Understanding the Heat Transfer and Solidification of Plasma-Sprayed Yttria-Partially Stabilized Zirconia Coatings

A variety of microstructures have been observed in plasma-sprayed yttria-partially stabilized zirconia (YSZ) thermal barrier coatings. Control of the coating microstructures requires a good understanding of the heat transfer and solidification during the process. This article presents a quantitative analysis of heat transfer and solidification of plasma-sprayed YSZ splats. The analysis is based on a simple heat transfer and solidification model that solves a one-dimensional moving boundary problem with consideration of melt undercooling prior to solidification and nonequilibrium crystalline growth kinetics at the moving interface. The solidification morphology is first assumed to be planar, and the stability of the planar interface is examined against the absolute stability velocity calculated from the linear stability theory. Examining the temperature distribution in a solidifying YSZ zirconia splat indicates that a large positive temperature gradient exists in front of the interface, which leads to a stable planar interface and a segregation-free columnar structure, agreeing well with experimental observation. The model also finds that a low interface velocity results from poor heat transfer, which leads to a formation of cells and, therefore, the segregation of yttria. A steady-state dendrite tip growth model is then employed to calculate the radius of the cell tips and thus the cell spacings, which is then compared with experimental observations.     

LaMeAl11O19/YSZ热障涂层热力学性能和热循环寿命

LaMeAl₁₁O₁₉陶瓷具有独特的晶体结构, 优异的热力学性能, 低热导率, 高温相稳定性等特点, 是一类非常有应用前景的热障涂层(TBC)材料。本研究通过大气等离子喷涂(APS)制备了LaMeAl₁₁O₁₉/YSZ (Me=Mg, Cu, Zn)双陶瓷层热障涂层。通过对涂层进行火焰热循环测试并结合扫描电子显微镜、X射线衍射仪等分析技术对涂层进行失效分析。结果表明, LaMgAl₁₁O₁₉ (LMA)、LaZnAl₁₁O₁₉ (LZA)和LaCuAl₁₁O₁₉ (LCA)粉末在等离子喷涂过程中发生了分解, 导致三种涂层中磁铅石相含量的差异, 从而影响三种涂层的热循环寿命。由于LaMeAl₁₁O₁₉层与YSZ层的热膨胀系数不匹配以及非晶相重结晶产生的体积收缩, LaMeAl₁₁O₁₉层从YSZ层上剥落。YSZ层暴露在高温下, 加速了烧结和TGO的生长, 又促进了YSZ层剥落。低温下, LaMeAl₁₁O₁₉的热导率随着Me原子序数增加而降低; 高温下, 与LMA和LZA相比, LCA涂层红外发射率最高(0.88, 600 ℃), 削弱了光子传导对热导率的贡献, 导致热导率降低, LCA在高温红外辐射涂层中具有潜在的应用价值。     

Lanthanzirkonat: ein innovatives Wärmedämmschichtmaterial. Lanthanum zirconate: an innovative Thermal Barrier Material

This paper presents the work of the research project AiF‐13114 N/1. Within the scope of this project coating systems from Lanthanum Zirconate and Y ttria s tabilised Z irconia (YSZ) were developed by use of E lectron B eam P hysical V apour D eposition (EB‐PVD). In addition, the potentials of Lanthanum Zirconate and YSZ as thermal barrier coatings within gas turbines were examined. Basis of the coating development was the use of powdery Lanthanum and YSZ, that were vaporised in a PVD‐machine from a double‐grooved cupreous crucible. Process parameters are evaluated to gain long lasting, columnar Lanthanum Zirconate EB‐PVD coatings with high stability, low heat conductivity a higher sintering inertness, that offer the opportunity to increase the temperature within the first stage of a gas turbine system [1, 2, 3, 4, 5, 6]. Therefor YSZ ‐ commonly used as conventional thermal barrier coating ‐ was used as a reference system during the tests. As base material Inconel Alloy 600 (a nickel‐based superalloy) was applied. The microstructure and the topography of the developed coating systems were characterized with the help of scanning electron microscopy. Nanoindentation proved to be a measurement method in order to define the stiffness distribution along the columns. The thermal cycle durability was determined via thermal cycle test.     

Design and modeling of multiple layered TBC system with high reflectance

Ceramic thermal barrier coatings (TBCs) are playing an increasingly critical role in advanced gas turbine engines due to their ability to sustain further increases in operating temperatures. However, these increases in temperature could raise considerable issues associated with increased radiative heat transfer into the TBC systems. This study was conducted to design a ceramic based multiple layered TBC system with high reflectance to radiation. Mathematical modeling was used to calculate the potential temperature reduction on the substrate surface when the multiple layered TBC is applied. The result of the simulation shows that a temperature reduction up to 90 °C is possible when utilizing the designed multiple layered TBC coatings.     

Comparative analysis of steady state heat transfer in a TBC and functionally graded air cooled gas turbine blade

Internal cooling passages and thermal barrier coatings (TBCs) are presently used to control metal temperatures in gas turbine blades. Functionally graded materials (FGMs), which are typically mixtures of ceramic and metal, have been proposed for use in turbine blades because they possess smooth property gradients thereby rendering them more durable under thermal loads. In the present work, a functionally graded model of an air-cooled turbine blade with airfoil geometry conforming to the NACA0012 is developed which is then used in a finite element algorithm to obtain a non-linear steady state solution to the heat equation for the blade under convection and radiation boundary conditions. The effects of external gas temperature, coolant temperature, surface emissivity changes and different average ceramic/metal content of the blade on the temperature distributions are examined. Simulations are also carried out to compare cooling effectiveness of functionally graded blades with that of blades having TBC. The results highlight the effect of including radiation in the simulation and also indicate that external gas temperature influences the blade heat transfer more strongly. It is also seen that graded blades with about 70% ceramic content can deliver better cooling effectiveness than conventional blades with TBC.     

Finite Element Simulation on Thermal Fatigue of a Turbine Blade with Thermal Barrier Coatings

In this paper, a finite element model was developed for a turbine blade with thermal barrier coatings to investigate its failure behavior under cyclic thermal loading. Based on temperature and stress fields obtained from finite element simulations, dangerous regions in ceramic coating were determined in terms of the maximum principal stress criterion. The results show that damage preferentially occurs in the chamfer and rabbet of a turbine blade with thermal barrier coatings and its thermal fatigue life decreases with the increase of thermal stress induced by high service temperature.     

Measurement and Modeling of the Bidirectional Reflectance of SiO2 Coated Si Surfaces

An understanding of the variation of directional radiative properties of rough surfaces with dielectric coatings is important for temperature measurements and heat transfer analysis in many industrial processes. An experimental study has been conducted to investigate the effect of coating thickness on the bidirectional reflectance distribution function (BRDF) of rough silicon surfaces.Silicon dioxide films with thicknesses of 107.2, 216.5, and 324.6 nm were deposited using plasma-enhanced chemical vapor deposition onto the rough side of two Si wafers. The wafer surfaces exhibit distinct anisotropic characteristics as a result of chemical etching during the manufacturing process. A laser scatterometer measures the BRDF at a wavelength of 635 nm, after improvement of the signal-to-noise ratio. The slope distribution function obtained from the measured BRDF of uncoated Si surfaces was used in an analytical model based on geometric optics for rough surface scattering and thin-film optics for microfacet reflectance. The predicted BRDFs are in reasonable agreement with experimental results for a large range of coating thicknesses. The limitations of the geometric optics for modeling the BRDF of coated anisotropic rough surfaces in the specular direction are demonstrated. The results may benefit future radiative transfer analysis involving complicated surface microstructures with thin-film coatings.     

热处理对热障涂层孔隙率及热导率的影响

热障涂层高温使用过程中不可避免发生烧结,引起涂层失效。为探索烧结过程对涂层可靠性的影响,采用等离子喷涂制备氧化钇稳定氧化锆(YSZ)涂层,并对其进行1 000,1 100,1 200,1 300℃高温烧结试验,研究其高温烧结过程中的微观结构及热导率演变规律。结果表明:高温热处理引起热障涂层组织结构和热导率均发生变化,稳定状态的孔隙率为12%~14%,在热障涂层服役温度范围内热导率增加到1.25~1.45 W/(m·K)。     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

An experimental technique to evaluate the effective thermal conductivity of Y2O3 stabilized ZrO2 coatings

An experimental device was set up to determine thermal resistance and conductivity of 8% yttria-stabilized zirconia deposited by plasma spray method on cylindrical specimen. In this experimental setup, coated surface of the sample was exposed to a high temperature environment and inner metal surface was cooled by flowing air, simulating actual gas turbine applications. Overall heat resistance at the outside surface of thermal barrier coating was adopted to assess thermal advantage due to the thermal barrier coating deposited on air-cooled cylindrical specimen. 28% less heat was extracted at 1000°C by applying 1.2 mm thick thermal barrier coating. Temperatures of the outside surface of the coated samples increased with increasing coating thickness with respect to the same furnace temperature since the sample with thicker coating was less thermally conductive and retarded heat transfer. The overall heat resistances of samples between the outside surface of sample and the flowing air inside the sample assembly were estimated. Then, the thermal conductivity of coating could be determined from the difference of overall thermal resistances of two selected samples with varying coating thickness.     

ITO coated quartz fibers for heat radiative applications

Radiation heat transfer through fibrous materials is very strong at high temperatures (up to 1000 °C). Indium tin oxide (ITO) thin films were sol-gel deposited onto the surfaces of fibers to reduce the radiation heat transfer as radiation reflective coatings. SEM, XRD and FT-IR techniques were used to characterize the microstructure and performance of films. Results show that ITO thin film is uniformly deposited on fibers with a thickness of about 200 nm and can be used to apply a radiative reflective coating. Moreover, the efficiency of radiation reflective properties of films is improved as the annealing temperature increases. Results prove that ITO film is an excellent candidate to reduce the radiation heat transfer as radiation reflective coatings on fibrous materials.     

Tailored microstructure of zirconia and hafnia-based thermal barrier coatings with low thermal conductivity and high hemispherical reflectance by EB-PVD

Zirconia and hafnia based thermal barrier coating materials were produced by industrial prototype electron beam-physical vapor deposition (EB-PVD). Columnar microstructure of the thermal barrier coatings were modified with controlled microporosity and diffuse sub-interfaces resulting in lower thermal conductivity (20–30% depending up on microporosity volume fraction), higher thermal reflectance (15–20%) and more strain tolerance as compared with standard thermal barrier coatings (TBC). The novel processed coating systems were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction, thermal conductivity by laser technique, and hemispherical reflectance.     

等离子喷涂-物理气相沉积制备7YSZ热障涂层及其热导率研究

通过等离子喷涂-物理气相沉积(PS-PVD)技术在3种不同工艺参数下制备7YSZ热障涂层。采用XRD和SEM分析涂层的相结构和微观组织,利用激光脉冲法测量涂层不同温度下的热导率。结果表明:通过调整工艺参数中电流的大小和等离子气体成分,可以制备截面呈柱状、致密层状和柱-颗粒状混合组织结构,表面呈"菜花"状或起伏的多峰状的YSZ热障涂层。涂层的相结构由粉末的单斜相氧化锆(m-ZrO_2)转变为涂层中的四方相氧化锆(t-ZrO_2),并保留至室温。在700~1100℃时,YSZ涂层的热导率随着温度的升高而增大。柱状晶结构涂层因具有较大的孔隙率,可以有效降低涂层的热导率,其热导率为1.0~1.2W·m~(-1)·K~(-1);而层状结构涂层由于比较致密,其热导率相对较高。     

ITO的热辐射性质与等离子波长关系的实验研究

热镜膜具有广泛的应用背景;特别是将其沉积在柔性的黑色底衬上,就能获得不同于通常的、必须用发射率很低的金属做衬底的太阳能选择性吸收表面．采用衬底不加温的普通ＲＦ溅射技术制备单层ＩＴＯ热镜膜,并对它们的热辐射性质与等离子波长之间的关系进行了实验研究．研究表明,对于单层ＩＴＯ热镜膜,同时具有最小的发射率和最小的对太阳辐射的反射率是困难的．但是,等离子波长可以作为预测太阳能热利用领域使用的单层ＩＴＯ热镜膜热辐射性质的一个判据．     

Microstructure and indentation mechanical properties of plasma sprayed nano-bimodal and conventional ZrO2–8wt%Y2O3 thermal barrier coatings

The nanostructured agglomerated feedstock used for plasma spraying was obtained by the nanoparticle reconstituting technique. Nanostructured and conventional ZrO₂–8wt%Y₂O₃ (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on 45# steel substrates with the NiCrAlY as the bond-layer. The microstructure and phase composition of feedstocks and corresponding coatings were characterized. The top layer of nanostructured 8YSZ TBCs is denser and has fewer defects than that of conventional TBCs. The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ TBCs exhibit bimodal distribution while the conventional 8YSZ exhibit mono-modal distribution. The elastic modulus and elastic recoverability were also obtained by the nanoindentation test. The results indicate that the elastic modulus of nanostructured 8YSZ coating is lower than that of conventional 8YSZ coating, but the nanostructured 8YSZ coating has higher elastic recoverability than that of the conventional 8YSZ coating. The prediction of the average elastic modulus was established by the mixture law and weibull distribution according to the fraction of phases with different molten characteristic.     

Nano /spl alpha/-Fe/epoxy resin composite absorber coatings fabricated by thermal spraying technique

In this paper, the low temperature high velocity air fuel (LTHVAF) spraying technique was applied to prepare the /spl alpha/-Fe/epoxy resin nanocomposite coatings. The composite powders were mixed with different mass fractions, and the microstructure and reflectivity coefficient of coatings were tested. The results show that the microstructure of coatings is dense and low porosity; nano metal particles are dispersed in the coatings. The coatings are closely combined with substrate. In these coatings, the volume fraction calculated with density, component distribution, properties of metal particle, and coating thickness can affect the microwave absorption ability of the coatings. The reflectance coefficient of 70 mass% nano /spl alpha/-Fe/epoxy resin composite coatings is lower than others. In these nanocompsite absorber coatings, the relationship of the reflectivity coefficient and the coating structure were constructed with permittivity, permeability, and thickness. The optimal mass fraction of absorber coatings is about 0.3. It is analyzed with self-bonding strength and reflectivity coefficient. This means that the change of the coating structure affects the performance of the nanocomposite coatings.     

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

Increase 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.     

A sputtered zirconia primer for improved thermal shock resistance of plasma-sprayed ceramic turbine seals

Considerable success has been achieved in the development of plasma-sprayed Y₂O₃-stabilized ZrO₂ (YSZ) ceramic turbine blade tip seal components. The YSZ layers are quite thick (0.040–0.090 in) in comparison with typical thermal barrier coating layers applied to airfoils, which are more effectively cooled than the seal. The service potential of seal components with such thick ceramic layers are cyclic thermal shock limited. The most usual failure mode is ceramic layer delamination at or very near the interface between the plasma-sprayed YSZ layer and the Ni-Cr-Al- Y bondcoat. Deposition of a thin r.f.-sputtered YSZ primer to the bondcoat before deposition of the thick plasma-sprayed YSZ layer has been found to reduce laminar cracking in cyclic thermal shock testing. The cyclic thermal shock life of one ceramic seal design was increased by a factor of 5–6 when the sputtered YSZ primer was incorporated. A model based on thermal response of plasma-sprayed YSZ particles impinging on the bondcoat surface with and without the sputtered YSZ primer provides a basis for understanding the function of the primer.     

Effect of the composition profile and density of LPPS sprayed functionally graded coating on the thermal shock resistance

Low Pressure Plasma Spraying (LPPS) is a promising coating method for Functionally Graded Material (FGM) expected to beable to reduce the thermal stress in high temperature environments such as a gas turbine. In this paper, we report the effect of the composition profile and coating density of LPPS sprayed FGM, consisting of ZrO₂–8 wt%Y₂O₃ (YSZ) top coating, YSZ–Ni–20 wt%Cr (NiCr) FGM coating, NiCr under coating and copper substrate, on the thermal shock resistance evaluated by a modified temperature difference test. The density of YSZ and NiCr coating was successfully controlled by the chamber pressure and initial particle size in the range from 5.43 to 5.79 g/cm³ and from 7.89 to 8.09 g/cm³, respectively. For an YSZ composition profile from NiCr under coating to YSZ top coating (in FGM), the highest thermal shock resistance was obtained when the fraction of YSZ increased with gentleslope just over NiCr coating and acute slope just under YSZ coating. Also, the higher density coatings tended to perform the higher thermal shock resistance. Initial cracks formed in the YSZ top coating propagated into YSZ parts in FGM coating through the grain boundary of YSZ and/or the interface between flattened NiCr and YSZ particles. After the cracks connected, the coupled cracks caused the coating spallation.

©2003 Elsevier Science Ltd. All rights reserved.     

Investigation on Failure in Thermal Barrier Coatings on Gas Turbine First-Stage Rotor Blade

Failure in turbine blades can affect the safety and performance of the gas turbine engine. Results of coating decohesion, erosion and cracking at the first-stage high-pressure (HPT) blade working in gas turbine engine are being reported in this paper. This investigation was carried out for the possibility of various failure mechanisms in the thermal barrier coating exposed to high operating temperature. The blade was made of nickel-based superalloy, having directionally solidified grain structure coated with thermal barrier coatings of yttria-stabilized zirconia with EB-PVD process and platinum-modified aluminum (Pt–Al) bond coat with electro-deposition. The starting point of analysis was apparent coating decohesion close to the leading edge on the suction side of blade. The coating decohesion was found to be widening of interdiffusion zone toward the bond coat at higher operating temperature which could change the composition and induce thermal stresses in the bond coat. The erosion, cracking and decohesion of the coating on the pressure side was also observed during failure investigation. The erosion of the coating was coupled by two factors: one by increase in temperature as demonstrated by change in microstructure of the substrate and second by increase in coating inclination toward the trailing side. As a result of high operating temperature, swelling and thickening of TGO was observed due to outward diffusion of aluminum from the bond coat to form alumina (non-protective oxide) which causes internal stresses that leads to top coat decohesion and cracking. The possibility of hot corrosion was also investigated, and it was found that top coat decohesion did not involve this failure mechanism. Visual inspection, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy have been used as characterization tools.