Performance Of Thermal Barrier Coatings Research Articles

Effect of graphene-based nano-electronic film on the insulation performance of thermal barrier coatings

In order to accurately analyse the effect of graphene on the thermal insulation performance of thermal barrier coatings, the effect of graphene nano-electronic films on the thermal insulation performance of thermal barrier coatings was studied. Graphene, graphene nano-electronic thin films and thermal barrier coatings were prepared by using the initial materials in the order of preparation. The experimental samples were divided into a control group and an experimental group, and the structure and content of the graphene nano-electronic thin films were changed to conduct an insulation performance experiment. The experimental results show that the thermal insulation coefficient is 0.32 when the film content is 3.0 μg/mg, and the maximum thermal insulation temperature of the thermal barrier coating is 800°C. When the graphene nano-electronic film has a double-layer structure and a content of 3.0 μg/mg, the thermal insulation effect is the best.

Effect of graphene-based nano-electronic film on the insulation performance of thermal barrier coatings

Effect of graphene-based nano-electronic film on the insulation performance of thermal barrier coatings

Effect of substrate rotation on the microstructure of 8YSZ thermal barrier coatings by EB-PVD

Thermal Barrier Coatings (TBCs) is an essential requirement to protect aero engines and gas turbines against high inlet gas temperatures. The microstructure plays an important role in governing the performance and thermal life cycle of the TBC structure. The TBC deposition process is carried using a combination of the Electron Beam Physical Vapor Deposition (EB-PVD) and Atmospheric Plasma Spray (APS) process. The microstructure obtained by EB-PVD represents columnar structure whereas APS represents splat layer structure. The combined deposition process is used to study the grain growth behavior which can affect the performance of the TBCs. The 8YSZ TBC was deposited by the in-house EB-PVD system onto the Ti-6Al-4V substrate with the NiCrAlY bond coat by the APS process. In this paper, the effect of substrate rotation on the microstructure of 8YSZ TBCs has been investigated and discussed in detail. The dissimilar grain growth behavior of the TBC was observed with the stationary and rotating substrate at 20 rpm for microstructural analysis. It was found that the stationary substrate with 8YSZ TBC has dense and fine grain growth behavior compared to a rotating substrate. In stationary condition, the substrate is always exposed to uniform deposition, therefore the grains with less shadow region is present which results in low porosity. Whereas, a substrate with rotation consists of distributed grains resulting in more shadow region contributing towards high porosity. Therefore, the substrate rotation affects the grain distribution resulting in the shadow region which leads to the formation of pores affecting the performance of TBCs.

Microstructure and failure analysis of suspension plasma sprayed thermal barrier coatings

Improvements in performance of thermal barrier coatings (TBCs) used in gas turbine engines are highly desired as they can result in higher engine efficiency leading to reduction of harmful emissions. Suspension plasma spraying (SPS) has been shown to produce high performance porous columnar TBCs that can provide low thermal conductivity and high durability. Apart from the topcoat microstructure and chemistry, the lifetime of TBCs is also dependent on bondcoat microstructure and chemistry, and topcoat-bondcoat interface roughness. In case of SPS TBCs, the interface roughness can significantly affect the columnar topcoat microstructure, thus making the bondcoat selection even more crucial.In this work, six different sets of samples were produced by fabricating bondcoats with conventional atmospheric plasma spraying (APS), high velocity air fuel (HVAF) spraying, or hybrid water/argon stabilised plasma (WSP-H) gun, and SPS topcoats using axial SPS (ASPS) or WSP-H spray guns. The objective of this study was to investigate the influence of varying the topcoat microstructure, bondcoat microstructure and topcoat-bondcoat interface roughness on oxide growth behaviour and thermal cyclic fatigue (TCF) lifetime of SPS TBCs. Samples after failure were investigated to understand the failure mechanism in each case.The results showed that changing the bondcoat spray process and spray gun resulted in significant variation in bondcoat surface roughness. A porous columnar structure was created by the ASPS process, while a feathery columnar structure was created by the WSP-H spray gun in this study. Samples with WSP-H bondcoat resulted in highest cyclic lifetime in this study, despite showing severe oxidation of the bondcoat as compared to APS and HVAF bondcoats. This result could be attributed to the very high bondcoat surface roughness in these samples that could have resulted in improved mechanical anchoring of the topcoat. The HVAF bondcoats showed the best oxidation resistance in this study.

Experimental and numerical investigation on gas turbine blade with the application of thermal barrier coatings

The engine parts material used in gas turbines (GTs) should be resistant to high-temperature variations. Thermal barrier coatings (TBCs) for gas turbine blades are found to have a significant effect on prolonging the life cycle of turbine blades by providing additional heat resistance. This work is to study the performance of TBCs on the high-temperature environment of the turbine blades. It is understood that this coating will increase the lifecycles of blade parts and decrease maintenance and repair costs. Experiments were performed on the gas turbine blade to see the effect of TBCs in different combinations of materials through the air plasma method. Three-layered coatings using materials INCONEL 718 as base coating, NiCoCrAIY as middle coating, and La2Ce2O7 as the top coating was applied. Finite element analysis was performed using a two-dimensional method to optimize the suitable formulation of coatings on the blade. Temperature distributions for different combinations of coatings layers with different materials and thickness were studied. Additionally, three-dimensional thermal stress analysis was performed on the blade with a commercial code. Results on the effect of TBCs shows a significant improvement in thermal resistance compared to the uncoated gas turbine blade.

Effect of Lanthanum Zirconate on High Temperature Resistance of Thermal Barrier Coatings

Thermal barrier coatings (TBCs) typically have low thermal conductivity and high resistance to oxidation at elevated temperatures. TBCs are used to protect the turbine blades of aircraft engines operating at high temperature. The commonly used YSZ (8 wt% Y2O3-stabilized ZrO2) cannot function at temperatures higher than 1200°C for an extended period of time, which hinders the necessary improvement in the aircraft thrust-to-weight ratio. In this work, thermal barrier coatings consisting of a YSZ ceramic layer and of a lanthanum zirconate (LZ) + YSZ composite structure were prepared on a nickel-based high-temperature substrate using atmospheric plasma spraying. Thermal insulation properties and high temperature oxidation performance of TBCs with single layer structure and double layer structure were compared. The growth mechanism and influencing factors of high-temperature oxidation of thermally grown oxide (TGO) were discussed. The results showed that the LZ had good thermal insulation performance and oxygen isolation compared to the YSZ layer. This performance effectively reduces the growth of TGO in TBCs, thereby delaying the coating failure.

Fracture performance and crack growth prediction of SPS TBCs in isothermal experiments by crack numbering density

This study examined the application of crack numerical density theory (CND) to determine the damage state of a thermal barrier coating (TBC) produced using the suspension plasma spray (SPS) technique. A TBC consisting of a partially stabilized zirconia topcoat was deposited upon an APS NiCoCrAl bondcoat and INCONEL 625 substrate. The TBC was exposed to an isothermal temperature of 1080 °C for a duration of up to 1300 h with specimens removed for examination at, 400 h, 800 h and 1300 h. Material properties after each exposure were determined and utilized to fit a CND model to predict the crack distribution within the topcoat following isothermal exposure. Probabilities of cracks developing within the topcoat that can bridge asperities (0.01% at 400 h, 2% at 800 h, and 11% at 1300 h) were determined indicating that CND may be a useful tool for predicting lifetimes of SPS-TBCs.

In-situ evaluation of porosity in thermal barrier coatings based on the broadening of terahertz time-domain pulses: simulation and experimental investigations.

Porosity is one of the most important indicators for the characterization of the comprehensive performance of thermal barrier coatings (TBCs). Herein, we explored a fast, nondestructive porosity evaluation method based on the terahertz time-domain broadening effect. Different preparation process parameters were used to deposit the ceramic coatings, and the porosity ranged from 9.09% to 21.68%. Monte Carlo simulations were conducted to reveal the transitive relation between porosity and the terahertz time-domain broadening at different extinction coefficients and transmission distances. A transmission mode with an incidence angle of 0° was used to estimate the terahertz dielectric properties of ceramic coatings and the relative broadening ratio of terahertz pulses at different porosities. As a result, the Monte Carlo simulations showed that the time-domain broadening effect was enhanced when the extinction coefficient and transmission distances increased. As the porosity increased, the refractive index decreased and the extinction coefficient increased. The latter was more sensitive to minor porosity changes as demonstrated by linear fitting comparisons. Meanwhile, the relative broadening ratio increased when the porosity increased, and reserved the sensitivity of the extinction coefficient to porosity changes. The effect was more obvious on the relative broadening ratio which experienced multiple transmissions and reflections. Moreover, the relative broadening ratio could be obtained faster and in an easier manner compared to the dielectric parameters in both the transmission and reflection modes, based on single-step tests with the use of actual terahertz wave inspection. Finally, this study proposed a novel, convenient, online, nondestructive, and noncontact porosity evaluation method that could be potentially utilized to evaluate the integrity of TBCs in gas turbine blades.

Effect of APS flash bond coatings and curvature on TBC performance on rod specimens

The addition of an air plasma sprayed (APS) “flash” bond coating layer on top of a high velocity oxy-fuel (HVOF) bond coating significantly extended the lifetime of APS yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on rod specimens of superalloy 247 tested using 100-h cycles in air+10%H2O at 1100 °C. The flash bond coatings of both NiCoCrAlY and NiCoCrAlYHfSi powder were compared to an HVOF-only and a vacuum plasma sprayed (VPS) NiCoCrAlYHfSi bond coating. The flash coatings appear to form a mixed oxide-metal zone, which likely inhibits crack formation and therefore extends lifetime compared to conventional bond coatings. The underlying dense HVOF layer appeared to act as a source of Al for this intermixed zone and prevented the oxide from penetrating deeper into the bond coating. Residual stress in the thermally-grown Al2O3 scale was measured using photo-luminescence piezospectroscopy (PLPS) as a function of time for each coating variation, including a comparison of concave and convex surfaces on a specially designed rod.

Corrosion resistance and thermal-mechanical properties of ceramic pellets to molten calcium-magnesium-alumina-silicate (CMAS)

Because gas turbine engines must operate under increasingly harsh conditions, the degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) is becoming an urgent issue. Mullite (3Al 2 O 3 ·2SiO 2 ) is considered a potential material for CMAS resistance; however, the performance of mullite in the presence of CMAS is still unclear. In this study, mullite and Al 2 O 3 –SiO 2 were premixed with yttria stabilized zirconia (YSZ) in different proportions, respectively. Porous ceramic pellets were used to conduct CMAS hot corrosion tests, and the penetration of molten CMAS and its mechanism were investigated. The thermal and mechanical properties of the samples were also characterized. It was found that the introduction of mullite and Al 2 O 3 –SiO 2 mitigated the penetration of molten CMAS into the pellets owing to the formation of anorthite, especially at 45 wt% mullite/55 wt% YSZ. Compared with Al 2 O 3 –SiO 2 , mullite possesses a higher chemical activity and undergoes a faster reaction with CMAS, thus forming a sealing layer in a short time. Additionally, the thermal expansion coefficient, thermal conductivity, and fracture toughness of different samples were considered to guide the architectural design. Considering the CMAS corrosion resistance, thermal and mechanical performance of TBCs systematically, a TBC system with a multilayer architecture is proposed to provide a theoretical and practical basis for the design and optimization of the TBC microstructure.

Analysis of performance and emission characteristics of TBC coated low heat rejection engine

ABSTRACT Present investigation is focused on the effect of thermal barrier coated piston on the performance and emission characteristics of diesel engine using diesel as a fuel. The piston crown top surface were fully coated with three different combinations of thermal barrier coatings (TBC) materials like 7% YSZ, 2% Gd2O3 + YSZ, 5% Gd2O3 + YSZ with thickness of 0.25 mm and in all combinations the bond coat NiCrAlY were used with a thickness of 0.05 mm which is deposited on the target surface using plasma spray technique. Experimental analysis is carried out under different loading conditions in a four-stroke single cylinder diesel engine with diesel as a fuel. Results of this study reveal that the brake thermal efficiency of the coated engine is increased by 12.1% with reduction of brake specific fuel consumption by up to 16.2% compared to baseline engine. Experimental results also revealed that in coated engine Hydro carbon (HC) and carbon monoxide (CO) emissions were reduced by 17.8% and 18.4% respectively and also a significant increment of Nitrogen oxide (NOx) emission by 14.87% compare to baseline engine.

Structural evolution of Al-modified PS-PVD 7YSZ TBCs in thermal cycling

Feather-like columnar 7YSZ thermal barrier coatings (TBCs) were prepared by plasma spray-physical vapor deposition (PS-PVD). In order to improve the thermal cycle performance of TBCs, Al-modification was carried out with the as-sprayed TBCs. Al film was deposited on the surface of TBCs. Subsequently, the Al-deposited TBCs were conducted with vacuum heat treatment. After that, a dense overlay consisting of α-Al2O3, Al3Zr phases was generated on the TBCs surface due to the reactions of Al and O2, Al and ZrO2. The Al-modified TBCs show a better thermal cycle performance than the as-sprayed TBCs because of the overlay generation. Moreover, the structural evolution of overlay was investigated emphatically before and after thermal cycle testing, showing good phase stability. No crack and spallation were observed in the Al-modified TBCs.

Adhesion strength studies on zirconia based pyrochlore and functionally gradient thermal barrier coatings

Abstract Thermal Barrier Coating (TBC) plays a major role in the improvement of gas turbine and engine components in terms of their service life and performance. Generally, all coatings must possess certain primary properties to perform in the intended applications. However, regardless of applications, suitable adhesion strength is one major characteristic they must have to adequately protect the basic components on which they are applied upon. In TBCs, adhesion (or Bond) strength is a parameter that helps to illustrate the resistance of the ceramic top coat against spallation either from the bond coat (and component) or within the TBC layers itself. The performance of TBCs are reliant upon the adhesion between the coating and the metal substrate and also adhesion (or cohesion) between the bond coat and the overlying ceramic top coat layer. The de-bonding of the top coat layer or the inter-metallic bond coat layers are the main reasons of the failure of the overall TBC system. Some of the prominent problems associated with coatings applications are residual stresses, micro-cracks and pores etc. These and many other factors influence the adhesion of the coatings in addition to service environment conditions and pre coating substrate preparations such as substrate cleaning, grit blasting and very importantly plasma spray parameters. In the present work, results obtained from adhesion strength measurements carried out by following the ASTM C 633 standard test method, on various types of TBCs are being shared. Thermal barrier coatings (TBCs) were synthesized with NiCrAlY bond coat deposited on SS 304L substrate by using air plasma spray and different ceramic top coats (a) commercial 8%Yttria Stabilized Zirconia (8YSZ) (b) lab synthesized plasma spray powders of (i) Lanthanum Zirconate (La2Zr2O7) (ii) Lanthanum Ceria Zirconate (La2 (Zr0.7Ce0.3)2O7) and (iii) Lanthanum cerate (La2Ce2O7). The coating depositions were carried out in different configurations i.e. two layers, three layers and gradient layers (Functionally gradient materials). The evaluation of properties includes the studies of morphology of the strength (adhesive/cohesive failure mode) tested specimen as well. General conclusions drawn from the studies on several specimen in various configurations are that cohesive failures (between the ceramic top coat layers) is the predominant mechanisms followed by few adhesive failures in bond coat coat/ceramic interface.

A comparative study of oxidation kinetics and thermal cyclic performance of thermal barrier coatings (TBCs)

The present study was carried out to investigate the isothermal oxidation behavior, thermal cyclic performance and oxidation kinetics of thermal barrier coatings (TBCs) systems consisting of CoNiCrAlY metallic bondcoats produced by Atmospheric plasma spraying (APS) and high velocity oxy fuel spraying (HVOF) on Inconel 718 substrate. Yttria stabilized zirconia (YSZ) ceramic top coats was produced by APS process as for the insulation layer. The oxidation behavior of the coatings were investigated under isothermal oxidation conditions at 1000 °C, 1100 °C and 1200 °C for 8, 24, 50 and 100 h. The results of the experimental studies on the oxidation kinetics and microstructural evolution of APS and HVOF sprayed thermal barrier coatings (TBCs) are compared and discussed in the paper. The microstructural changes at the metallic/ceramic interface and growth behavior of the thermally grown oxide (TGO) layers during the isothermal oxidation and thermal cyclic tests were analyzed in detail.

The Effect of Coating Composition and Geometry on Thermal Barrier Coatings Lifetime

Several factors are being investigated that affect the performance of thermal barrier coatings (TBC) for use in land-based gas turbines where coatings are mainly thermally sprayed. This study examined high velocity oxygen fuel (HVOF), air plasma-sprayed (APS), and vacuum plasma-sprayed (VPS) MCrAlYHfSi bond coatings with APS YSZ top coatings at 900–1100 °C. For superalloy 247 substrates and VPS coatings tested in 1 h cycles at 1100 °C, removing 0.6 wt %Si had no effect on average lifetime in 1 h cycles at 1100 °C, but adding 0.3%Ti had a negative effect. Rod specimens were coated with APS, HVOF, and HVOF with an outer APS layer bond coating and tested in 100 h cycles in air + 10%H2O at 1100 °C. With an HVOF bond coating, initial results indicate that 12.5 mm diameter rod specimens have much shorter 100 h cycle lifetimes than disk specimens. Much longer lifetimes were obtained when the bond coating had an inner HVOF layer and outer APS layer.

On the Microstructures of Platinum Aluminide Bond Coatings Produced by Different Aluminizing Methods: Effects on the Performance of Thermal Barrier Coatings on Ni-Based Superalloys

It is shown that aluminizing of platinum-modified aluminide bond coatings by an external vapor source of aluminum in an open retort (chemical vapor deposition) can substantially enhance the performance of thermal barrier coatings in comparison with aluminizing by an internal vapor source in a closed retort (pack cementation). This is demonstrated by about twofold increase in life as determined from cyclic oxidation tests at 1150 °C. The difference in behavior is explained with reference to variations in the initial microstructure of each bond coating, particularly the platinum distribution and thickness of the interdiffusion zone. However, in both cases, it is found that decohesion of the top ceramic coatings during cyclic oxidation occurs by spallation of the oxides developed by the bond coatings. It is concluded that aluminizing by chemical vapor deposition improves the thermal stability and resistance to oxidation of the bond coating.

Substrate-constrained effect on the stiffening behavior of lamellar thermal barrier coatings

Thermal exposure would compromises the compliance and thermal insulating performance of thermal barrier coatings (TBCs). However, most publications were based on free-standing coatings in which the stress resulting from substrate is essentially different from TBCs on superalloy substrate. In this paper, the constrained effect of substrate on the ceramic top-coat of plasma sprayed lamellar TBCs was investigated. Results showed that the structural changes evolve from micro-scale to macro-scale during thermal exposure. In a relatively shorter thermal exposure stage, the inter-splat pores became narrowed, whereas the intra-splat cracks became widened. Consequently, the healing kinetics of inter-splat pores was much faster than that of the intra-splat cracks. In a relatively longer thermal exposure stage, some macroscale cracks appeared in coating surface owing to the gradually stiffening coatings. As a result, the microscale intra-splat cracks near the macroscale cracks were healed rapidly. In brief, the substrate constraint induced structural changes were stage sensitive.

3D characterization of porosity in an air plasma‐sprayed thermal barrier coating and its effect on thermal conductivity

AbstractAir plasma spray (APS) thermal barrier coatings (TBCs) contain pores, cracks and splat interfaces that are preferentially aligned normal to the heat flux direction. These significantly reduce (by as much as 50%) the thermal conductivity over a fully dense coating. Here the microstructures of APS thermal barrier coatings (TBCs) have been characterized in 3D using X‐ray microcomputed tomography (μ‐CT). Pores larger than 2.8 μm3 are resolved and their contribution to thermal conductivity reduction is evaluated using image‐based microstructurally realistic numerical models. However, the models overestimate (by almost 50%) the measured thermal conductivity of the as‐deposited TBC sample. It is shown that this discrepancy is due to very fine (micrometer and submicrometer) cracks which readily sinter after short‐term exposure to temperatures representative of TBC operating conditions causing the measured thermal conductivity to rise to within 15% of that predicted. This suggests that under the realistic service conditions, the 3D image‐based models based on CT images provide a good indicator of the likely long‐term TBC performance. Virtual experiments showed that for the retained pores, the larger pores which are flatter and more oriented in the plane of the APS splats, contribute disproportionately to the beneficial reduction in thermal conductivity. Our results demonstrate that X‐ray imaging is a useful tool in establishing APS process conditions that give rise to a beneficial distribution of such pores.

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.

Effect of layer thickness on thermal shock behavior in double-layer micro- and nano-structured ceramic top coat APS TBCs

Abstract The performance of thermal barrier coatings with the double-layer top coat in thermal shock condition has been investigated. Used powders to produce coatings are as follows: Yttria Stabilized Zirconia (YSZ, Y), Ceria-Yttria Stabilized Zirconium Oxide (CSZ, C) and nano-structured YSZ (YSZ-N, YN). The samples were classified into four double-layer families, including Y-C, YN-C, Y-YN and YN-Y. At the end of each cycle, samples were photographed, and the surface and edge damage were determined. Furthermore, scanning electron microscope (SEM) Images and energy-dispersive spectrometer (EDS) analysis of sample's cross-section were taken before and after the test. After collecting experiment's data, the effects of various factors on the outputs were checked. The results showed that Y-YN, YN-Y and YN-C families, have the best performance, respectively. Moreover, it was found that using YSZ-N as the top layer, reduces the thickness of TGO, and it has a great effect on performance and the amount of damage.