Long-Term Behavior and Application Limits of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO₂-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO₂, 19.5 mol% CeO₂/1.5 mol% Y₂O₃, 35 mol% CeO₂, and 4.5 mol% Y₂O₃ after long-term heat treatments at typical operation temperatures (1000°–1400°C) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO₂ contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1 h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ or 35 mol% CeO₂ reveal a smaller amount of monoclinic phase after long-term heat treatments (1250°C/1000 h) compared with TBCs containing 4.5 mol% Y₂O₃. TBCs containing 35 mol% CeO₂ show a higher degree of sintering than the TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y₂O₃ exhibit the highest resistance to failure in thermal-cycling experiments.     

Creep Behavior of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often demonstrated improved properties relative to coatings produced from conventional powders. This type of coating has been reported to exhibit better wear resistance and higher adhesion strength compared with conventional deposits. Powder consisting of hollow spherical particles has been reported to produce coating with lower unmelted particles and lower thermal conductivity. In this study, the thermo-mechanical properties of plasma-sprayed yttria-stabilized zirconia coatings deposited using each of these types of powder were investigated. Creep strain and creep rate were measured using free-standing thick coatings loaded in a four-point bend configuration at temperatures ranging from 800° to 1200°C in air under a range of loads. The creep exponent and activation energy were determined.     

Microstructure–Thermal Conductivity Relationships for Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The microstructures of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings are complex, contributing to challenges in establishing microstructure–thermal conductivity relationships. Furthermore, the dynamic evolution of microstructure and properties during service offers a significant challenge in defining design strategies and extended coating performance. In this paper, the relationship between microstructure and thermal conductivity is investigated for three sets of plasma-sprayed YSZ coating systems prepared using different morphology powders, different particle size distributions, and controlled modification of particle states through plasma torch parameters. Both ambient and temperature-dependent thermal conductivity were conducted in the as-sprayed and thermally aged states. The results suggest that a range of thermal conductivities can be achieved from the coatings, offering potential for microstructural tailoring for desired performance. The results also demonstrate that different as-deposited microstructures display varying propensity for sintering and these attributes need to be considered in the design and manufacturing cycle. This expansive study of a range of coatings has also allowed synthesis of the results through thermal conductivity–porosity maps and has allowed elucidation of the contributing microstructural components for both the ambient and high-temperature thermal conductivity. Considering that the operating thermal transport mechanisms are different at these two temperature extremes, such mapping strategies are of value to both science and technology.     

Effects of Yttria-Stabilized Zirconia on Plasma-Sprayed Hydroxyapatite/Yttria-Stabilized Zirconia Composite Coatings

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma-sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria-stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10–50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma-sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.     

Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y₂O₃–ZrO₂ thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.     

Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directional-hemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 μm. Spectra were collected over a wide range of coating thickness from 60 to almost 900 μm. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surface-temperature pyrometer measurements and on (2) applying mid-infrared reflectance to monitor TBC delamination are discussed.     

Process-Controlled Plasma-Sprayed Yttria-Stabilized Zirconia Coatings: New Insights from Ultrasmall-Angle X-ray Scattering

A multicomponent microstructure model is applied in ultrasmall-angle X-ray scattering studies of two groups of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs). One group was sprayed from a single powder feedstock using controlled processing conditions. The other group included three different feedstock morphologies (obtained from different manufacturing methods), each with a similar particle size distribution and sprayed under the same average controlled processing conditions. The microstructure is quantitatively related to the feedstock morphology and processing conditions. Relationships are explored among these microstructures and the coating properties (e.g., thermal conductivity, elastic modulus). The degree of microstructural anisotropy is demonstrated to be pore-size dependent, being more pronounced for larger pores, and more sensitive to feedstock morphology ( powder processing ) than to spray processing. The microstructure analysis indicates two broad distributions of interlamellar pores, which combined, account for 70%–80% of the pore volume. The total porosity is found to increase with decreasing particle temperature or velocity. For all coatings, a negative linear relationship exists between thermal conductivity and total porosity. Comparison of the new analysis is made with earlier small-angle neutron scattering results, and implications are considered for a more general application of this metrology in TBC microstructure design.     

Thermal Conductivity of Plasma-Sprayed Monolithic and Multilayer Coatings of Alumina and Yttria-Stabilized Zirconia

The temperature dependence of the thermal conductivity of plasma-spray-deposited monolithic coatings, as well as multilayer coatings that consisted of Al₂O₃ and ZrO₂ that was stabilized by 8% Y₂O₃ (YSZ), was investigated. The coatings exhibited a large reduction in thermal conductivity at all temperatures, when compared to the bulk monolithic Al₂O₃ and YSZ. This reduction was due to porosity as well as thermal resistance that was caused by interfaces in the coatings. The largest decrease in the thermal conductivity of the coatings, relative to monolithic fully dense materials, was due to splat interfaces within each layer, as well as the coating/substrate interface. On the other hand, the multilayer coatings showed little variation in the thermal conductivity, relative to the number of layers, which suggests that the influence of interlayer interfaces on heat transfer is relatively small. A one-dimensional analysis of steady-state heat transfer has been presented to illustrate the significance of porosity, splat interfaces, and interlayer interfaces, with respect to the overall thermal conductivity of multilayer coatings.     

Origin of Darkening in 8 wt% Yttria—Zirconia Plasma-Sprayed Thermal Barrier Coatings

The origins of darkening of 8 wt% Y₂O₃-ZrO₂ air plasmasprayed (APS) and low-pressure plasma-sprayed (LPPS) thermal barrier coatings (TBC) have been studied using X-ray photoelectron spectroscopy. The change of valence states of zirconium, due to the reduction of ZrO₂ to Zr₂O₃, was responsible for darkening of TBC. Quantification of Zr³⁺ oxide was related both to the black color of TBC and to the spraying technologies and parameters. Furthermore, impurity (Fe, Al, Si, and Na) segregation and exsolution phenomena were monitored as a function of the air thermal treatment (up to 1473 K) and it was demonstrated not to be the origin of darkening.     

Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings at Elevated Temperatures

The temperature dependence of the scattering and absorption coefficients for a set of freestanding plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) was determined at temperatures up to 1360°C in a wavelength range from 1.2 μm up to the 8YSZ absorption edge. The scattering and absorption coefficients were determined by fitting the directional-hemispherical reflectance and transmittance values calculated by a four-flux Kubelka–Munk method to the experimentally measured hemispherical-directional reflectance and transmittance values obtained for five 8YSZ thicknesses. The scattering coefficient exhibited a continuous decrease with increasing wavelength and showed no significant temperature dependence. The scattering is primarily attributed to the relatively temperature-insensitive refractive index mismatch between the 8YSZ and its internal voids. The absorption coefficient was very low (<1 cm⁻¹) at wavelengths between 2 μm and the absorption edge and showed a definite temperature dependence that consisted of a shift of the absorption edge to shorter wavelengths and an increase in the weak absorption below the absorption edge with increasing temperature. The shift in the absorption edge with temperature is attributed to strongly temperature-dependent multiphonon absorption. While TBC hemispherical transmittance beyond the absorption edge can be predicted by a simple exponential decrease with thickness, below the absorption edge, typical TBC thicknesses are well below the thickness range where a simple exponential decrease in hemispherical transmittance with TBC thickness is expected. [Correction added after online publication August 11, 2009: "edge to a shorter wavelengths" has been updated as "edge to shorter wavelengths."]     

Fractal Perimeters of Polishing-Induced Pull-Outs Present on Polished Cross Sections of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Polished cross sections of plasma-sprayed yttria-stabilized zirconia coatings deposited using different process parameters were prepared with both hot- and vacuum-mounting techniques and investigated by image analysis. It was found that polishing-induced pull-outs were evidently present on the hot-mounted cross sections, and that the perimeter of these pull-outs could be described statistically by means of fractal analysis. In this work, values of the corresponding fractal dimension range from 1.45–1.54; they increase linearly while increasing fracture toughness, and decrease with the increase in porosity of the coatings. Thus, this fractal dimension may be regarded as a measure of the fracture toughness of the coatings, but only for hot-mounted samples.     

人工神经网络在污水处理中的应用

文章对污水处理系统的特点进行了分析,并针对时变性、非线性、复杂性和不确定性的特点,提出应用人工智能技术对其进行智能控制是实现城市污水处理系统自动控制的重要方法。对人工神经网络控制特点进行了介绍,综述了国内外人工神经网络在污水处理不同领域中的应用研究,并结合国内外研究动态,简要分析了人工神经网络污水处理今后的发展方向。     

Influence of Tetragonality on Tetragonal-to-Monoclinic Phase Transformation during Hydrothermal Aging in Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The influence of tetragonality, which is defined as the lattice parameter ratio c / a , on the tetragonal-to-monoclinic phase transformation during hydrothermal aging was investigated in yttria-stabilized zirconia coatings. The yttria content was adjusted in the range of 4–8 mass% (denoted as x YZ, where x = 4–8 and YZ represents the yttria-stabilized zirconia). The tetragonality of the zirconia in the as-sprayed coatings was less than that in the powders. To change the tetragonality for each yttria content, the coatings were annealed at 1273 K before aging. Without annealing, the phase transformation was prevented only in 8YZ. When annealing was applied, an increase of the tetragonality (i.e., recovery of the tetragonality) was observed, and transformation during hydrothermal aging was also suppressed in 6YZ. It was concluded that the increase in tetragonality that occurred without a change in the yttria content was suggested to be caused by the lattice relaxation of the tetragonal phase, and this relaxation is believed to cause a reduction of the critical yttria concentration, thus preventing the phase transformation.     

Investigation of Plasma-Sprayed Dysprosia Coatings

Plasma-sprayed coatings of dysprosia were made and characterized. The coatings consisted mostly of the metastable, monoclinic phase and minor amounts of the cubic phase. Grinding did not transform the high-temperature monoclinic phase, but, on annealing in air at 800° for 5 h, 50% reversion to the cubic phase occurred. Commercial yttria-stabilized zirconia thermal barrier coatings out-performed unannealed plasma-sprayed dysprosia coatings in terms of wear resistance.     

人工神经网络在聚苯乙烯乳胶球制备中的应用

利用人工神经网络很强的非线性映射功能，关联了聚苯乙烯乳胶球制备工艺参数与球尺寸相对标准偏差之间的关系，预报相对标准偏差效果较好.     

人工神经网络在注塑参数预测中的应用

应用人工神经网络技术建立了注塑工艺参数值快速预测的双层BP网络模型。利用CAE软件进行模拟，获取训练样本。用Matlab语言编制了应用程序，对网络的训练及参数预测的过程进行求解。将网络预测结果与模拟所得验证数据进行比较和误差分析，显示出BP网络的稳定性和可靠性。经实例验证，表明人工神经网络技术应用于注塑参数值的快速预测和选取方面是可行的。     

神经元网络在化学工程中的应用与发展

综述了神经元网络在化工领域中的应用状况，并指出了今后的发展趋势。     

人工神经网络用于电除盐产水水质预测研究

为了实现化工行业高纯水全自动生产,本文利用电除盐（electrodeionization,EDI）技术替代传统的混床技术,并研究了人工神经网络对EDI产水过程模拟仿真的可行性,采用误差反向传播网络（BP网络）建立了进水流量、电导率、pH值以及工作电压与EDI透过水电阻率之间关系的动态模型,并对不同的训练样本归一化方法和训练方法进行比较。结果表明,在网络隐含层层数为1、节点数为13时,采用归一化方法三能够较好的预测EDI透过水电阻率,且该模型可用于EDI除盐过程的动态描述,为实现化工行业高纯水全自动生产奠定了基础。     

人工神经网络在蛋白质结构预测和QSAR中的应用

人工神经网络（ANN）作为一种新型的信息处理系统和计算系统,近年来被广泛的应用于蛋白质结构预测、谱图分析、药物分子药效预测、定量构效关系（QSAR）研究等方面。文章论述了人工神经网络的的工作原理和基本特点,列举了国内研究者运用人工神经网络在蛋白质结构预测和QSAR中的主要应用,并对以后的应用进行了展望。