Neodymium-cerium oxide as new thermal barrier coating material

Neodymium-cerium oxide (Nd₂Ce₂O₇) was proposed as a new thermal barrier coating material in this work. Monolithic Nd₂Ce₂O₇ powder was prepared by the solid-state reaction at 1400 °C. The phase composition, thermal stability and thermophysical properties of Nd₂Ce₂O₇ were investigated. Nd₂Ce₂O₇ with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd₂Ce₂O₇ was higher than that of YSZ (6-8 wt.% Y₂O₃ + ZrO₂) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd₂Ce₂O₇ is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd₂Ce₂O₇ was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.     

Thermal-shock resistance of LnMgAl11O19 (Ln = La, Nd, Sm, Gd) with magnetoplumbite structure

Lanthanide hexaaluminates including LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉ and GdMgAl₁₁O₁₉ were synthesized via Sol–Gel method. Due to the anisotropic crystal growth, these oxides crystallize in the form of platelets and the platelet thickness increases with the decrease of rare-earth ionic radius. It was observed that the thermal-shock resistances of LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉ and SmMgAl₁₁O₁₉ oxides were superior to 8YSZ as proved by water quenching tests. In addition, the thinner the platelet, the more interstices are retained in the sintered specimen, and the better thermal-shock resistance the oxide has. Based on SEM images, it can be seen that the SmMgAl₁₁O₁₉ sample exhibits a mixture of the intergranular and transgranular fracture after thermal cycling failure.     

Thermal properties of oxides with magnetoplumbite structure for advanced thermal barrier coatings

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl₁₁O₁₉, GdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉, and Gd_0.7Yb_0.3MgAl₁₁O₁₉ magnetoplumbite oxides were synthesized by citric acid sol-gel method and hot-pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 °C. The average CTE value was found to be ∼ 9.6 × 10^− 6/C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd_0.7Yb_0.3MgAl₁₁O₁₉ composition was found to be lower than that of the undoped GdMgAl₁₁O₁₉. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Preliminary results of thermal conductivity testing at 1600 °C for LaMgAl₁₁O₁₉ and LaMnAl₁₁O₁₉ magnetoplumbite oxide coatings plasma-sprayed on NiCrAlY/Rene N5 superalloy substrates are also presented. The plasma-sprayed coatings did not sinter even at temperatures as high as 1600 °C.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Preparation and thermal conductivities of Gd2Ce2O7 and (Gd0.9Ca0.1)2Ce2O6.9 ceramics for thermal barrier coatings

Two kinds of rare earth cerium oxides Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 were prepared by solid state reaction method at 1600 °C for 10 h. The phase compositions, microstructures, and their thermal conductivities of these materials were investigated. XRD results revealed that single phase Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 with fluorite structure were synthesized. Results of SEM and EDS showed that the microstructures of these materials were dense and no other phases existed among grains. Because of phonon scattering resulted by the oxygen vacancies and difference in atomic mass between substitutional atoms and host atoms, thermal conductivities of Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 are lower than that of 8YSZ at 800 °C, and thermal conductivity of (Gd_0.9Ca_0.1)₂Ce₂O_6.9 is lower than that of Gd₂Ce₂O₇. These results imply the Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 can be used as novel candidate materials for thermal barrier coatings in the future.     

Finite element simulation of thermal barrier coating performance under thermal cycling

In this paper, a numerical simulation of stress development within the air plasma-sprayed thermal barrier coating system incorporating nonlinear behavior under compression for the top coat ceramic layer is presented. The nonlinear behavior as well as its evolution with sintering at high temperature is simulated using a microstructure based model. The simulation results indicate that this nonlinearity has a significant role on distribution of the residual stresses in this layer resulting from the thermal cycling. A parametric study is carried out to investigate the effects of the microstructural features of the top coat ceramic layer on residual stress distribution. It is revealed from the simulation results that the variation of porosity has only a negligible effect on the residual stress distribution. In addition, the stresses accountable for the crack growth can be lowered by changing the microcrack densities of the top coat layer within a specified range.     

The hot corrosion resistance of 20 mol% YTaO4 stabilized tetragonal zirconia and 14 mol% Ta2O5 stabilized orthorhombic zirconia for thermal barrier coating applications

Zirconia stabilized with 3.2–4.2 mol% (6–8 wt.%) yttria (3–4YSZ), the current material of choice for thermal barrier coating applications, is susceptible to hot corrosion by acidic oxides such as vanadia in the 700–900 °C range. The current study is a preliminary examination of the hot corrosion resistance to NaVO₃–V₂O₅ mixtures in the above temperature range of two alternative materials: a tetragonal zirconia co-doped with 10 mol% yttria+10 mol% tantala (20YTaO₄Z) and an orthorhombic zirconia doped with 14 mol% tantala (14TZ). Results show that the 20YTaO₄SZ is resistant to destabilization by NaVO_3, but is attacked at higher V₂O₅ activities, resulting in the formation of YVO₄ and orthorhombic zirconia. Studies on the 14TZ itself then indicated that it is substantially more resistant than the YSZ to attack by environments more acidic (specifically V₂O₅ rich) than pure NaVO₃. However, it is less suitable than either 20YTaO₄SZ or 3–4YSZ for environments that are more basic. A comparison of the resistance of the 14TZ, the 3–4YSZ and the 20YTaO₄Z shows that the 20YTaO₄SZ is more resistant to acidic oxides than the YSZ and more resistant to the basic oxides than the 14TZ.     

Thermal fracture of zirconia-mullite composite thermal barrier coatings under thermal shock: An experimental study

Thermal barrier coatings (TBCs) are used in applications that involve high temperatures and severe temperature gradients in order to improve product performance. The understanding of the mechanisms resulting in coating delamination allows the development of materials that can prolong component life. The goal of this study was to demonstrate that single layer mullite-YSZ composites resulted in reduced interface fracture under the application of a thermal shock. This was accomplished by comparing the thermal shock behavior of three coating architectures: monolithic YSZ, monolithic mullite and a mullite-YSZ composite. The coating architectures were chosen to optimize material properties to reduce the driving force for coating failure. It was found that under thermal loads that result in similar surface temperatures, the mullite-YSZ composite developed shorter multiple surface cracks along with shorter horizontal cracks compared to the monolithic YSZ. The composite coating was able to combine advantageous material properties from both the constituent ceramics.     

Thermal cycling resistance of modified thick thermal barrier coatings

The thermal cycling properties of several modified thick thermal barrier coatings (TTBC) were studied in three test series in which the maximum coating temperature was fixed to 1000, 1150 and 1300 °C. The modified coating structures were all segmentation-cracked coatings and some of these coatings were surface-sealed. The segmentation-cracked coatings were produced by laser glazing or by using appropriate plasma spray parameters. The sealing treatments were made by using aluminium phosphate or sol–gel-based sealant. In this paper, it was demonstrated that regardless of whether the segmentation-cracked TTBCs were made by using specific plasma spray parameters or by laser glazing, the strain tolerance of the coating improved significantly. Instead, both sealing treatments reduced the thermal cycling resistance of the TTBCs to some degree, especially in the case of aluminium phosphate sealing. Coating microstructures, their mechanical and elastic properties and residual stresses were taken into consideration when estimating the thermal cycling properties and failure modes of the coatings.     

热喷涂纳米结构La_2Zr_2O_7(LZ)/8YSZ双陶瓷热障涂层

为了满足发动机以及涡轮机越来越高的性能要求,双陶瓷热障涂层逐渐取代单陶瓷层的8YSZ涂层,成为可以长期使用温度高于1 200℃的新型陶瓷涂层。采用等离子喷涂方法制备了La2Zr2O7(LZ)/8YSZ双陶瓷热障涂层,并同时制备了微米结构和纳米结构的单层8YSZ陶瓷涂层作为对比。通过X射线衍射仪和扫描电子显微镜研究了粉体喂料和涂层的组织结构。采用对偶拉伸试验法、水淬方法和日本工业标准等研究了涂层的结合强度、隔热效果、热震行为以及高温抗氧化行为。结果表明,与单层的8YSZ陶瓷层相比,双陶瓷型n-LZ/8YSZ涂层的隔热效果提高了35%,热震次数增加了一倍,热氧化失效时间延长了100多小时,具有较佳的隔热效果、抗热震性能以及抗高温氧化性能。     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

热障涂层的CMAS腐蚀失效及对策研究综述

研制具有长寿命、高性能的热障涂层（TBCs）,是制造我国大功率航空发动机、发展新一代超音速战机的一项十分紧迫的任务。由于外来沉积物CaO-MgO-Al2O3-SiO2（CMAS）渗入涂层而导致TBCs失效的现象越来越受到人们的重视,在过去的一段时间里,很多学者对CMAS渗入后涂层的失效机理进行了大量的研究并尝试了一些缓解CMAS渗入涂层的方法。本文针对当前应用最为广泛的“层状多孔结构”的等离子喷涂涂层及“细长柱晶结构”的电子束辅助物理气相沉积涂层,系统地梳理了近年来国内外学者对TBCs在CMAS渗入条件下的失效机制及涂层缓解CMAS渗入方面的最新研究成果,为制备高性能的TBCs提供帮助。     

Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.     

The development of new bond coat compositions for thermal barrier coating systems operating under industrial gas turbine conditions

Environmental and economic issues are driving the development of increasingly efficient gas turbines. An important step in achieving this is to engineer components which can operate with longer lifetimes and at higher metal temperatures. Inlet temperatures for gas turbines now exceed the melting temperatures of nickel-based superalloys (i.e. 1300–1350 °C). The use of advanced air cooling systems coupled with thermal barrier coatings (TBCs) reduces the temperature of the underlying superalloy substrate. The bond coating, an important part of the TBC system, oxidizes to form a slow growing protective oxide layer, while also providing adhesion between the ceramic topcoat and the substrate. NiCoCrAlY overlay coatings are some of the most commonly used bond coatings for industrial gas turbines and extensive research has been undertaken over many years to find the best bond coating composition.This paper reports upon the production of new, model bond coatings with a wide range of different compositions. The focus is on their oxidation behavior at a temperature typically experienced by bond coatings on industrial turbine blades (950 °C). A physical vapor deposition technique, magnetron sputtering, has been used to deposit a range of Ni–Co–Cr–Al coatings onto 10 mm diameter sapphire substrates. This was achieved through co-sputtering two targets: a Ni–10%Cr, Ni–20%Cr, Ni–50%Cr, Ni–20%Co–40%Cr or Ni–40%Co–20%Cr target and a pure Al target. About a hundred samples with varying compositions were produced by this method. The coatings were then oxidized in air for 500 h at 950 °C.All samples were assessed by measuring the change in coating thickness, using pre- and post-exposure metrology only, and also the change in specimen weight. This approach has shown that magnetron sputtering successfully deposited 20 to 30 μm thick coatings and allowed the calculation of oxide growth rates. Energy dispersive X-ray (EDX) analysis was used to characterize the exact composition of each sample. Additionally, X-ray diffraction (XRD) has been used to identify the major oxides formed during exposure. The selective growth of protective Cr₂O₃ or Al₂O₃ or other less protective mixed oxides (depending on the initial coating composition) was observed. This influenced the oxide scale growth rate, indicating which coatings produced more protective oxides and allowing future optimization of the bond coating composition, for service within the turbine section of industrial gas turbines to be planned.     

Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior

Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400 μm-500 μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9 cracks mm^− 1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350 °C) and bondcoat temperatures (up to 1085 °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.     

等离子喷涂梯度热障涂层的抗热震性能

对比研究了等离子喷涂梯度热障涂层与双层热障涂层,试验中梯度热障涂层选用不同比例的NiCoCrAlY与ZrO₂-8%Y₂O₃复合粉末作为梯度过渡层材料,并对两种结构的热障涂层进行了抗热震性能试验。抗热震试验结果表明,梯度热障涂层的抗热震寿命明显高于双层热障涂层的抗热震寿命。     

Evaluation of LaAlO3 as top coat material for thermal barrier coatings

Perovskite is a versatile group of oxide materials allowing their properties to be tailored by composition towards specific requirements. LaAlO₃ was prepared to study and report its properties in the context of its potential in thermal barrier coatings (TBCs) technology. A citric acid method was used for synthesis and the perovskite structure was confirmed using XRD and FT-IR. Viscosity of the solution precursor was checked as well as the particle size by laser particle size analysis. Densification behavior of the material was followed by conventional sintering and by spark plasma sintering. Apparent porosity by the Archimedes method, thermal conductivity and thermal expansion coefficient were studied. Mechanical and fracture properties were measured at elevated temperatures up to 1300 °C. For samples sintered at 1200?1400 °C, coefficient of thermal expansion ranged from 5.5×10^?6 to 6.5×10^?6 K^?1 and thermal conductivity ranged between 2.2 and 3.4 W/(m·K). Elastic modulus and ultimate stress were measured at 1000?1300 °C, while by micro-indentation, fracture toughness was found to be 3 MPa·m^1/2. As the sintering temperature increased from 1200 to 1500 °C, significant densification from 3.21 to 5.81 g/cm³ was found, indicating that material annealing should be made at least at 1400 °C. Under this condition, negligible dimensional change in phase transition temperature of LaAlO₃ from the rhombohedral (R3c) to the ideal cubic (Pm3m) is found. Data reported in this work can be useful for comparing the mechanical and fracture behaviours of different TBCs developed involving LaAlO₃ as well as input for numerical simulations.     

Correlation between spraying conditions and microcrack density and their influence on thermal cycling life of thermal barrier coatings

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific morphology of porosity, such as microcracks and typically larger pores, on the performance of the coatings. Both features are usually formed during plasma spraying of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation, the influence of microcracks on the thermal cycling behavior was studied. The amount of microcracks within YSZ thermal barrier coatings was changed by changing the powder-feeding rate. Only small changes of the total porosity were observed. Mercury porosimetry served as a tool to investigate both the amount of microcracks and pores in the coating. Additionally, microcrack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling TBC specimens with different microcrack densities were produced and tested in a gas burner test facility. At high surface temperatures (above 1300 °C), failure occurred in the ceramic close to the surface. Under these conditions, the samples with increased horizontal microcrack densities showed a significant increase of thermal cycling life.     

Microstructural evolution of 7 wt.% Y2O3-ZrO2 thermal barrier coatings due to stress relaxation at elevated temperatures and the concomitant changes in thermal conductivity

The purpose of this study was to evaluate the combined effect of stress and temperature on the microstructure of air plasma-sprayed 7 wt.% Y₂O₃-ZrO₂ thermal barrier coatings, and relate microstructural changes to the thermal conductivity, k_th. To simulate TBC service conditions, stand-alone tubes of YSZ were stress relaxed, starting from a compressive stress of 60 MPa, at temperatures of 1000 °C or 1200 °C. The duration of the stress relaxation test was either 5 min or 3 h. Detailed scanning electron microscopy (SEM) and Porod's specific surface area (SSA) analysis of small angle neutron scattering (SANS) results were used to determine which void systems, either interlamellar pores or intralamellar cracks, contributed to the observed relaxation of stress in the coatings. SEM investigations revealed closure of intralamellar cracks located perpendicular to the stress direction. For thinner YSZ coatings, SANS measurements indicated a statistically significant reduction in the total SSA and SSA associated with intralamellar cracks after stress relaxation at the times, temperatures, and stress investigated compared to those samples that were exposed to identical times and temperatures, but no stress. The SSA associated with the interlamellar pores was not significantly smaller in YSZ coatings stress relaxed from 60 MPa at 1200 °C for 3 h compared to as-sprayed coatings. The thermal conductivity of the coatings was strongly influenced by stress, with increases in k_th observed after only 5 min at 60 MPa and 1200 °C. Reductions in the total SSA were directly linked to increases in k_th.