Multilayer Thermal Barrier Coatings Research Articles

The effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings

The aim of the present research was to investigate the effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings. In this regard, specimens of Inconel 718 were sprayed with Ni–22Cr–10Al–Y bond coat and Al2O3/YSZ top coat using an atmospheric plasma spraying approach. Then, top surfaces of specimens were under exposed to Nd-YAG laser radiation and thereby were glazed. Thermal stabilities of both coatings were evaluated by rising the environment temperature up to 1000 °C, keeping at this temperature for 15 min, and also rapid cooling of specimens in cold water. Using FESEM with EDS and XRD, the microstructure and morphology observations, elemental investigations and also phase identification were carried out. Findings displayed that both sprayed and glazed coatings were damaged as a spallation of top coats. Furthermore, by checking the life-time of coatings, the number of thermal cycles led to 20% destruction of top surfaces of coatings, increased from 232 cycles in sprayed coating to 325 cycles in laser glazed Al2O3/YSZ multilayer coating which displayed 40% improvement of thermal shock resistance after laser glazing process. This enhancement in resistance to thermal shock was due to strain adaptability improvement via a network of segmented cracks on the top surface produced by laser glazing process.

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

Thermo-mechanical evaluation of plasma sprayed YSZ-based multi-layered thermal barrier coatings

The multi-layered YSZ, NiCoCrAlY, Cr2O3 and Al2O3 coatings of varying thickness (200 μm-400 μm) have been investigated for thermal barrier coating (TBC) application. Plasma spray technique has been utilised on piston crown to optimise the thermal fatigue life in view to reduce the heat losses. Four types of TBC overlay each with a 100 μm NiCrAlY bond coat were deposited on the A336 aluminium alloy substrate cut out of the diesel engine piston. The phase composition of coatings before and after the thermal shock testing was analysed by XRD and the lattice strain analysis was performed by Williamson-Hall analysis. It was observed that among the considered multi-layered coating configurations, those with 300 μm YSZ and 200 μm Cr2O3 top coat exhibited acceptable thermal shock resistance as the specimens sustain up to 298 and 325 thermal cycles respectively. The microstructural analysis suggested against the formation of any major deformity or structural changes at the higher temperature as investigated by thermal shock experiment.

Tailored microstructures of gadolinium zirconate/YSZ multi-layered thermal barrier coatings produced by suspension plasma spray: Durability and erosion testing

This work employed an axial suspension plasma spray (SPS) process to deposit two different gadolinium zirconate (GZ) based triple layered thermal barrier coatings (TBCs). The first was a ‘layered’ TBC (GZ dense/GZ/YSZ) where the base layer was YSZ, intermediate layer was a relatively porous GZ and the top layer was a relatively dense GZ. The second triple layered TBC was a ‘composite’ TBC (GZ dense/GZ + YSZ/YSZ) comprising of an YSZ base layer, a GZ + YSZ intermediate layer and a dense GZ top layer. The as sprayed TBCs (layered and composite) were characterized using SEM/EDS and XRD. Two different methods (water intrusion and image analysis) were used to measure the porosity content of the as sprayed TBCs. Fracture toughness measurements were made on the intermediate layers (GZ + YSZ layer of the composite TBC and porous GZ layer of the layered TBC respectively) using micro indentation tests. The GZ + YSZ layer in the composite TBC was shown to have a slightly higher fracture toughness than the relatively porous GZ layer in the layered TBC. Erosion performance of the as sprayed TBCs was evaluated at room temperature where the composite TBC showed higher erosion resistance than the layered TBC. However, in the burner rig test conducted at 1400 °C, the layered TBC showed higher thermal cyclic lifetime than the composite TBC. Failure analysis of the thermally cycled and eroded TBCs was performed using SEM and XRD.

Hot corrosion performance and electrochemical study of CoNiCrAlY/YSZ/YSZ-La2O3 multilayer thermal barrier coatings in the presence of molten salt

One of the effective methods to prevent materials degradation at high temperatures is thermal barrier coating (TBC). In this research, a multilayer (CoNiCrAlY/YSZ/YSZ-10 wt%La2O3) TBC system was deposited on the IN738 superalloy using the high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) techniques. Hot corrosion behaviors were studied in the presence of Na2SO4-20 wt% V2O5 molten salt at 880 °C for 400 h. Hot corrosion kinetics were determined by measuring the thermal grown oxide (TGO) thickness of the specimens at regular intervals. Afterward, the electrochemical behavior of the coated and hot corroded samples was investigated by electrochemical polarization method. Phase identification, TGO, surface morphologies and elemental distribution of the coated samples were also studied during hot corrosion by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The results showed that the multilayer YSZ/YSZ-10 wt%La2O3 ceramic top coat led to a reduction in the penetration of corrosive components into the coating; thereby, delaying the formation of destructive YVO4 phases and postponing the monoclinic-ZrO2 phase transformation.

Oxidation Behavior of a High-Nb-Containing TiAl Alloy with Multilayered Thermal Barrier Coatings

Oxidation of TiAl alloys has been recognized as an obstacle for high-temperature applications such as aero engine and gas turbine. Substantial efforts have been made to improve oxidation resistance of TiAl alloys at elevated temperatures. In this study, multilayered thermal barrier coatings are prepared to protect a high-Nb-containing TiAl alloy from oxidation by air plasma spraying. The combination of Al2O3-13wt.%TiO2 ceramic coatings and NiCoCrAlY metallic coatings can improve thermal stability and increase the service lifetime of coatings. The fully melted TiO2 particles distribute in ceramic coatings uniformly and act as sealing pores and microcracks, which decrease porosity of the ceramic coatings and reduce diffusion channels of oxygen atoms. The porosity of surface and cross-section morphology are 5.5 ± 0.8 and 5.1 ± 0.8%, respectively. The results of oxidation experiment carried out at 800 and 900 °C for 100 h indicate that the coatings can effectively protect a high-Nb-containing TiAl alloy from oxidation. The mass gain of the high-Nb-containing TiAl alloys with coatings is lower than that of the one without coatings. The ceramic coatings retard diffusion of large amount of oxygen atoms, and bond coatings avoid to be excessively oxidized. Thus, the multilayered thermal barrier coatings exhibit an excellent long-term stability.

Hot Corrosion Behavior of Multilayered Titanium Aluminum Carbide/Yttria-Stabilized Zirconia Coatings for Silicon Carbide

Economic and environmental concerns are the main driving force behind the ever increasing demand for higher gas turbine engine inlet temperatures critical for meeting higher efficiency goals. Turbine materials, while in operation, are exposed to corrosive deposits of molten Na2SO4 produced from the high temperature reaction of environmental NaCl salt with sulphur resulting from fuel combustion. This phenomenon is known as “hot corrosion” and may lead to catastrophic failure of structural components. New light-weight and high-strength, high-temperature materials are needed to replace performance-limited metallic-based turbine components. Ceramic matrix composite (CMC) materials are attractive candidates for turbine hot-section components; however, direct exposure to combustion environments leads to degradation via corrosion, volatilization and associated surface recession. To protect against these phenomena, multilayered thermal and environmental barrier coatings are required to increase durability through cyclic operation to temperatures in excess of 1000°C. Mn+1AXn (abbreviated as MAX, where M is an early transition metal, A is an IIIA- or IVA-group element, and X is C or N) phases have received increasing attention because of their unique combination of properties including excellent thermal shock resistance and damage tolerance. Among them, Ti2AlC has attracted considerable interest because of its excellent high-temperature oxidation and hot corrosion resistance. Yttria Stabilized Zirconia (YSZ) is instead recognized as an efficient thermal barrier coating (TBC). The main objective of this study is to determine the effectiveness of a combined thin film coating (~5 μm) comprised of a MAX phase sublayer and a TBC top layer, specifically Ti2AlC coupled with YSZ. The thin film coatings were deposited via electron beam evaporation, physical vapor deposition (EB-PVD) on bulk SiC substrates used as prototypical material. Uncoated and coated samples were exposed to a simulated combustion environment gas mixture of air with 100 ppm SO2 after the deposition of 1 mg/cm2 of prototypical salt (Na2SO4) at 700 °C for up to 100 hours. The corrosion behavior of uncoated, Ti2AlC coated and Ti2AlC/YSZ coated samples were compared. Thermogravimetric analyses were performed to evaluate the development of the surface oxide layer in order to quantify the entity of the corrosion process. Results were then compared with those collected for samples exposed to simple oxidation conditions (air only). Phase compositions of coated and uncoated samples before and after exposure were assessed using X-ray diffraction (XRD) and their corresponding morphologies (surfaces and cross sections) analyzed with a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS) system. The corrosion mechanisms of the uncoated and coated specimens were proposed and compared based on the experimental results. Results and interpretations will be presented and discussed in the context of improving mechanistic understanding of hot corrosion behaviors of protective thin film coatings.

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.

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

AbstractIncrease of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La2Zr2O7) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.

Failure analysis of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings subjected to thermal cyclic fatigue

8 wt.% yttria stabilized zirconia (8YSZ) is the standard ceramic top coat material used in thermal barrier coatings (TBCs) due to its excellent thermo-physical and thermo-mechanical properties. However, above 1200 °C, YSZ has issues such as susceptibility to CMAS (Calcium Magnesium Alumino Silicates) attack and enhanced sintering which could lead to catastrophic failure of the TBC. Pyrochlores of rare earth zirconate composition such as gadolinium zirconate have shown to be resistant to CMAS attack and at the same time possess several other attractive properties. However, poor thermal cycling life of single layer gadolinium zirconate (GZ) TBC compared to single layer YSZ has been reported. Therefore, a double layered GZ/YSZ TBC with YSZ as the intermediate coating and GZ as the top coat and a single layer 8YSZ were deposited by the axial suspension plasma spray process. Additionally, a triple layer TBC (GZdense/GZ/YSZ) comprising of denser GZ coating on top of GZ/YSZ TBC was deposited. SEM analysis revealed a columnar microstructure in the single, double and triple layer TBCs. XRD analysis confirmed the presence of tetragonal prime and defect fluorite phases in the top surface of YSZ and GZ based as sprayed TBCs respectively. The single layer YSZ and GZ/YSZ multi-layered TBCs were subjected to thermal cyclic fatigue (TCF) testing at 1100 °C and 1200 °C. The triple layer TBC showed a higher thermal cyclic life at both the temperatures compared to the single and double layer TBCs. The failed TBCs at 1100 °C were analyzed by SEM/EDS and image analysis. It was found that the failure modes in single layer YSZ and GZ based TBCs were different.

High temperature corrosion resistance of silicate based nanostructured thermal barrier coatings using Al2O3–(Y2O3) ZrO2/SiO2 nanocomposite

Abstract In this study, amorphous nanocomposite of Al 2 O 3 –(Y 2 O 3 ) ZrO 2 /SiO 2 was used for multilayer nanostructured thermal barrier coatings (TBCs) on SS 316L specimens. Thermal stability of the TBCs was investigated in terms of direct heat treatment, isothermal cyclic oxidation, and hot corrosion molten salt (50 V 2 O 5 + 25 Na 2 SO 4 + 25 NaCl) wt.% at different temperatures (800, 900 and 1000 °C) for 100 h. The X-ray diffraction patterns for the heat-treated and hot corrosion-tested specimens showed the stable structural phase composition of t-ZrO 2 , Al 2 SiO 5 , ZrSiO 4 , and Y 2 SiO 5 with an absence of m -ZrO 2 . Similarly, the scanning electron microscopic images showed dense and crack-free surface coating with a random porosity. However, the molten salt-deposited specimen at 1000 °C showed the surface deformation with cracks and patches. In addition, Brunauer–Emmett–Teller surface area and pore size distribution values of TBCs were in the range of 634–493 m 2 g − 1 and 2.64–2.49 nm, respectively. The rate of TBC oxidation was found to increase with an increase in temperature and resulted in a cyclic oxidation which has higher oxidation resistance those of hot corrosion. The elastic properties of TBCs were investigated by nanoindentation technique where average hardness ( H ) and elastic modulus ( E r ) were in the range of 16.36 ± 0.03 to 21.67 ± 0.02 GPa and 70.01 ± 0.07 to 62.87 ± 0.09 GPa, respectively, whereas the same for molten salt-tested specimen, it was decreased in their range from 14.88 ± 0.12 to 14.04 ± 0.29 GPa and 75.81 ± 0.30 to 92.80 ± 0.21 GPa, respectively.

Deposition and properties of a multilayered thermal barrier coating

Abstract The widely used air plasma sprayed thermal barrier coatings (TBCs) in industrial gas turbine engines are limited by its durability. The failure of plasma sprayed TBCs in thermal cycling is usually induced by the spallation of ceramic coating within the ceramic coating near the top coat/bond coat interface. In this research, a multilayered ceramic coating architecture is introduced comprised of the layers of segmentation cracks, porous and dense. The segmented yttria stabilized zirconia (YSZ) layer was deposited at the substrate temperature of 750 °C without any artificial cooling, which resulted in the formation of YSZ layer with obvious columnar crystal structure and segmentation cracks. And a dense top YSZ layer was also included on the top to restrain the penetration of oxygen and the molten calcium–magnesium–alumina-silicate (CMAS) glass deposits. Additionally, the conventional porous YSZ coating was preserved in order to get the low thermal conductivity. The thermal cycling tests show that the lifetime of the multilayered TBCs increases by a factor of 2 compared with that of the conventional one, which could attribute to the high strain tolerance and fracture toughness of the segmented and columnar microstructure. Oxidation resistance and CMAS corrosion resistance are also better for the multilayered TBCs compared with that of the conventional one, due to the suppression function of dense top YSZ layer. It is potential to obtain the high performance TBCs by adjusting plasma spraying.

Development of a new TBC system for more efficient gas turbine engine application

To protect the superalloy blades in industrial gas turbine engines from failure Atmospheric plasma-sprayed YSZ (yttria-stabilized zirconia) thermal barrier coatings (TBCs) are widely used. The spalling of YSZ coating is the main failure of TBCs. This failure usually occurs within the YSZ coating near the YSZ/bond coat interface. In the present feasibility study, a novel durable multilayer TBC was fabricated using atmospheric plasma spray (APS) on Ni718 superalloy substrate consisting of a NiCrAlY-20%YSZ- MoSi2 top coat. The developed system introduces a protective top layer of MoSi2 for preventing diffusion of oxygen, oxidation of the bond coating, provides thermal insulation and protection against corrosion and high temperature erosion. The results of hot gas flow exposure test (H2+O2) to the triple layered coating, a kinetic heating study, showed that a decreased temperature 400 °C for heating of 145 second achieved without any visible damage or delamination in such drastic combustion condition exposure even after 27 cycles of such repeated exposure.

Design of functionally graded multilayer thermal barrier coatings for gas turbine application

Progress in design of thermal barrier coatings (TBCs) is based upon application of new materials and deposition techniques. In this work traditional NiCrAl bond coat YSZ top coat were deposited on Ni superalloy substrate by inexpensive dry detonation spraying, while finishing layers of oxide nanocomposites (LaAlO3–La2Zr2O7, LaAlO3–LaCuAl11O19) were deposited on YSZ by slip casting. Complex oxides were prepared via Pechini route. Thin (~ 20 μm) finishing layers were deposited by slip casting of suspensions of oxides mixture in isopropanol with addition of polyvinylbutyral. Genesis of the texture, composition and real/defect structure of bulk nanocomposite materials and deposited layers after annealing in air up to 1300 °C as well as after series of thermal shocks by heating up to 1200 °C by H2–O2 burner were studied by combination of diffraction (high resolution SEM and TEM with EDX, XRD on synchrotron radiation) and spectroscopic (UV–Vis, EXAFS, laser-excited Dy3 + luminescence spectra) methods. Thermal conductivity of nanocomposites and TBCs was determined by using NETZSCH LFA 457 MicroFlash. Nanocomposites were shown to retain porosity as well as nanosizes of disordered domains of oxide phases even after sintering at high temperatures. A good adhesion and compatibility of all layers in TBCs were demonstrated, while in general disordering of the oxides structure in deposited layers was higher than that in bulk materials due to the effect of depositing procedure and interaction between layers. This provides a low thermal conductivity of nanocomposites and functionally graded TBC. After 90 thermal shocks neither layers spallation nor cracks were revealed, 8YSZ and finishing layers retaining porosity, nanocrystallinity and disordering required for a low thermal conductivity and cracks trapping.

Phase composition and residual stresses in thermal barrier coatings

The phase composition and the residual stresses in multilayer thermal barrier coatings, which consist of an external ZrO2–8Y2O3 ceramic layer, an intermediate gradient (metal ceramic) layer, and a transient metallic NiCrAlY sublayer, are studied. It is shown that an increase in the specific volume of the metallic sublayer as a result of the formation of thermal growing oxide Al2O3 generates high compressive stresses in this sublayer. The ceramic layer undergoes tensile stresses in this case. A method is proposed to estimate the stresses in gradient coatings from X-ray diffraction results.

Heat transfer through nanoscale multilayered thermal barrier coatings at elevated temperatures

Heat transfer through thermal barrier coatings (TBCs) composed of alternating nanometer thick layers of aluminum oxide and 7% yttria stabilized zirconia (7YSZ) was studied by pulsed heating at temperatures in the range 1275K to 1375K. The thermal diffusivity of the TBCs, deposited on thin metal foils by electron beam evaporation and coated with an opaque, submicrometer metal capping layer, was studied by applying a sub-μs duration heating pulse from a Q-switched laser to the substrate and then monitoring the temperature rise on the opposing, metal-capped surface of the TBC. The recorded temperature transients were modeled using properties of the constituent materials in order to obtain an upper bound of the thermal resistance associated with the interfaces between layers. The results thus provide insight into the feasibility of using interfacial thermal resistance in this material system to improve TBC performance by decreasing thermal conductivity.

Monitoring Local Strain in a Thermal Barrier Coating System Under Thermal Mechanical Gas Turbine Operating Conditions

Advances in aircraft and land-based turbine engines have been increasing the extreme loading conditions on traditional engine components and have incited the need for improved performance with the use of protective coatings. These protective coatings shield the load-bearing super alloy blades from the high-temperature combustion gases by creating a thermal gradient over their thickness. This addition extends the life and performance of blades. A more complete understanding of the behavior, failure mechanics, and life expectancy for turbine blades and their coatings is needed to enhance and validate simulation models. As new thermal-barrier-coated materials and deposition methods are developed, strides to effectively test, evaluate, and prepare the technology for industry deployment are of paramount interest. Coupling the experience and expertise of researchers at the University of Central Florida, The German Aerospace Center, and Cleveland State University with the world-class synchrotron x-ray beam at the Advanced Photon Source in Argonne National Laboratory, the synergistic collaboration has yielded previously unseen measurements to look inside the coating layer system for in situ strain measurements during representative service loading. These findings quantify the in situ strain response on multilayer thermal barrier coatings and shed light on the elastic and nonelastic properties of the layers and the role of mechanical load and internal cooling variations on the response. The article discusses the experimental configuration and development of equipment to perform in situ strain measurements on multilayer thin coatings and provides an overview of the achievements thus far.

Thermal conductivity and stability of multilayered thermal barrier coatings under high temperature annealing conditions

Pyrochlore oxides have most of the relevant attributes for use as next generation thermal barrier coatings (TBCs) such as phase stability, low sintering kinetics and low thermal conductivity. One of the issues with the pyrochlore oxides is their lower toughness and therefore higher erosion rate compared to the current state of the art TBC material, yttria (6–8wt.%) stabilized zirconia (YSZ). In this work, sintering characteristics were investigated for novel multilayered coating consisted of alternating layers of pyrochlore oxide viz. Gd2Zr2O7 and t′ low k (rare earth oxide doped YSZ). Thermal gradient and isothermal high temperature (1316°C) annealing conditions were used to investigate sintering and cracking in these coatings. The results are then compared with that of relevant monolayered coatings and a baseline YSZ coating.

Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

This paper investigates the feasibility of manufacturing sol-gel multilayer thermal barrier coatings (TBC) functionalized with different lanthanide ions Ln3+ having distinct photo-luminescence emission wavelengths (Ln=Sm, Eu, Dy, Er, Tm) for future applications in temperature gradient sensing. Ln3+ doped 9.75mol% yttria stabilized zirconia (YSZ) powders were produced to study the effect of activator concentration on luminescence intensity and host matrix crystal structure. Self-quenching was found to limit the maximum signal-to-noise ratio achievable with Sm3+, Dy3+, Er3+ and Tm3+ activators, which was not the case for Eu3+ in the 1–10mol% range. The increase in activator was found to affect the crystal structure of YSZ. A solution was proposed that suppressed this effect while significantly increasing the luminescence intensity of all activators. Finally a TBC sensor prototype integrating Eu3+, Er3+ and Dy3+ doped layers distributed throughout the thickness was successfully deposited by a dip-coating sol-gel process and showed promising through-thickness luminescence sensing capabilities.

Processing and characterization of CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ multilayered thermal barrier coatings

CYSZ/Al2O3 and CYSZ/Al2O3+YSZ multilayer ceramic coatings were produced in 4, 8 and 12 layered with a total thickness of ~400μm by high-velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) processes. Microstructure, thermal and mechanical properties were evaluated. The thickness of metallic bonding layer and total thickness of ceramic top layer were measured as 100±10μm and 400±20μm, respectively. Porosity levels of multilayer coatings were observed between 6 and 13%. Thermal conductivity values of the CYSZ/Al2O3 and CYSZ/Al2O3+YSZ coatings were in the range of 0.99 to 1.50W/mK. The bonding strength of as-sprayed coatings was increased from 5.4 to 10.1MPa for CYSZ/Al2O3, and 8.7 to 11.5MPa for CYSZ/Al2O3+YSZ coatings with the increasing number of layers. Existence of phase transformation (γ→α) for Al2O3 coating was observed after the thermal cycle test. The results indicated that the thermal conductivity and thermal cycle strength of CYSZ/Al2O3+YSZ TBCs were higher than CYSZ/Al2O3 based thermal barrier coatings.