Performance Of Thermal Barrier Coatings Research Articles

Micro-chemical and -morphological features of heat treated plasma sprayed zirconia-based thermal barrier coatings

Zirconia-based plasma-sprayed coatings are extensively used in jet and land-based engines as thermal barrier coatings (TBCs) for protecting and insulating gas turbine metal components from the extreme temperature in the hot gas extending the engine life capabilities and service performances as well as reducing fuel consumption. Zirconia-based thermal barrier coatings stabilized with yttria and ceria were prepared by means of atmospheric plasma spray (APS) and thermal treated at different temperatures. The resulting fractured heated surfaces have been studied by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) combined with energy dispersive spectrometry (EDS) and secondary ion mass spectrometry (SIMS) in order to study the surface micro-chemical composition and morphology. The results disclose the variation of the stabilizing oxide amount, the occurrence of valence state modifications of cerium, impurity segregation phenomena and sintering. High temperature sintering influenced the porous microstructure leading to structural changes of the surface. This information confirmed that chemical and morphological aspects in plasma sprayed TBCs must be known in order to understand and predict relationships between the parameters of plasma spray process and TBC features, properties and performances for a better design of reliable TBCs.

Effect of post heat treatment on thermal durability of thermal barrier coatings in thermal fatigue tests

The effects of post heat treatment and coating parameters on the microstructural development of thermal barrier coatings (TBCs) have been investigated and the thermal durability related to the microstructural evolution has been evaluated through cyclic thermal exposure tests. TBC systems with thicknesses of 2000 and 200μm in the top and bond coats, respectively, were prepared with the air–plasma spray system using a 9MB gun with ZrO2–8wt.% Y2O3 (METCO 204 C-NS) for the top coat and Ni-based metallic powder (AMDRY 962) for the bond coat. The post heating was performed at a temperature of 1000°C for 3h with a heating rate of 5°C/min in flowing argon (Ar) gas at 200ml/min. The thermal exposure tests were performed with a dwell time of 60min for 874cycles using a specially designed apparatus—one side of the sample was exposed to a high temperature of 1100°C and the other side was air cooled to 950°C. The post heat treatment is an efficient process in improving the thermal durability of thick TBCs, and the TBC with porous microstructure obtained by controlling coating parameters shows a better thermal durability than those with intermediate or dense microstructures. Results indicate that post heat treatment and microstructural control are important in proposing efficient processes to improve the lifetime performance of thick TBCs in high-temperature environments.

Effect of Laser Remelting on the Tribological Performance of Thermal Barrier Coatings

Gas turbine's efficiency improves as operating temperature is increased. For this reason, metallic components used in turbine engines, for propulsion and power generation, are protected by thermal barrier coatings (TBC). Laser glazing has been used to enhance the oxidation and corrosion resistance of thermally sprayed TBC, but there is no information about the effect of this treatment on the tribological performance. ZrO2(CaO) top coat and NiAIMo bond coating were flame sprayed onto an AlSI 1045 carbon steel. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. Both, the as sprayed and the laser remelted top coatings, were formed by cubic ZrO2 with some tetragonal precipitates. The grain size was reduced by the laser treatment. The mechanical properties and the local wear rate were evaluated by depth sensing indentation and scratch tests respectively. The nanoscale wear behaviour was always improved by the laser treatment.

Study of the high temperature oxidation performance of Thermal Barrier Coatings with HVOF sprayed bond coat and incorporating a PVD ceramic interlayer

The performance of an intermediate Cr3C2 ceramic layer applied by PVD between the bond coat and the ceramic top coat in a TBC system was evaluated. The thickness of the transitional layer was kept around 1–2μm. Two substrate materials and two distinct bond coats were combined in the tests. High Velocity Oxygen Fuel (HVOF) and Atmospheric Plasma Spraying (APS) were used respectively for bond coat and top coat deposition. Isothermal oxidation tests were performed at 1000°C in static air atmosphere. Thermal grown oxide (TGO) was measured and correlated to the exposition times. Results are discussed in terms of the TGO growth rate and changes in residual stresses. The results suggest an improvement in the oxidation resistance of the bond coat because of the presence of the intermediate layer.

Deposition and Characteristics of Submicrometer-Structured Thermal Barrier Coatings by Suspension Plasma Spraying

In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young’s modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400 °C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400 °C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening.

Isothermal Oxidation Behavior of NiCoCrAlTaY Coating Deposited by High Velocity Air-Fuel Spraying

The performance of thermal barrier coatings is influenced by the high temperature oxidation behavior of the bond coat. In this paper, NiCoCrAlTaY bond coat was deposited by high velocity air-fuel (HVAF) spraying, and the microstructure and surface morphology of the bond coat before and after oxidation were examined to aim at developing high performance thermal barrier coatings. Results showed that the HVAF sprayed NiCoCrAlTaY coating presented a dense microstructure and some partially melted particles with a near spherical morphology were deposited on the coating surface. A uniform α-Al2O3 scale was formed on the HVAF sprayed MCrAlY coating surface after the pre-oxidation treatment in an argon atmosphere. A small fraction of nodular-shaped mixed oxides was formed when the MCrAlY coating was oxidized for 100 h at 1000 °C. The amount of the mixed oxides increased less significantly after 200 h oxidation. A homogeneous α-Al2O3 oxide scale was maintained over the large particles on the bond coat surface after 200 h oxidation at 1000 °C in air. A model is proposed to explain the formation of nodular-shaped mixed oxides.

Performance of thermal barrier coatings in industrial gas turbine conditions

The effect of aerofoil geometry on the oxidative degradation mechanisms experienced by thermal barrier coatings (TBCs) used on industrial turbine blades has been investigated. Modified aerofoil-shaped samples (CMSX4 coated with high-velocity oxy-fuel sprayed AMDRY 995 and air plasma sprayed TBC) were oxidised at five temperatures in furnaces from 900 to 1000°C. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterise details of the microstructural evolution of the thermally grown oxide and to monitor inter-diffusion between the bond coating and substrate. Additionally, a novel non-destructive examination technique (flash thermography) was used to detect and track the spread of cracks beneath the TBCs. Multiple samples cracking in identical locations suggested an effect of geometry in the failure of coatings. Furthermore, it was observed that coating curvature influenced spinel formation.

Comparative Performance of a Thermal Barrier Coating System Utilizing Platinum Aluminide Bond Coat on Alloys CMSX-4® and MAR M® 002DS

It is known that the relative performance of thermal barrier coatings is largely dependent upon the oxidation properties of the bond coat utilized in the system. Also, the oxidation properties of diffusion-type bond coats (aluminides and their modifications) are functions of the superalloy substrate used in blade applications. Therefore, the performance of a given coating system utilizing a diffusion-type bond coat can significantly vary from one superalloy to another. Toward the objective of developing coating systems with more universal applicability, it is essential to understand the mechanisms by which the superalloy substrate can influence the coating performance. In this study, we examined the relative performance of yttria-stabilized zirconia/platinum aluminide coating system on alloys CMSX-4 and MAR M 002DS representing single-crystal and directionally-solidified alloy systems respectively using thermal exposure tests at 1150 °C with a 24-h cycling period to room temperature. Changes in coating microstructure were characterized by various electron-optical techniques. Experiment showed that the coating system on alloy MAR M 002DS had outperformed that on alloy CMSX-4, which could be related to the high thermal stability of the bond coat on alloy MAR M 002DS. From a detailed microstructural characterization, this difference in behavior could be explained at least partially in terms of variation in chemical composition of the two alloys, which was also reflected on the exact failure mechanism of the coating system.

Preparation and characteristics of three-layer YSZ–(YSZ/Al 2O 3)–YSZ TBCs

Novel three-layer YSZ–(YSZ/Al 2O 3)–YSZ (6 wt.% Y 2O 3 partially stabilized ZrO 2) thermal barrier coatings (TBCs) were successfully prepared on Ni-based superalloy substrate using composite sol–gel and pressure filtration microwave sintering (PFMS) techniques. The coatings were evaluated for the cyclic oxidation resistance, thermal barrier effect and the presences of phases and microstructures. FE-SEM results indicate that the coatings were dense and crack-free. The coatings maintained their structural integrity when they were exposed at 1100 for 100 h. They exhibited superior oxidation resistance, spallation resistance and thermal insulation property compared with single-layer YSZ coatings. Moreover, the detailed mechanisms were discussed in order to understand the improved performance of the three-layer TBCs.

Performance of thermal barrier coatings in industrial gas turbine conditions

Performance of thermal barrier coatings in industrial gas turbine conditions

Comparison of the Failures during Cyclic Oxidation of Yttria-Stabilized (7 to 8 Weight Percent) Zirconia Thermal Barrier Coatings Fabricated via Electron Beam Physical Vapor Deposition and Air Plasma Spray

The failures during oxidation of electron beam physical vapor deposition (EBPVD) and air plasma spray (APS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) on different bond coats, namely, platinum-modified aluminide and NiCoCrAlY, are described. It is shown that oxidation of the bond coats, along with defects existing near the TBC/bond coat interface, plays a very important role in TBC failures. Procedures to improve TBC performance via modifying the oxidation characteristics of the bond coats and removing the as-processed defects are discussed. The influence of exposure conditions on TBC lives is described and factors such as cycle frequency and thermal gradients are discussed.

Thermal-gradient testing of thermal barrier coatings under simultaneous attack by molten glassy deposits and its mitigation

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines by molten calcium–magnesium–aluminosilicate (CMAS) glassy deposits is becoming a pressing issue, as engines are required to operate under increasingly harsh conditions. While new approaches for mitigating CMAS attack of TBCs are starting to emerge, there is a need for appropriate tests for evaluating the efficacy of those approaches. To that end, we present here a new thermal-cycling test for the evaluation of TBC performance, where a thermal gradient is applied across the TBC, with simultaneous injection of CMAS. The conditions simulated in this new test are closer to actual conditions in an engine, as compared to the conventional furnace test without thermal gradient. This new test is applied to a TBC with a new composition designed for mitigating CMAS attack, where the mitigation mechanisms are observed to be similar to those found in the conventional furnace test. However, based on a previously established mechanics model it is shown here that thermal-gradient cyclic testing is essential for evaluating thermomechanical performance of TBCs under attack by CMAS, and that the use of just the conventional furnace test can lead to the underestimation of the TBC performance.

Characterization of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings by Cathodoluminescence

The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs.

CMAS corrosion of EB PVD TBCs: Identifying the minimum level to initiate damage

Abstract Over the last decade a significant amount of research has been conducted into the durability of thermal barrier coatings (TBCs) focusing mainly on issues of oxidation, erosion and foreign object damage (FOD). However, as the performance and durability of TBCs has improved the temperatures at which they operate has increased. This increase in temperature has resulted in another lifing issue for EB PVD TBCs, namely that of CMAS attack. Calcium–magnesium–alumino-silicate (CMAS) attack occurs when atmospheric dust that has deposited on the surface of turbine blades melts and wicks into the columns of the TBC. This occurs at temperatures above 1240–1260 °C and results in the degradation of the columnar microstructure of the TBCs. Due to the fact that TBCs operate in a temperature gradient CMAS only infiltrates part of the coating before solidifying. There are a number of issues associated with CMAS attack, both chemical and mechanical. From a chemical point of view CMAS attack of electron beam (EB) physical vapour deposited (PVD) TBCs can be considered as a form of corrosion; when there is a lot of excess CMAS on the surface of a coated component Yttria diffuses out of the TBC into the molten CMAS resulting in a t′ to monoclinic phase transformation in the yttria stabilised zirconia (YSZ), CMAS attack also results in localised melting and subsequent re-precipitation of the coating resulting in a loss of the defined columnar microstructure. While from a mechanical point of view the CMAS, once re-solidified, reduces the strain compliance of the EB PVD and can result in spallation of the TBC on cooling. Furthermore, current studies have indicated that small amount of CMAS infiltration significantly increases the erosion rate of EB PVD TBCs. This paper covers various aspects of CMAS attack of EB PVD TBCs, specifically looking at minimum levels of CMAS required to initiate damage, as well as investigating it from an erosion–corrosion perspective.

Damage growth triggered by interface irregularities in thermal barrier coatings

The efficiency and reliability of modern jet engines strongly depend on the performance of thermal barrier coatings (TBCs), which prevent melting and oxidation of the turbine blades’ structural core. The system’s lifetime is limited by cracks appearing in and in the vicinity of an oxide layer that grows in the TBC under thermal cycling. High replacement costs have led to an increased demand to identify, quantify and remedy damage in TBCs. An integrated experimental–numerical approach is presented for studying the main factors that contribute to damage, particularly interfacial irregularities. Damage at several stages of oxidation in TBCs is analyzed in samples with predefined interfacial irregularities. The model predicts the experimentally observed crack patterns, clearly quantifying the influence of imperfections and indicating that damage can be delayed by surface treatment.

Comparison of the Thermal Cycling Performance of Thermal Barrier Coatings under Isothermal and Heat Flux Conditions

Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The integrity and reliability of these coatings is of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. The main cause of failure of coatings is through debonding which develops as a consequence of thermally induced strains between the metallic bondcoat and the alumina TGO layers due to the differences in the thermal expansion coefficients of the individual layers. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach here employs a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Tests have been conducted on a TBC system consisting of an IN738 substrate with a CN334 bondcoat and EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. This paper reports on the performance of the flame rig equipment and the results from the exposures on the TBC system.

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.

Enhancement of TBC (Thermal Barrier Coatings) Characteristics by Gas Tunnel Type Plasma Spraying

Zirconia (ZrO2) coating formed by plasma spray method is widely used industrially as a thermal barrier coating (TBC). Presently, there are some problems such as spallation and cracks inside the coating. As one solution given by the development of new spaying processing, the gas tunnel type plasma spraying is one of excellent method to enhance the TBC performances. The zirconia-alumina (ZrO2-Al2O3) composite coating formed by this method has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness, and is superior as a TBC. In this paper, the performance of such high hardness ZrO2-Al2O3 composite coating was investigated and the merit as TBC was clarified. The Vickers hardness of the high hardness layer near the coating surface increased by the thermal process of high energy plasma, which corresponded to the result that the coating became denser. Also, the effect of alumina mixing was discussed about the microstructure of this composite coating. The combination of high hardness of Al2O3 with the low thermal conductivity of ZrO2 resulted to the development of high performance TBC. The transverse thermal conductivity of such ZrO2-Al2O3 composite coatings was proved to be much smaller than that in the longitudinal direction.

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.

Microstructure and mechanical behavior of Pt-modified NiAl diffusion coatings

Abstract The performance of thermal barrier coatings (TBCs) in jet engines is strongly influenced by thermal and mechanical properties of the bond coat. In this investigation two different types of Pt-modified diffusion coatings developed for application as oxidation protection or as a bond coat in TBC systems were studied with respect to their microstructure and mechanical properties. The microstructural analyses based on different microscopy techniques and X-ray diffraction measurements revealed distinct differences between the two coatings: the coating aluminized by a gas phase process consisted solely of a (NiPt)Al phase, while the coating fabricated by pack-cementation consisted mainly of the two phases (NiPt)Al and Al2Pt, leading to a higher strength in comparison with the single-phase coating. In contrast, the ductility determined with 3-point bend tests between room temperature and 700°C was clearly better for the single-phase coating. Additionally, the substrate curvature of coated sheets was measured to gain some information about coating stresses.