Thermal conductivity of a zirconia thermal barrier coating

The conductivity of a thermal-barrier coating composed of atmospheric plasma sprayed 8 mass percent yttria partially stabilized zirconia has been measured. This coating was sprayed on a substrate of 410 stainless steel. An absolute, steady-state measurement method was used to measure thermal conductivity from 400 to 800 K. The thermal conductivity of the coating is 0.62 W/(m×K). This measurement has shown to be temperature independent.     

Thermal modeling of various thermal barrier coatings in a high heat flux rocket engine

One- and two-dimensional thermal models were developed to predict the thermal response of tubes with and without thermal barrier coatings (TBCs) tested for short durations in a H₂/O₂ rocket engine. Temperatures were predicted using median thermophysical property data for traditional air plasma sprayed ZrO₂–Y₂O₃ TBCs, as well as air plasma sprayed and low pressure plasma sprayed ZrO₂–Y₂O₃/NiCrAlY cermet coatings. Good agreement was observed between predicted and measured metal temperatures. It was also shown that the variation in the reported values of the thermal conductivity of plasma sprayed ZrO₂–Y₂O₃ coatings can result in temperature differences of up to 180°C at the ceramic/metal interface. In contrast, accounting for the presence of the bond coat or radiation from the ceramic layer had only a small effect on substrate temperatures (<20°C). The thermal models were also used to show that for the short duration test conditions of this study, a 100 μm thick ZrO₂–Y₂O₃ coating would provide a metal temperature benefit of approximately 300°C over an uncoated tube while a 200 μm thick coating would provide a benefit greater than 500°C. The difference in the thermal response between tubes and rods was also predicted and used to explain the previously-observed increased life of TBCs on rods over that on tubes.     

Anisotropic thermal conductivities of plasma-sprayed thermal barrier coatings in relation to the microstructure

Anisotropic thermal conductivities of the plasma-sprayed ceramic coating are explicitly expressed in terms of the microstructural parameters. The dominant features of the porous space are identified as strongly oblate (cracklike) pores that tend to be either parallel or normal to the substrate. The scatter in pore orientations is shown to have a pronounced effect on the effective conductivities. The established quantitative microstructure-property relations, if combined with the knowledge of the processing parameters-resulting microstructure connections, can be utilized for controlling the conductivities in the desired way.     

等离子喷涂和激光熔覆热障涂层隔热性能比较

采用多种方法制备不同类型的Al2O3-13％TiO2热障涂层,即等离子喷涂常规涂层、纳米结构涂层及激光熔覆纳米结构涂层．在分析三类涂层微观组织的基础上,对其隔热性能进行了比较．结果表明,即等离子喷涂常规陶瓷涂层呈典型的层状堆积特征,纳米结构涂层都为特殊的两相结构,其中部分熔化区由类似的残留纳米粒子组成,等离子喷涂纳米结构涂层的完全熔化区为片层状结构,而相应的激光熔覆涂层的完全熔化区则为细小等轴晶．在相同条件下,等离子喷涂纳米结构热障涂层具有最好的隔热性能,而激光熔覆纳米结构涂层的隔热性能要好于等离子喷涂常规涂层．     

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.     

Furnace cyclic oxidation behavior of multicomponent low conductivity thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in advanced gas turbine engines due to their ability to further increase engine operating temperatures and reduce cooling, thus helping achieve future engine low emission, high efficiency, and improved reliability goals. Advanced multicomponent zirconia (ZrO₂)-based TBCs are being developed using an oxide defect clustering design approach to achieve the required coating low thermal conductivity and high-temperature stability. Although the new composition coatings were not yet optimized for cyclic durability, an initial durability screening of the candidate coating materials was conducted using conventional furnace cyclic oxidation tests. In this paper, furnace cyclic oxidation behavior of plasma-sprayed ZrO₂-based defect cluster TBCs was investigated at 1163°C using 45 min hot-time cycles. The ceramic coating failure mechanisms were studied using scanning electron microscopy (SEM) combined with x-ray diffraction (XRD) phase analysis after the furnace tests. The coating cyclic lifetime is also discussed in relation to coating processing, phase structures, dopant concentration, and other thermo-physical properties.     

热障涂层高温TGO生长变化

通过Abaqus有限元分析软件对热障涂层在高温氧化过程中的热氧化物层(themally growth oxide,TGO)生长机制进行研究.结果表明,当高温氧化到100 h时,TGO厚度由初始的0.5μm生长至6.7μm且在不同位置TGO的厚度略微不同.随着高温时间的增加,热障涂层在TGO的波峰、波谷以及涂层边界处容易出现应力较大值,且和周围材料相比应力明显较大,此时,这些位置容易达到材料开裂临界应力,形成裂纹萌生点,使得涂层失效.在高温氧化过程中,涂层吸收总能量为43.6 J,其中少部分转化为涂层变形所消耗的能量,剩下的能量为高温氧化过程中涂层成分改变,微观组织改变以及裂纹萌生扩展提供能量.     

Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.     

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.     

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.     

三种热障涂层的组织性能

采用等离子喷涂技术制备了三种不同材料的热障涂层(TBC),对涂层进行了组织性能的分析比较.结果表明,Al-1075的TBC结合强度最高,为 24.66 MPa,具有良好的抗热震性能;KF-230的TBC结合强度最低,为 16.06 MPa;LG-210的TBC结合强度居中,抗热震性能最差.分析认为,氧化物层(TGO)在热障涂层中的失效起至关重要的作用,TGO是裂纹的产生源,是裂纹扩展的通道,是热障涂层系统中的最薄弱环节.因此抑制TGO是提高涂层结合强度、改善涂层抗热震性能的重要措施.     

An analytical model for simulation of heat flow in plasma-sprayed thermal barrier coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.     

Thermal stability and mechanical properties of thick thermal barrier coatings with vertical type cracks

The thermal stability and failure mechanism of thick thermal barrier coatings (TBCs) with and without vertical type cracks were investigated through the cyclic thermal exposure and thermal-shock tests. The TBC systems with thickness of about 2000 µm in the top coat were prepared by an air plasma spray (APS) on the bond coat of about 150 µm in thickness prepared by APS. The adhesive strength values of the as-prepared TBCs with and without vertical type cracks were determined to be 24.7 and 11.0 MPa, respectively, indicating the better interface stability in the TBC with vertical type cracks. The TBC with vertical type cracks shows a better thermal durability than that without vertical type cracks in the thermal cyclic exposure and thermal-shock tests. The hardness values of the as-prepared TBCs with and without vertical type cracks were found to be 6.6 and 5.3 GPa, respectively, which were increased to 9.5 and 5.5 GPa, respectively, after the cyclic thermal exposure tests. These results indicate that the vertical type cracks developed in the top coat are important in improving the lifetime performance of thick TBC in high temperature environment.     

纳米氧化锆热障涂层组织结构和高温稳定性能分析

采用大气等离子喷涂技术制备了纳米氧化锆热障涂层.利用FESEM和XRD对纳米氧化锆热障涂层的微观组织和物相组成进行研究.微观组织分析结果表明,纳米氧化锆热障涂层展现出独特的微观复合结构,包括未熔纳米颗粒和柱状晶组织.物相分析结果表明,纳米氧化锆热障涂层主要由非平衡四方相组成.纳米氧化锆热障涂层高温稳定性能试验结果表明,涂层晶粒度随着服役温度和服役时间的增加而增加,但仍保持纳米结构;涂层物相组成不随服役环境的变化而变化.     

The effect of a high thermal gradient on sintering and stiffening in the top coat of a thermal barrier coating system

Superalloy substrates coated with plasma-sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats (TCs) have been subjected to a high heat flux under a controlled atmosphere. The sintering exhibited by the TC under these conditions has been studied and compared with the behavior observed during isothermal heating. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both the in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical model has been used to explore the stress state under isothermal and thermal gradient conditions. Dilatometry data indicate significant linear contractions during holding at elevated temperatures, particularly in the through-thickness direction. This is largely attributed to microstructural changes associated with sintering, with any volume changes due to phase transformations making relatively small contributions. Sintering proceeds faster at higher temperatures but is retarded by the presence of tensile stresses (from differential thermal expansion between the coating and substrate) within the TC. Thus, it occurs preferentially near the free surface of the TC under gradient conditions, not only due to the higher temperature, but also because the in-plane stress is more compressive in that region.     

Processing of yttria partially stabilized zirconia thermal barrier coatings implementing a high-power laser diode

Several studies have been undertaken recently to adapt yttria partially stabilized zirconia (YPSZ) thermal barrier coating (TBC) characteristics during their manufacturing process. Thermal spraying implementing laser irradiation appears to be a possibility for modifying the coating morphology. This study aims to present the results of in situ (i.e., simultaneous treatment) and a posteriori (i.e., post-treatment) laser treatments implementing a high-power laser diode. In both cases, the coatings underwent atmospheric plasma spraying (APS). Laser irradiation was achieved using a 3 kW, average-power laser diode exhibiting an 848 nm wavelength. Experiments were performed to reach two goals. First, laser post-treatments aimed at building a map of the laser-processing parameter effects on the coating microstructure to estimate the laser-processing parameters, which seem to be suited to the change into in situ coating remelting. Second, in situ coating remelting aimed at quantifying the involved phenomena. In that case, the coating was treated layer by layer as it was manufactured. The input energy effect was studied by varying the scanning velocity (i.e., between 35 and 60 m/min), and consequently the irradiation time (i.e., between 1.8 and 3.1 ms, respectively). Experiments showed that coating thermal conductivity was lowered by more than 20% and that coating resistance to isothermal shocks was increased very significantly.     

Overview of thermal barrier coatings in diesel engines

An understanding of delamination mechanisms in thermal barrier coatings (TBCs) has been developed for diesel engine applications through rig tests, structural analysis modeling, nondestructive evaluation, and engine evaluation of various TBCs. This knowledge has resulted in improved TBCs that survive se-vere cyclic fatigue tests in high-output diesel engines. Although much conflicting literature now exists regarding the impact of TBCs on engine performance and fuel consumption, changes in fuel consumption appear to be less than a few percent and can be nega-tive for state-of-the-art diesel engines. The ability of the TBC to improve fuel economy depends on a num-ber of factors, including the fuel injection system, combustion chamber design, and initial engine fuel economy. Limited investigations on state-of-the-art diesel engines have indicated that surface- connected porosity and coating surface roughness may influence engine fuel economy. Current research efforts on TBCs are primarily directed at reduction of in-cylinder heat rejection, ther-mal fatigue protection of underlying metal surfaces, and possible reduction of diesel engine emissions. Significant efforts are still required to improve the plasma spray processing capability and the economics for complex-geometry diesel engine components.     

Influence of heat treatment on nanocrystalline zirconia powder and plasma-sprayed thermal barrier coatings

Nanostructured zirconia top coat was deposited by air plasma spray and NiCoCrAlTaY bond coat was deposited on Ni substrate by low pressure plasma spray.Nanostructured and conventional thermal barrier coatings were heat-treated at temperature varying from 1050 to 1 250oC for 2-20 h.The results show that obvious grain growth was found in both nanostructured and conventional thermal barrier coatings(TBCs)after high temperature heat treatment.Monoclinic/tetragonal phases were transformed into cubic phase in the agglomerated nano-powder after calcination.The cubic phase content increased with increasing calcination temperature.Calcination of the powder made the yttria distributed on the surface of the nanocrystalline particles dissolve in zirconia when grains grew.Different from the phase constituent of the as-sprayed conventional TBC which consisted of diffusionlesstransformed tetragonal,the as-sprayed nanostructured TBC consisted of cubic phase.     

Strain analysis of plasma sprayed thermal barrier coatings under mechanical stress

The stiffness of air plasma sprayed (APS) thermal barrier coatings (TBCs) was determined from bending experiments combining strain analysis on a microstructural level with macroscopic mechanical parameters. Tests were performed with freestanding and attached TBCs, the latter either loaded in tension or in compression. Relationships are derived, which describe the TBC stiffness in a multilayer composite (attached TBC) and for a bimodular material that possess a lower stiffness in tension than in compression (stand-alone TBC). The increase of in-plane stiffness with increasing compressive stress emphasizes the importance of the spraying defects for the elastic response of the coating.     

无冷却喷涂工艺对其制备的热障涂层的裂纹系统和寿命的影响

无冷却喷涂形成的热障涂层裂纹体系,可提高陶瓷顶层应变容限.但目前缺乏对裂纹体系的系统研究,特别是横向分叉裂纹.因此,文中研究送粉率和基体预热温度对陶瓷顶层裂纹系统的定量影响,并比较不同裂纹系统的热循环寿命.结果表明,增加送粉率,垂直裂纹密度和横向分叉裂纹长度均呈现先大后小的趋势.预热温度的提高可增加涂层中垂直裂纹数量,但横向分叉裂纹长度呈现先增后降的趋势.热循环试验表明,维持一定垂直裂纹的同时,降低横向分叉裂纹可提高涂层热循环寿命.