Air Plasma-Sprayed Yttria and Yttria-Stabilized Zirconia Thermal Barrier Coatings Subjected to Calcium-Magnesium-Alumino-Silicate (CMAS)

Yttria (Y₂O₃) and zirconia (ZrO₂) stabilized by 8 and 20 wt.%Y₂O₃ thermal barrier coatings (TBCs) subjected to calcium-magnesium-alumino-silicate (CMAS) have been investigated. Free-standing Y₂O₃, 8 and 20 wt.%YSZ coatings covered with synthetic CMAS slurry were heated at 1300 °C in air for 24 h in order to assess the effect of Y₂O₃ on the corrosion resistance of the coatings subjected to CMAS. The microstructures and phase compositions of the coatings were characterized by SEM, EDS, XRD, RS, and TEM. TBCs with higher Y₂O₃ content exhibited better CMAS corrosion resistance. Phase transformation of ZrO₂ from tetragonal (t) to monoclinic (m) occurred during the interaction of 8YSZ TBCs and CMAS, due to the depletion of Y₂O₃ in the coating. Some amounts of original c-ZrO₂ still survived in 20YSZ TBCs along with a small amount of m-ZrO₂ that appeared after reaction with CMAS. Furthermore, Y₂O₃ coating was found to be particularly highly effective in resisting the penetration of molten CMAS glass at high temperature (1300 °C). This may be ascribed to the formation of sealing layers composed of Y-apatite phase [based on Ca₄Y₆ (SiO₄)₆O and Y_4.67(SiO₄)₃O] by the high-temperature chemical interactions of Y₂O₃ coating and CMAS glass.     

基于磁过滤技术的热障涂层表面Al2O3 覆盖层抗CMAS腐蚀性能

为了提高热障涂层（TBC）的抗沉积物（主要成分为CaO、MgO、Al₂O₃和SiO₂,简称CMAS）腐蚀性能,采用磁过滤阴极真空电弧（FCVA）技术在TBC表面上制备了致密的Al₂O₃覆盖层,比较和分析了Al₂O₃改性TBC和沉积态TBC的润湿行为和抗CMAS腐蚀性能。结果表明：使用FCVA技术制备Al₂O₃覆盖层的过程对7%（质量分数）氧化钇稳定的氧化锆（7YSZ）相的结构无明显影响,且经Al₂O₃改性的TBC综合性能均优于沉积态TBC。在1250 ℃、CMAS腐蚀条件下,Al₂O₃覆盖层有效地限制了熔融CMAS在TBC表面上的扩散行为。同时,Al₂O₃填充了7YSZ柱状晶之间的间隔并且阻碍了熔融CMAS的渗透,证明了FCVA可作为一种制备Al₂O₃涂层的新方法以提高TBC的抗CMAS腐蚀性能,且Al₂O₃涂层及其制备过程对TBC的热震性能均无消极影响。     

Hot corrosion behaviour of plasma sprayed alumina + YSZ particle composite coating

Hot corrosion is one of the damage mechanisms in thermal barrier coatings (TBCs) due to the molten salt effects as a result of combustion of low quality fuel. In this study, the hot corrosion behaviour of alumina–yttria stabilized zirconia particle composite coatings produced by thermal spraying for use as a thermal barriers on industrial gas turbines and in jet engines was evaluated. Plasma sprayed coatings with three different amounts of alumina- yttria stabilized zirconia particle composite have been exposed to 50 wt % Na₂SO₄ + 50 wt % V₂O₅ corrosive molten salt temperatures at 1050°C for 60 hours. Damages in the coatings surface and cross section after hot corrosion tests have been studied by using a scanning electron microscope to observe the microstructure and x-ray diffraction techniques to analyze the phase composition. The results have shown that the amount of YVO₄ crystals on the surface of YSZ coatings decrease while Al₂O₃ increases in YSZ + Al₂O₃ composition, therefore, the hot corrosion resistance of TBC improves with the addition of Al₂O₃.     

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.     

Effect of Microstructure on the Thermal Conductivity of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-YSZ Coatings

The microstructures of three atmospheric plasma-sprayed (APS) Al₂O₃-ZrO₂ coatings were investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The differences in the microstructures of the three Al₂O₃-ZrO₂ coatings, including their phase compositions, cracks, pores, grain sizes, and solid solutions, were analyzed in detail. A close relationship was observed between the thermal conductivities of the coatings and the microstructures, and the Al₂O₃-YSZ coatings with more spherical pores, fewer vertical cracks, and finer grains exhibited the lowest thermal conductivity of 0.91 W/m·K. Compared with YSZ coatings, Al₂O₃-YSZ coatings can exhibit lower thermal conductivity, which may be attributed to the formation of an amorphous phase, smaller grains, and Al₂O₃-YSZ solid solution.     

Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V2O5 and Na2SO4

Plasma‐sprayed 8YSZ (zirconia stabilized with 8 wt% yttria)/NiCoCrAlYTa thermal barrier coatings (TBCs) were laser‐glazed using a continuous‐wave CO₂ laser. Open pores within the coating surface were eliminated and an external densified layer was generated by laser‐glazing. The hot corrosion resistances of the plasma‐sprayed and laser‐glazed coatings were investigated. The two specimens were exposed for the same period of 100 h at 900 °C to a salt mixture of vanadium pentoxide (V₂O₅) and sodium sulfate (Na₂SO₄). Serious crack and spallation occurred in the as‐sprayed coating, while the as‐glazed coating exhibited good hot corrosion behavior and consequently achieved a prolonged lifetime. The results showed that the as‐sprayed 8YSZ coating achieved remarkably improved hot corrosion resistance by laser‐glazing. Changes in the coatings were studied by scanning electron microscopy (SEM) to observe the microstructure and X‐ray diffraction (XRD) technique to analyze the phase composition. XRD results showed that the reaction between yttria (Y₂O₃) and V₂O₅ produced yttrium vanadate (YVO₄), leaching Y₂O₃ from YSZ and causing the progressive destabilization transformation from the tetragonal (t) to monoclinic (m) phase. The external dense layer produced by laser‐glazing restrained the penetration of the molten salt, to a certain extent, into the coating, which led to a relatively low m‐ZrO₂ content in the coating after the hot corrosion test. Additionally, the segmented cracks in the coating surface induced by laser‐glazing were helpful to the improvement of strain tolerance of the coating. The two factors were important contributions to the significant enhancement of hot corrosion resistance of the as‐glazed YSZ coating.     

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.     

Yttria stabilized zirconia corrosion destabilization followed by Raman mapping

The incidence of V₂O₅ corrosion on yttria stabilized zirconia (YSZ) thermal barrier coatings has been studied as a function of application methods and powder initial granulometry. Commercial fused-and-crushed 8 wt.% yttria-stabilized zirconia was sprayed by Atmospheric Plasma Spraying (APS) and High Frequency Pulse Detonation. Hollow Spherical Powder (HOSP™) with the same composition was sprayed by APS. The coatings where covered with V₂O₅ powder and treated at 1000 °C for different times. The extent of corrosion was followed by X-ray diffraction, scanning electron microscopy and Raman micro-spectrometry. A relationship between coating porosity and corrosion resistance is explored: the HOSP coating presented deeper penetration of corrosion than the other coatings. The authors present the extended capabilities of Raman semi-quantitative phase analysis to describe the depth and density of yttria leaching by vanadia leading to YSZ destabilization.     

Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray

7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM.     

Phase Stability of Ce-Modified La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Coatings and Chemical Compatibility with YSZ

Ce-modified La₂Zr₂O₇ powders, i.e., La₂Zr₂O₇ (LZ), La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3), and La₂(Zr_0.3Ce_0.7)₂O₇ (LZ3C7), were used to produce thermal barrier coatings by atmospheric plasma spray process. The chemical compatibility of the CeO₂-doped La₂Zr₂O₇ with the traditional YSZ was investigated in LZ-YSZ powder mixtures and LZ-YSZ bilayer coatings by x-ray diffraction and scanning electron microscope. The powder mixtures and coatings were aged at 1200 and 1300 °C for 100 h. The results showed that LZ and LZ7C3 presented single pyrochlore structure after the heat treatments at both 1200 and 1300 °C. For LZ3C7, however, fluorite structure was observed at 1300 °C, indicating a poor phase stability of LZ3C7 at the elevated temperature. The results further showed that La₂(Zr_0.3Ce_0.7)₂O₇ reacted with YSZ in the bilayer ceramic coatings due to the diffusion of cerium, zirconium, and yttrium. While for La₂Zr₂O₇(LZ) and La₂(Zr_0.7Ce_0.3)₂O₇, a better chemical compatibility with YSZ was shown.     

Microstructure and Thermal Cycling Behavior of Air Plasma-Sprayed YSZ/LaMgAl11O19 Composite Coatings

Yttria-stabilized zirconia (YSZ) based composite coatings with the addition of LaMgAl₁₁O₁₉ (LaMA) as the secondary phase, were prepared by air plasma spraying in order to improve the performances of the traditional YSZ coating. Results indicate that the newly developed composite coating shows increased vertical crack density with the enhancement of the LaMA content during thermal cycling process, which results in increased strain tolerance and service lifetime. However, such composite coatings about 200 ??m thick, exhibit inferior thermal cycling lifetimes with respect to the typical YSZ coating for surface temperatures above 1400 °C. The presence of amorphous LaMA phase in the composite coating results in increased thermal conductivity and a relative thin top coat leading to a reduced thermal insulation efficiency. These are believed to be responsible for the premature degradation of bond coat and final top coat spallation failure. Such an investigation gives useful guidelines to develop advanced composite coatings based on YSZ/LaMA systems.     

CMAS对热障涂层界面裂纹和残余应力的影响

随着航空发动机涡轮叶片工作温度的提升,使得一种主要由CaO,MgO,Al₂O₃和SiO₂组成的玻璃态物质(CMAS)对热障涂层的危害越来越严重,从而对热障涂层的性能和耐久性有了更高的要求。本文以电子束物理气相沉积热障涂层为研究对象,利用有限元方法研究了CMAS的渗入对界面裂纹扩展及CMAS对陶瓷层(TC)内部残余应力的影响规律。采用波长固定、振幅变化的正弦曲线表示不同粗糙度的涂层界面,同时考虑了CMAS的弹性模量变化的影响及不同界面形貌与CMAS之间的相互作用。结果表明：CMAS弹性模量的增加对界面裂纹具有抑制作用,并且TGO幅值和厚度越小,抑制作用越明显。CMAS弹性模量对TC层最大残余应力S₂₂的影响存在临界点,在临界点之前,CMAS弹性模量的变化对TC层最大残余应力的影响较大,随着CMAS弹性模量的增加,TC层最大残余应力大幅度减小;在临界点之后,TC层最大残余应力基本不受CMAS弹性模量变化的影响。这些结果对电子束物理气相沉积喷涂的热障涂层失效机理的研究具有重要意义,可以为热障涂层界面的优化提供指导。     

地面重型燃气轮机及其热障涂层的研究进展与发展趋势

国际公认的重型燃气轮机制造尖端技术之一—热障涂层技术,高温下通常面临CMAS(CaO-MgO-Al₂O₃-SiO₂)腐蚀、氧化、相变与烧结等问题,其抗CMAS腐蚀性等关键性能极大地影响涂层寿命,提高热障涂层的性能刻不容缓。对重型燃气轮机用热障涂层的研究进展与发展趋势进行全面总结与分析。首先介绍国内外重型燃气轮机的现状及发展趋势、热障涂层的系统结构、材料和几种典型的制备工艺,然后针对高温下燃气轮机热障涂层遇到的一些问题,对其隔热性、抗氧化性及抗热震性等关键性能的研究进展进行综述,最后分类详述热障涂层的CMAS腐蚀机理及其防护研究进展。综述热障涂层的几种关键性能,提出热障涂层的性能与其材料、结构及制备工艺密切相关,据此总结归纳提高热障涂层性能的方法,为热障涂层性能的提高提供参考依据,以弥补燃气轮机热障涂层领域目前缺乏这类综述文章的不足。     

Microstructural Characterization and Cyclic Hot Corrosion Behaviour of Sputtered Co�CAl Nanostructured Coatings on Superalloy

Nanostructured Co?CAl coatings on Superni-718 superalloy substrate were deposited by DC/RF magnetron sputtering in the present work. The microstructure and cyclic hot-corrosion behavior of nanostructured Co?CAl coatings on Superni-718 superalloy were investigated in molten salt of 40 wt% Na₂SO₄ + 60 wt% V₂O₅ at 900 °C. The results showed that a dense scale formed on the coated samples exposed to corrosive environment during thermal cycling. The spinel phases of CoCr₂O₄, CoAl₂O₄ and NiCr₂O₄ were found in the corroded scale of the coatings, resulting in an effective inhibition of O and S diffusion. The sputtered Co?CAl coatings exhibited high hot corrosion resistance due to the formation of ??-CoAl phases in the coating. The relevant corrosion mechanisms substantiating the role of coatings are discussed.     

Al2O3/YSZ Composite Coatings Prepared by a Novel Sol–Gel Process and Their High-Temperature Oxidation Resistance

In this study, a novel sol–gel process has been utilized to fabricate Al₂O₃/YSZ (6 wt% yttria partially stabilized zirconia) composite coatings on Ni-based superalloy. The green coatings were obtained by electrophoretic deposition (EPD) in a suspension containing aluminium oxide sol, nano-Al₂O₃ and micro-YSZ particles, and then treated by so-called pressure filtration microwave sintering (PFMS) process. The as-sintered composite coatings were dense, uniform and crack-free and the phases mainly present α-Al₂O₃, m-ZrO₂ and t-ZrO₂ as aluminium oxide sol content decreasing. The cyclic oxidation tests at 1000 °C for 200 h demonstrate that both of high-temperature oxidation and spallation resistance for the coated samples were remarkably improved. These beneficial effects could be attributed to the special microstructure that micro-YSZ particles embedded in nano/submicron Al₂O₃ matrix. Meanwhile, the mechanisms of the inhibition of the oxygen diffusion and thermal match are further discussed.     

Comparison of corrosion resistance between Al2O3 and YSZ coatings against high temperature LiCl-Li2O molten salt

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li₂O molten salt under an oxidizing environment. Al₂O₃ and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al₂O₃ and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li₂O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al₂O₃ coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li₂O molten salt to form a porous reaction layer of LiAl, Li₅AlO₄ and LiAl₅O₈ for the Al₂O₃ coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.     

Effect of heat treatment on the thermal conductivity of plasma-sprayed thermal barrier coatings

The effect of heat treatment on the thermal conductivity of plasma-sprayed Y₂O₃ stabilized ZrO₂ (YSZ) and Al₂O₃ coatings was investigated. A heat treatment of 1300 °C in flowing argon for 50 h was found to significantly increase the thermal conductivity of the coatings when compared to measurements in the assprayed condition. Transmission electron microscopy (TEM) examination of the microstructures of the coatings in the as-sprayed and heat-treated conditions revealed that sintering of microcracks at the splat interfaces was the main cause for the increase in thermal conductivity. In the YSZ coatings, complete closure of microcracks was frequently observed. In contrast, microcrack closure in the Al₂O₃ coatings was characterized by the isolated necking of particles across a microcrack rather than complete closure. A model for thermal conductivity in a solid containing oriented penny-shaped cracks was used to explain the observed increase in thermal conductivity after heat treatment.     

Hot Corrosion Behavior of YSZ and CaZrO<Subscript>3</Subscript>/YSZ Composite Thermal Barrier Coatings in Contact with 50V<Subscript>2</Subscript>O<Subscript>5</Subscript> + 50Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> Salts

A novel thermal barrier coating system was formulated to resist hot corrosion environments. In this coating system, 5% CaZrO₃ was added to conventional yttria-stabilized zirconia (YSZ). The above composite coating system was compared with the standard YSZ system in the presence of a mixture of 50% Na₂SO₄ and 50% V₂O₅ at 950 °C. The results demonstrated that the lifetime of the CaZrO₃-added composite system in this highly hostile environment was longer compared with the standard YSZ system. This is due to the fact that the preferential reaction of NaVO₃ shifted from yttria to calcia, forming CaV₂O₄ instead of YVO₄. The preferential reactions are discussed in terms of free energy changes and acid–base theory of molten salts with ceramic oxides. Furthermore, calculations of lattice distortion also proved that the CaZrO₃-added composite system demonstrated less distortion, thus increasing the overall lifetime of the coating system.     

Microstructure and Thermal Characterization of Plasma-Sprayed Nanostructured La2Ce2O7-Doped YSZ Coatings

Nanostructured La₂Ce₂O₇-doped YSZ coatings were developed using atmospheric plasma-spraying technique by optimizing various process parameters. To ensure the retention of nanostructure, the molten state of nanoagglomerates was monitored using plasma and particle diagnostic tools. It was observed that the morphology of the coating exhibits a bimodal microstructure consisting of nanozones reinforced in a matrix of fully-molten particles. The thermal diffusivity of nano-LaCeYSZ coatings is lower than that of nano and bulk YSZ. The reason for this change in thermal diffusivity may be attributed to scattering of phonons at grain boundaries, point defect scattering and higher inter-splat porosity. Also, the thermal conductivity of the nanocomposite coatings was lower than those of nanostructured and bulk YSZ coatings. XRD results show cubic zirconia with a small amount of tetragonal zirconia. The average grain size of the as-sprayed La₂Ce₂O₇-YSZ nanocomposite coatings is ~150-200 nm. The improved thermal behavior is mainly due to a dense, packed, and more compact structure of the coatings.     

Electrophoretic deposition of 3YSZ coating on AZ91D using an aluminum interlayer

The zirconia stabilized by 3 mol % Y₂O₃ (3YSZ) was applied onto the surface of the magnesium alloy AZ91D using electrophoretic deposition (EPD) from a non- aqueous solvent. An interlayer of aluminum between the substrate and YSZ coating was also prepared by EPD. The preparation, microstructure and corrosion resistance of the coatings were investigated. The surface morphologies of the coatings were studied by scanning electron microscopy (SEM) and their compositions were determined by X-ray diffraction (XRD). The corrosion resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. The results indicate that the aluminum interlayer has a favorable effect on the densification of the coating by formation of aluminum oxide. In addition, the corrosion resistance of coated AZ91D alloy in chloride solution is significantly improved because of the aluminum interlayer and an increase in charge-transfer resistance of the AZ91D surface in chloride solution was observed which was attributed to YSZ.