SPS TBCs produced using varied suspensions resulted in a wide-ranging overall TBC performance. All TBCs exhibited thermal conductivity lower than 1 W/(m. K) except water-ethanol mixed suspension produced TBC. The TS lifetime was also affected to a large extent where 10 wt % solid loaded ethanol and 25 wt % solid loaded water suspensions produced TBCs exhibited the highest and the lowest lifetime, respectively. On the contrary, TCF lifetime was not as significantly affected as thermal conductivity and TS lifetime, and all ethanol suspensions showed marginally better TCF lifetime than water and ethanol-water mixed suspensions deposited TBCs.

Investigation of the bond coat interface topography effect on lifetime, microstructure and mechanical properties of air-plasma sprayed thermal barrier coatings

The effect of bond coat/thermal barrier coating (BC/TBC) interface topography on the lifetime of air-plasma sprayed (APS) TBCs was comprehensively investigated in the present work. A quantitative description of the interface topography was achieved by utilising multiple surface texture parameters obtained from confocal microscopy, including a newly formulated parameter, denominated as total thresholded summit area, Ssth. Thermal cycling fatigue (TCF) testing showed a clear correlation between the TBC lifetime and the interface topography, especially for the novel Ssth parameter. The topographical and microstructural analysis revealed that deposition of a TBC over tortuous BC topographies leads to a highly curved TBC splat morphology, which in turn affected the crack path configuration. Mechanical testing of as-deposited and heat-treated specimens evidenced that the aforementioned microstructural changes promoted a reduced local and global elastic moduli, effectively linking the more compliant TBC microstructures with more beneficial fracture mechanics behaviours and higher TCF lifetimes.

Atypical decomposition processes of Sm2Zr2O7 + 8YSZ dual-phase TBCs during hot corrosion

The accelerated decomposition of an Sm2Zr2O7+8YSZ system in the form of air plasma spraying (APS) thermal barrier coatings of composites, double ceramic layer (DCL), and functional graded system (FGS) types under hot corrosion conditions were analyzed. Hot corrosion tests were performed using liquid sodium sulfate salt deposits and then X-Ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS) were performed to identify the reaction products. The obtained data indicated that negligible amounts of sulfates or oxysulfates were formed, but significant decomposition of pyrochlore Sm2Zr2O7 to the fluorite phase was detected. These phenomena were also observed in a much higher temperature range during pure oxidation tests, but the lowest decomposition temperature (920 °C) was observed in the sodium sulfate assisted process. The described observations were confirmed during similar tests and realized using model powder mixtures. Microstructural analysis of thermal barrier coatings (TBC) cross-sections revealed a complex decomposition process that was localized to areas near the Sm2Zr2O7 splats. The decomposition of the observed pyrochlore phase was mainly related to interactions between Sm2Zr2O7 and 8YSZ in accordance with the fluxing mechanism typical for the hot corrosion process of Ni-based superalloys. Additionally, the effect of eutectic Na-Sm-O was determined.

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

The pore filling behavior in yttria-stabilized zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated. After fully infiltration, the total porosity of the coating decreased. However, the total porosity of the top region increased due to severe microstructural change. For the bottom region, whose microstructure did not change significantly, its total porosity dropped and almost all crack network disappeared, but 48 vol. % of globular pores still existed and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.

The effect of bond coating surface modification on the performance of atmospheric plasma spray thermal barrier coatings

Asperities in atmospheric plasma sprayed (APS) CoNiCrAlY bond coatings were observed to rapidly form spinel-type oxides growing into the APS yttria-stabilized zirconia (YSZ) top coating during isothermal exposures. To minimize these transient oxides, bond coatings were either lightly or heavily modified to remove asperities, prior to top coating deposition. After a 100 h isothermal exposure at 1100 °C in dry air, modified regions formed a thin, Al-rich thermally grown oxide (TGO) scale while unmodified regions formed Cr- and Ni-rich transient oxide. However, in 1-h furnace cycle testing (FCT) at 1100 °C, the modified bond coatings showed a reduced average lifetime compared to the specimens with unmodified bond coatings. The benefit of asperities may be to geometrically inhibit crack propagation, but the volume of transient oxide formed may also densify the adjacent YSZ and further inhibit cracking.

Determining Young’s modulus of coatings in vibrating reed experiments using irregularly shaped specimens

Knowing Young’s modulus of coatings is important to understand the stress evolution and failure of coating systems. However, such values are often not available for modern coatings and/or cannot be determined by existing methods for attaining a global stiffness. In this paper, a new method is described and validated that can be used even for brittle, highly porous, and irregularly shaped coatings, for example sputtered or thermally sprayed thermal barrier coatings: In vibrating reed experiments, the resonance frequencies of the specimens are determined. The specimen geometry is measured by a computer tomograph, and a finite element simulation based on that measured geometry is carried out to determine Young’s modulus. To validate this method, a monocrystalline irregularly shaped silicon plate with known Young’s modulus was measured. The method is tested on different metallic thermal spray coatings, for which other mechanical test methods for Young’s modulus were also applicable for comparison. Lastly, a very porous, gas flow sputtered zirconia thermal barrier coating was analyzed where other methods were not suitable.

The evolution of fracture process zones in as-received and oxidized air plasma sprayed TBCs

The evolution of the fracture process zones as precursor to fracture was elucidated in as-received and oxidized air plasma sprayed (APS) Thermal Barrier Coating (TBC) specimens. Multiple techniques of Digital Image Correlation (DIC), Acoustic Emission (AE), Finite Element Analysis (FEA) and Scanning Electron Microscope (SEM) micrographs were combined together to obtain in-depth understanding of the failure mechanisms involved in a TBC component under uniform bending stress field. Both sides of specimens were monitored simultaneously during loading and it is shown that within a uniform applied strain, the localized zones of non-symmetric strains with the different lengths were initiated and growth within the ceramic top coat perpendicular to the principle stresses. The process zones were initiated in both as-received and oxidized specimens upon yielding the substrate. However, different strain localization areas were observed on two sides of each specimen. It was observed that separations in each localized zone were resisted by cohesive tractions controlled by the material microstructural features and not the applied external load. One of these localized strains was then rotated parallel to the interface propagating until the final delamination. In the oxidized specimen, the delamination was faster and spallation of top coat ceramic layer followed immediately after cracks reached the bond coat interface. The cohesive properties were determined using a hybrid experimental-numerical approach.

Dominant effect of oriented 2D pores on heat flux in lamellar structured thermal barrier coatings

Thermal barrier coatings (TBCs) provide thermal insulation to metallic components served at high temperatures. The oriented 2D pores are primarily responsible for the efficient prevention of heat flux. Thus, structural design of TBCs with higher thermal insulation requires clear understanding on the thermal conduction inside coatings. Up to now, previous analytical models to investigate the heat conduction were mainly based on the concept of thermal contact resistance. However, the related assumption was far away from the real coating microstructure. In this study, the intrinsic structural characteristics of plasma sprayed TBCs were firstly investigated. Subsequently, an analytical model based on the structural features was developed to understand the dominant effect of oriented 2D pores on heat flux inside the coatings. Results showed that the insulative ratio of 2D pores dominantly determine the effective prevention of heat flux. Moreover, effects of the microstructural parameters, including splat thickness, bonding ratio and unit size, on the total thermal resistance was discussed. Overall, the understanding of the dominant effect of 2D pores would make it possible to design new TBCs with high performance in future applications.