YSZ Thermal Barrier Coatings Research Articles

Nanostructured YSZ Thermal Barrier Coatings Obtained by Atmospheric Plasma Spray

Nanostructured YSZ Thermal Barrier Coatings Obtained by Atmospheric Plasma Spray

Improving the Thermal Shock Resistance of Thermal Barrier Coatings Through Formation of an In Situ YSZ/Al2O3 Composite via Laser Cladding

In the present study, laser cladding of alumina on the top surface of YSZ thermal barrier coatings (TBC) was conducted via Nd:YAG pulsed laser. The thermal shock behavior of the TBC before and after laser cladding was modified by heating at 1000 °C for 15 min and quenching in cold water. Phase analysis, microstructural evaluation and elemental analysis were performed using x-ray diffractometry, scanning electron microscopy (SEM), and energy-dispersive spectroscopy. The results of thermal shock tests indicated that the failure in the conventional YSZ (not laser clad) and the laser clad coatings happened after 200 and 270 cycles, respectively. The SEM images of the samples showed that delamination and spallation occurred in both coatings as the main mechanism of failure. Formation of TGO was also observed in the fractured cross section of the samples, which is also a main reason for degradation. Thermal shock resistance in the laser clad coatings improved about 35% after cladding. The improvement is due to the presence of continuous network cracks perpendicular to the surface in the clad layer and also the thermal stability and high melting point of alumina in Al2O3/ZrO2 composite.

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

AbstractLifetime 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.

Insights on the High-Temperature Operational Limits of ZrO2-Y2O3 TBCs Manufactured via Air Plasma Spray

The effective high-temperature operation limit of a ZrO2-7-8 wt.%Y2O3 (YSZ) thermal barrier coating (TBC) manufactured via air plasma spray (APS) is considered to be ~1300 °C. This is related to the metastable tetragonal t′-phase formed during the rapid quenching of the YSZ particles during spraying. The t′-phase transforms into the equilibrium tetragonal and cubic phases at temperatures ≥~1300 °C, which can lead to the formation of the monoclinic phase of YSZ upon cooling to room temperature. This formation of the monoclinic phase is accompanied by a volume expansion that leads to TBC failure due to extensive micro-cracking. To further investigate this limitation, an APS YSZ TBC was sprayed on a CMSX-4 substrate. By using a thermal (laser) gradient cyclic testing, a temperature gradient was generated across the TBC/substrate system. The YSZ T-front and substrate backside T-back temperature levels were ~1500 and ~1000 °C, respectively. In cycle conditions (5-min or 1-h hot and 2-min cool), no TBC failure has been observed. This behavior was partially attributed to the unexpected absence of the monoclinic phase of the YSZ in the cycled coatings. Although preliminary, these results are promising regarding increasing the effective high-temperature operational limits of APS YSZ TBCs.

High temperature oxidation and insulation behavior of plasma-sprayed nanostructured thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) for more efficient gas turbine engines require higher operating temperature. Adopting pyrochlore structure materials such as rare-earth zirconate and double-ceramic-layer (DCL) structure can well satisfy service demands of TBCs at elevated temperature. The nanostructured single-ceramic-layer (SCL) 8YSZ, DCL Sm 2 Zr 2 O 7 (SZ)/8YSZ and doping 8 wt% CeO 2 nanoparticles in SZ (8CSZ)/8YSZ TBCs were prepared by atmospheric plasma spraying (APS) in this work. The high temperature oxidation behavior and thermal insulation property of the three kinds of as-sprayed TBCs were investigated systematically at 1000 °C and 1200 °C. The results indicate that the nanostructured DCL 8CSZ/8YSZ TBCs have the best oxidation resistance and thermal insulation property at different temperatures among others. The influence of microstructure on thermal insulation property of the coating has been discussed in detail.

Factors Affecting the Performance of Thermal Barrier Coatings in the Presence of V 2 O 5 and Na 2 SO 4

This study investigates the influence of temperature, salt concentration and thickness on the corrosion resistance of seven YSZ thermal barrier coatings in the presence of V2O5 and Na2SO4. For this ...

Thermal cycling behavior of nanostructured 8YSZ，SZ/8YSZ and 8CSZ/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The nanostructured single-ceramic-layer (SCL) 8YSZ thermal barrier coatings (TBCs), double-ceramic-layer (DCL) Sm2Zr2O7 (SZ)/8YSZ and SZ doped with 8wt% CeO2 nanoscale particles (8CSZ)/8YSZ TBCs were fabricated by atmospheric plasma spraying (APS) on nickel-based superalloy substrates with NiCoCrAlY as the bond coating. The thermal cycling behavior of the three as-sprayed TBCs was investigated systematically at 1000℃ and 1200℃. The results reveal that the thermal cycling lifetime of the nanostructured DCL 8CSZ/8YSZ TBCs is the longest among them, which is largely due to the fact that the intermediate layer buffer effect of the DCL structure, more porosity and improvement of thermal expansion coefficient from doping CeO2 nanoparticles can relieve thermal stress to a great extent at elevated temperature. The failure mechanism of the nanostructured TBCs has been discussed in detail.

Impedance Analysis of 7YSZ Thermal Barrier Coatings During High-Temperature Oxidation

ZrO2-7 wt.%Y2O3 (7YSZ) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying. High-temperature oxidation of 7YSZ TBCs was accomplished at 950 °C and characterized by impedance spectroscopy and scanning electron microscopy with energy-dispersive spectrometry. The results indicated that the thermally grown oxide (TGO) mainly contained alumina. The increase of the thickness of the TGO layer appeared to follow a parabolic law. Impedance analysis demonstrated that the resistance of the TGO increased with increasing oxidation time, also following a parabolic law, and that characterization of the TGO thickness based on fitting an equivalent circuit to its measured resistance is feasible. The YSZ grain-boundary resistance increased due to increasing cracks within the coating for oxidation time less than 50 h. However, beyond 150 h, the YSZ grain-boundary resistance slightly decreased, mainly due to sintering of the coating during the oxidation process.

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H2), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H2 as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.

Cyclic oxidation behavior of SiC-containing self-healing TBC systems fabricated by APS

It is well-known that thermally grown oxide (TGO) deteriorates thermal barrier coating (TBC) systems after extended operating time. In this study, SiC-self-healing coatings (called “SAZ coatings” here) were investigated as a possible solution to enhance the oxidation resistance and spallation resistance of TBC systems. The self-healing-related filling and sealing effects of the proposed coatings were investigated and the oxidation resistance of the system after SAZ coating treatment was examined. To fully understand the oxidation resistance and spallation resistance mechanisms, we also conducted high-temperature cyclic oxidation tests (1100 °C) and recorded the mass gain and mass loss of samples after each cycle. Results showed that the corrosion current of YSZ/SAZ TBC samples decreased 79.23% after oxidation at 720 °C for a period of 10 h, and that 25.77% less TGO grew in YSZ/SAZ TBC samples after 20 h at 1100 °C compared to YSZ TBC samples due to the self-healing coating layer. The lifetime of YSZ/SAZ TBC was improved 58.33% compared to YSZ TBC in the high-temperature cyclic oxidation test, owing to the higher oxidation resistance and toughening effect of the SAZ coating.

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

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.

Thermal and Metrological Studies on YTTRIA Stabilized Zirconia Thermal Barrier Coatings and a Simulated Model to Co-Relate the Findings

Thermal Barrier Coatings (TBCs), routinely prepared from Ceramic based compositions (typically 8%Y 2O3-ZrO 2or 8YSZ) are being engineered to protect the metallic components from degradation in applications like gas turbines, jet and automotive engines. With a goal of finding improved TBC materials a wide variety of ceramics are being researched worldwide. Before physically preparing the TBCs of uncommon compositions in the laboratory, their suitability to perform can be predicted. Limited accessibility to detailed and realistic information on the influence of newer compositions (other than 8YSZ) on TBCs warrants methods to obtain this information. In this paper, 8YSZ TBCs coated onto aluminium substratesare studied for thermal fatigue, thermal barrier and materials characteristics to determine the reliability of the coating configuration to withstand the harshness of test conditions under the framework of experiments. Thereafter, the results have been used to corroboratethe developed simulation model. Results obtained via thermal tests confirm the suitability of the model and we can predict the thermal barrier effects of TBCs when prepared from materials other than YSZ.

Effects of incorporation of micro and nano Al2O3 layers on thermal shock behaviour of YSZ thermal barrier coatings

In the present paper, thermal shock behaviour of usual YSZ and layered composite TBCs was evaluated. The layered composite TBCs used in this study were YSZ/micro Al2O3 and YSZ/nano Al2O3 in which Al2O3 was as a topcoat on the YSZ layer. In addition, coating microstructures were investigated using scanning electron microscope; EDS and X-ray diffraction techniques before and after thermal shock test. The results showed that incorporation of a nano Al2O3 oxygen barrier layer caused better thermal shock resistance, while micro Al2O3 layer on the top of YSZ sample resulted in sooner coating failure, compared to the usual TBC coating. Also, the thermal shock behaviour and failure mechanism of mentioned coatings were investigated.

Thermal stability and sintering behavior of plasma sprayed nanostructured 7YSZ, 15YSZ and 5.5SYSZ coatings at elevated temperatures

Nanostructured scandia, yttria doped zirconia (5.5SYSZ), 7wt% yttria stabilized zirconia (7YSZ) and 15YSZ thermal barrier coatings (TBCs) were produced by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal stability and sintering behavior of the three as-sprayed TBCs at 1480°C were investigated. The results indicated that the thermal stability of SYSZ and TBCs was longer than the 7YSZ TBCs due to higher amount of tetragonal phase. Furthermore, the results demonstrated that the nanostructured 7YSZ coating exhibits higher sintering resistance than 5.5SYSZ TBC.

Enhanced properties of Al-modified EB-PVD 7YSZ thermal barrier coatings

7wt% yttria-stabilized zirconia (7YSZ) thermal barrier coating (TBC) prepared by electron beam-physical vapor deposition (EB-PVD) has been used in gas turbine engines for many years, where the TBC must successfully withstands the damage caused by a variety of environmental and mechanical aspects. The primary failure modes for TBC are oxidation of bond coating, particle erosion and CMAS (calcium-magnesium-alumina-silicates) corrosion. The lifetime of TBC associated with above three failure factors will be reduced significantly. In order to prolong the operation time, an alternative approach depositing Al film on 7YSZ TBC surface by magnetron sputtering is proposed. An α-Al2O3 overlay was in-situ synthesized on each 7YSZ column through reaction of Al and ZrO2 during vacuum heat treatment. And the results indicate that the Al-modified EB-PVD 7YSZ TBC shows better oxidation resistance, as well as lower particulate erosion and CMAS corrosion.

Thermal cycling behavior and bonding strength of single-ceramic-layer Sm2Zr2O7 and double-ceramic-layer Sm2Zr2O7/8YSZ thermal barrier coatings deposited by atmospheric plasma spraying

The single-ceramic-layer (SCL) Sm2Zr2O7 (SZO) and double-ceramic-layer (DCL) Sm2Zr2O7 (SZO)/8YSZ thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying on nickel-based superalloy substrates with NiCoCrAlY as the bond coat. The mechanical properties of the coatings were evaluated using bonding strength and thermal cycling lifetime tests. The microstructures and phase compositions of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that both coatings demonstrate a well compact state. The DCL SZO/8YSZ TBCs exhibits an average bonding strength approximately 1.5 times higher when compared to the SCL SZO TBCs. The thermal cycling lifetime of DCL SZO/8YSZ TBCs is 660 cycles, which is much longer than that of SCL 8YSZ TBCs (150 cycles). After 660 thermal cycling, only a little spot spallation appears on the surface of the DCL SZO/8YSZ coating. The excellent mechanical properties of the DCL LZ/8YSZ TBCs can be attributed to the underlying 8YSZ coating with the combinational structures, which contributes to improve the toughness and relieve the thermal mismatch between the ceramic layer and the metallic bond coat at high temperature.

Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates

Gadolinium zirconate (GZO) with its lower thermal conductivity and higher thermal stability compared to the industrial standard 7YSZ is a new promising material for thermal barrier coating (TBC) applications. In this study, top coats of GZO and 7YSZ were deposited on NiCoCrAlY and Hf-doped NiCoCrAlY bond coats with CMSX-4 as substrate material. The bond coats as well as the ceramic top coats were manufactured by electron-beam physical vapor deposition (EB-PVD). The lifetimes of these new TBC systems were investigated by thermal cycling at 1100 °C. In comparison to the standard 7YSZ TBC, GZO deposited on the NiCoCrAlY bond coat exhibited a significantly enhanced lifetime. Doping NiCoCrAlY with 0.6 wt.% Hf resulted in around 10 times increase in the lifetime for the 7YSZ top coat. However, no significant difference in lifetimes was observed when 7YSZ is replaced by GZO on NiCoCrAlY-Hf bond coats. During thermal cycling, a chemical reaction between GZO and the thermally grown oxide (TGO) formed on the NiCoCrAlY bond coat was observed; however, such a chemical reaction did not occur when GZO was deposited on NiCoCrAlY-Hf bond coats. A faster TGO growth has been observed for the Hf-based systems, resulting in TGO thicknesses as large as 20 μm. The role of Hf-doping in the bond coat, the individual TGO microstructure, and the diffusion of refractory elements from the substrate into the bond coats are discussed along with the lifetime measurements of the different TBC systems.

Modeling the thermal radiation properties of thermal barrier coatings based on a random generation algorithm

Thermal barrier coatings (TBCs) are porous media in which many different pores and cracks are induced by different manufacturing procedures. Many studies have been conducted to investigate the impact of microstructures of TBCs on thermal conductivity; nevertheless, the influence of microstructures on the radiative properties of TBCs has not drawn significant attention. In addition, the working condition of thermal barrier coatings is at high temperatures at which the contribution of radiative heat transfer plays a very crucial role. Therefore, it is necessary to study the radiative properties of TBCs to characterize their insulation performance. In this work, the microstructures of air-plasma- sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are constructed by the quartet structure generation set (QSGS) algorithm. A finite-difference-time-domain (FDTD) method is carried out to simulate radiative heat transfer through TBCs. Three parameters—average pore size, directional growth probability Di (especially horizontal growth probability D13) and porosity—have been investigated to study the microstructural effect on the radiative properties of TBCs. The reflectance of freestanding 50-μm-thick thermal barrier coatings is studied using Lumerical FDTD Solutions in the wavelength range from 1 to 6μm at normal incidence. The absorption and scattering coefficient as a function of wavelength are extracted using the four-flux model. The results will help us to characterize the radiative heat transfer process across the TBCs and provide us with a theoretical guide to design TBCs with a high thermal insulation property.

Comparison of hot corrosion behavior of nanostructured ScYSZ and YSZ thermal barrier coatings

Hot corrosion behavior of nanostructured scandia (3.6mol%) and yttria (0.4mol%) stabilized zirconia (4ScYSZ) and yttria stabilized zirconia (4YSZ) were investigated. Thus, duplex TBCs (thermal barrier coatings), composed of a bond coat (NiCrAlY) and a top coat (4ScYSZ or 4YSZ), were deposited on the IN738LC Ni-based supperalloy by atmospheric plasma spraying (APS). Hot corrosion studies of plasma sprayed TBCs were conducted in 45wt% Na2SO4+55wt% V2O5 molten salt at 910°C for different times. Based on the results, it was revealed that ScYSZ coating has more hot corrosion resistance than YSZ coatings. Moreover, the mechanisms of degradation of both TBCs under corrosion salt were suggested.

Calcia-magnesia-alumino-silicate (CMAS)-induced degradation and failure of air plasma sprayed yttria-stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs) used in gas-turbine engines experience severe degradation by calcia-magnesia-alumino-silicate (CMAS) deposits during high-temperature operation. The present study identified and evaluated the chemical and microstructural changes in air plasma-sprayed (APS) 7 wt.% Y2O3 stabilized ZrO2 (7YSZ) TBCs caused by CMAS attack under isothermal conditions at 1340 °C. Additionally, a ‘model’ experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300 °C for different exposure times. The combined results from both studies highlight the importance of local CMAS glass composition on the 7YSZ/CMAS interaction. Specifically, low Y-content in the glass, caused by a relatively large glass 'sink,' produces Y-depleted ZrO2 grains that undergo tetragonal (t) → monoclinic (m) phase transformation upon cooling. Alternatively, small pockets of Y-enriched glass induce the formation of t″-ZrO2, a phase characterized by its high stabilizer content. After prolonged high-temperature exposure, solution-reprecipitation induces the formation of both m-ZrO2 and t″-ZrO2 throughout the APS 7YSZ TBC in accordance with the phase diagram. Using a thermomechanical model it is shown that the strain associated with the martensitic t→m phase transformation plays an important role in the delamination failure of TBCs attacked by CMAS.