Showing papers on "Thermal barrier coating published in 2017"

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

Abstract: 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.

Defect engineering in development of low thermal conductivity materials: A review

Abstract: Low thermal conductivity is the key property dominating the heat insulation ability of thermal barrier coatings (TBC). Reducing the intrinsic thermal conductivity is the major topic for developing advanced TBCs. Defect engineering has attracted much attention in seeking better TBC materials since lattice defects play a crucial role in phonon scattering and thermal conductivity reduction. Oxygen vacancies and substitutions are proven to be the most effective, while the accompanying lattice distortion is also of great importance. In this paper, recent advances of reducing the thermal conductivity of potential thermal barrier coating materials by defect engineering are comprehensively reviewed. Effects of the mass and size mismatch between the defects and the host lattice are quantitatively estimated and unconventional thermal conductivity reduction caused by the lattice distortions is also discussed. Finally, challenges and potential opportunities are briefly assessed to further minimize the thermal conductivity of TBC materials in the future.

Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress Toward Models and Materials Solutions

Abstract: Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the r...

Lanthanum zirconate based thermal barrier coatings: A review

Abstract: This review article summarizes the latest information about the manufacturing techniques of lanthanum zirconate (La2Zr2O7, LZ) powder and La2Zr2O7 based thermal barrier coatings (TBCs). Lanthanum zirconate is a promising candidate material for TBC applications, due to its lower thermal conductivity and higher thermal stability compared to other traditional TBC systems. In this work, the physical, thermal, and mechanical properties of the powder and coatings are evaluated. The durability experiments of the TBCs in various thermal, mechanical, and corrosive conditions are also reviewed. In addition, theoretical studies on the powder and coatings properties are presented. Finally, future research directions of lanthanum zirconate as TBC applications are proposed.

Thermal insulation properties of YSZ coatings: Suspension Plasma Spraying (SPS) versus Electron Beam Physical Vapor Deposition (EB-PVD) and Atmospheric Plasma Spraying (APS)

Abstract: Improving efficiency of hot section components of aero engines such as turbine blades or nozzle guide vanes is critical for the aircraft industry. Over many years, the development of advanced Thermal Barrier Coatings (TBCs) has been a field of active research to achieve this purpose. Electron Beam Physical Vapor Deposition (EB-PVD) and Atmospheric Plasma Spraying (APS) processes are widely used to apply TBCs on metal substrates. High costs and rather high thermal conductivities of EB-PVD coatings, as well as low thermal lifetime of APS ones, are real drawbacks for next generations of turbine engines. In this study, Suspension Plasma Spraying (SPS) was assessed to improve TBC thermal properties. It was shown that the SPS process allows to perform columnar microstructure easily tunable in terms of both compaction of columnar structure and thermal conductivity. Thermal conductivities were in the 0.7–1.25 W·m− 1·K− 1 range for SPS coatings while values of 0.9 and 1.5 W·m− 1·K− 1 were measured for APS and EB-PVD coatings, respectively. The effect of heat conduction paths, which impact thermal diffusivity values, was highlighted for the columnar structure.

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

Abstract: The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 °C and 1225 °C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.

Laser surface texturing to enhance adhesion bond strength of spray coatings – Cold spraying, wire-arc spraying, and atmospheric plasma spraying

Abstract: The aim of this study is to promote significant bond strength with laser surface texturing tools for different coating deposited by spray processes. Pulsed nanosecond laser has been used to improve the ultimate adhesion strength of thermal spray coating designed for specific applications, and the full potential of this technology must be further explored. This technology proposes several benefits such as free of grit-particle inclusions, limited affected zone and the interface contact quality. The most important improvement is the coating anchoring in the substrate by laser patterned surface. Adhesion bond strength has been improved and evaluated with the contact area. Fracture mechanic analysis has been studied and it showed that the pattern morphology has an impact on crack propagation. A mixed-mode failure has been defined and chosen to explain adhesion strength improvements for the different applications. Laser surface texturing was performed on light metal alloys substrates before cold spraying of light metal alloys powder, wire-arc metallization and atmospheric plasma spraying of thermal barrier coating without bond coat. This study has highlighted laser potential to enhance adhesion bond strength in the dry deposition field.

Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray

Abstract: 7–8 wt.% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas turbines to insulate the underlying metallic substrate. However, at higher temperatures (> 1200 °C), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multi-layered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300 °C. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM.

A comprehensive sintering mechanism for TBCs‐Part I: An overall evolution with two‐stage kinetics

Abstract: A comprehensive sintering mechanism for lamellar thermal barrier coatings was reported experimentally and theoretically in this study. To begin with, an overall property evolution with two-stage kinetics was presented during thermal exposure. The increase in mechanical property at initial thermal exposure duration (stage-I) was much faster with respect to that in the following longer duration (stage-II). At the stage-I, the in situ pore healing behavior revealed that the significant faster sintering kinetics was attributed to the rapid healing induced by multipoint connection at the intersplat pore tips, as well as a small quantity of the narrow intrasplat cracks. At the following stage-II, the residual wide intersplat pore parts and the wide intrasplat cracks decreased the possibility of multiconnection at their counter-surfaces, resulting in a much lower sintering kinetic. Moreover, a structural model based on the microstructure of plasma sprayed YSZ coatings was developed to correlate the microstructural evolution with mechanical property. Consequently, the model predicted a two-stage evolutionary trend of mechanical property, which is well consistent with experiments. In brief, by revealing the pore healing behavior, this comprehensive sintering mechanism shed light to the structure tailoring toward the advanced TBCs with both higher thermal-insulating effect and longer life time.

Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs)

Abstract: Yttria Stabilized Zirconia (YSZ) is widely used as a traditional ceramic top coat material in gas turbine engine components. Nowadays, rare earth zirconates, as alternative materials to YSZ, are enhanced for use as top coat layer of TBC owing to their better thermal isolation properties. In the present study, metallic CoNiCrAlY bond coat powder was sprayed on Inconel 718 superalloy substrate by cold gas dynamic spray (CGDS) technique. After the deposition of bond coats, YSZ and Gd 2 Zr 2 O 7 top coats were produced using EB-PVD process. TBCs were exposed to isothermal oxidation tests at 1100 °C for 8, 24, 50 and 100 h. Oxidation and growth behaviors of thermally grown oxide (TGO) layer were observed. TBC samples were investigated using scanning electron microscope (SEM), EDS elemental mapping and X-ray diffractometer (XRD) analysis before and after the oxidation tests. Oxidation performances of two different TBC systems were compared to each other according to the analysis results.

Development of YSZ Thermal Barrier Coatings Using Axial Suspension Plasma Spraying

Abstract: The axial injection of the suspension in the atmospheric plasma spraying process (here called axial suspension plasma spraying) is an attractive and advanced thermal spraying technology especially for the deposition of thermal barrier coatings (TBCs). It enables the growth of columnar-like structures and, hence, combines advantages of electron beam-physical vapor deposition (EB-PVD) technology with the considerably cheaper atmospheric plasma spraying (APS). In the first part of this study, the effects of spraying conditions on the microstructure of yttria partially-stabilized zirconia (YSZ) top coats and the deposition efficiency were investigated. YSZ coatings deposited on as-sprayed bond coats with 5 wt % solid content suspension appeared to have nicely-developed columnar structures. Based on the preliminary results, the nicely developed columnar coatings with variations of the stand-off distances and yttria content were subjected to thermal cycling tests in a gas burner rig. In these tests, all columnar structured TBCs showed relatively short lifetimes compared with porous APS coatings. Indentation measurements for Young’s modulus and fracture toughness on the columns of the SPS coatings indicated a correlation between mechanical properties and lifetime for the SPS samples. A simplified model is presented which correlates mechanical properties and lifetime of SPS coatings.

Sintering-induced delamination of thermal barrier coatings by gradient thermal cyclic test

Abstract: Lifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma-sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering-induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high-temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering-induced delamination of the top coat. Based on the above-described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

Abstract: In this study, first, Gd2Zr2O7/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd2Zr2O7 coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO2 laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd2Zr2O7 was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Abstract: Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m−1 K−1 for EB-PVD YSZ coatings to about 0.7 W m−1 K−1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni3Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.

CMAS behavior of yttrium aluminum garnet (YAG) and yttria-stabilized zirconia (YSZ) thermal barrier coatings

Abstract: Calcium magnesium aluminosilicate (CMAS) that is formed from the ingested deposits in gas turbines degrades thermal barrier coatings (TBCs), especially for the most widely used material; yttria-stabilized zirconia (YSZ). In the present work, we examine the behavior of yttrium aluminum garnet (YAG) as an alternative material for TBCs. CMAS interaction studies were conducted by making composite pellets of YAG-CMAS and YSZ-CMAS powders. These pellets, after being subjected to heat treatment between 1100 °C and 1500 °C were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS), which showed YAG to be almost inert to CMAS whereas YSZ exhibited significant phase changes. To test the behavior of TBCs with YAG and 8YSZ as the topcoat material in a CMAS environment, cyclic furnace tests were conducted in which a controlled amount of CMAS was applied and then the samples were cycled to failure. In addition, to simulate the continuous accumulation of CMAS expected in service, a cyclic furnace test was devised in which a small dose of aqueous solution of CMAS was applied on TBC specimens at the start of every cycle until the samples were cycled to failure. In all these tests YAG TBCs outperformed YSZ in terms of durability. The mechanisms of CMAS attack are described and the relative resistance of YAG and YSZ is shown to be consistent with the Optical Basicity (OB) theory.

Modeling and simulation of the temperature and stress fields in a 3D turbine blade coated with thermal barrier coatings

Abstract: Prediction of the stress distribution on a 3D turbine blade coated with thermal barrier coatings (TBCs) plays a key role in analyzing the failure of TBCs. In this work, a 3D finite element model of turbine blade coated with multilayer-structure TBCs is developed, in which conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted. To obtain a closer to the actual temperature field, the external flow field performed by three turbulence model (RNG k - e , realizable k - e and SST k - ω turbulence model) are analyzed. It is found that the temperature and pressure distribution of the flow field performed by realizable k - e turbulence model are much closer to the experiment data. TBCs present an excellent insulating effect. The heat insulation performance at the leading and trailing edges is relatively better than the suction and pressure sides. Based on the thermal-stress simulation results, the dangerous regions of TBCs are predicted. It is shown that the maximum principal stress of ceramic layer locate at both suction and pressure surfaces near leading edge and trailing edge. Furthermore, the stress level of ceramic layer is higher than that of bond coating, TBCs may spall off at these regions, especially the ceramic layer.

The Correlation of the TBC Lifetimes in Burner Cycling Test with Thermal Gradient and Furnace Isothermal Cycling Test by TGO Effects

Abstract: Two types of typical thermal cycling tests are used for the evaluation of thermal cycling lifetime of thermal barrier coatings. Those are the burner cycling test with a thermal gradient and the isothermal furnace cycling test. There are diverse explanations to test results up to now. Although certain correlations should exist between the results obtained by two types of the tests, no evident parameters in two tests were directly related, possibly due to large range of difference test conditions. In this investigation, a series of TBC samples with carefully prepared Al2O3-based TGO of different thicknesses were used for both the burner cycling and the furnace cycling tests. The relationships between thermal cycling lifetime and TGO thickness were obtained for two types of the tests. It was found that TGO thickness presents the same influence tendency despite of different types of thermal cycling test. The results reveal the existence of the critical TGO thickness by which the transition of failure mode takes place. Moreover, the values of the critical TGO thickness for two tests are comparable. The results evidently suggest that the lifetimes during different thermal cycling tests can be correlated by TGO effects on failure behavior. However, it is clear that the apparent dominant driving factors to TBC failure are different in two types of tests. Accordingly, the burner cycling test could be used for optimizing the durability of ceramic top coat by separating the effect of individual factors through test condition design, while the furnace cycling test results represent the integrated TBC durable performance of the bond coat and top ceramic coating.