Degradation of EB-PVD thermal barrier coatings caused by CMAS deposits

In aero-turbine engines, thermal barrier coatings (TBCs) must be capable to withstand harsh environments, such as high-temperature oxidation and hot-corrosion. Recently, a new failure mode of TBCs caused by calcium–magnesium–alumina–silicate (CMAS) glass has attracted increasing attention. In this paper, yttria stabilized zirconia (YSZ) TBCs produced by electron beam physical vapor deposition (EB-PVD) were exposed to CMAS deposits at 1250 1C. The microstructure evolution and failure mechanism of the coatings were investigated. It has been shown that CMAS glass penetrated into the YSZ ceramic layer along the inter-columnar gaps and interacted with YSZ. As a result, an interaction zone of about 20 mm thickness, which was the mixture of CMAS and YSZ with equiaxial structure, was formed in the YSZ surface layer after 4 h heat-treatment at 1250 1C. Meanwhile, yttria in YSZ layer as a stabilizer was dissolved in CMAS glass and caused accelerated monoclinic phase transformation. After 8 h heat-treatment, degradation of YSZ TBC occurred by delamination cracking of YSZ layer, which is quite different from the traditional failure caused by interfacial cracking at the YSZ/metallic bond coat. Physical models have been built to describe the failure mechanism of EB-PVD TBCs attacked by CMAS deposits.     

Evaluation of plasma sprayed YSZ thermal barrier coating with the CMAS deposits infiltrati on using impedance spectroscopy

The yttria-stabilized zirconi a (YSZ) thermal barri er coatings (TBCs) prepared by atmo-sp heric plasma spraying with different heat treatment period at the temperature of 1250°C were studied in the present investig ation. Electrochemical impedance spectroscopy (EIS) was employed to non-destructively examine the impedance and capaitance behavior of free standi ng YSZ coatings deposited by plasma spray with CMAS (calcium–magnesium–alumino-silicate) infiltration. Equivalent circuit was established on the basis of the biomodal structure in coatings. The sintering behavior of the coatings can be reflected by the changes of resistance and capacitance of the coating . By EIS , the microstructure evolution of the coating with CMAS deposits was discussed in detail.     

Quality evaluation for air plasma spray thermal barrier coatings with pulsed thermography

Pulsed thermography(PT) as a non-contact non-destructive evaluation(NDE) technique was employed to examine as-sprayed air plasma spray(APS) thermal barrier coatings(TBCs) for quality evaluation.Thickness and micro-structural characteristics play a vital role in determining the quality.In this paper,PT logarithmic peak second-derivative method was adopted to measure the thickness of top coat.Time dependent thermal images were used to characterize the microstructure which was confirmed by scanning electron...     

Quality evaluation for air plasma spray thermal barrier coatings with pulsed thermography

Pulsed thermography (PT) as a non-contact non-destructiv e evaluation (NDE) technique was employed to examine as-sprayed air plasma spray (APS) thermal barrier coatings (TBCs) for quality eval uation. Thickness and micro-structural characteristics play a vital ro le in determining the quality. In this pape r, PT logarithmic peak second-derivative m ethod was adopted to measure the thickness of top coat. Time dependent ther mal images were used to characterize the microstructure which was confirmed by scanning electron microscope (SEM). The results showed that there was relationship between the temperature distribution of the surface and microstructu re change in TBCs. Temperature distribution in thermal images and measurement results of thickness were in fairly good agreement with the microstructure change. It can be concluded th at it was possible to employ these NDE methods as quality evaluation for as-sprayed TBCs.     

小波包变换在煤与瓦斯突出预测中的应用

针对目前滤噪技术不能很好地使声发射监测得以准确预报的情况,进行了基于小波包变换的去噪研究.该研究利用信号的小波包分析、计算和最优小波包基选取的方法,通过计算机数值计算,模拟了强噪声下声发射信号检测,并通过Sym8小波包与小波变换去噪的比较,证明前者优于后者.用小波包变换进行消噪处理,噪声消除彻底,提高了预警准确性.     

Porous α-Al2O3 thermal barrier coatings with dispersed Pt particles prepared by cathode plasma electrolytic deposition

Porous α-Al₂O₃ thermal barrier coatings (TBCs) containing dispersed Pt particles were prepared by cathode plasma electrolytic deposition (CPED). The influence of the Pt particles on the microstructure of the coatings and the CPED process were studied. The prepared coatings were mainly composed of α-Al₂O₃. The average thickness of the coatings was approximately 100 μm. Such single-layer TBCs exhibited not only excellent high-temperature cyclic oxidation and spallation resistance, but also good thermal insulation properties. Porous α-Al₂O₃ TBCs inhibit further oxidation of alloy substrates because of their extremely low oxygen diffusion rate, provide good thermal insulation because of their porous structure, and exhibit excellent mechanical properties because of the toughening effect of the Pt particles and because of stress relaxation induced by deformation of the porous structure.     

Phase transformation and crystal growth behavior of 8mol% (SmO1.5, GdO1.5, and YO1.5) stabilized ZrO2 powders

Nanocrystalline powders of ZrO2–8mol%SmO1.5(8SmSZ), ZrO2–8mol%GdO1.5 (8GdSZ), and ZrO2–8mol%YO1.5 (8YSZ) were prepared by a simple reverse-coprecipitation technique. Differential thermal analysis/thermogravimetry (DTA/TG), Fourier transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) were used to study the phase transformation and crystal growth behavior. The DTA results showed that the ZrO2 freeze-dried precipitates crystallized at 529, 465, and 467°C in the case of 8SmSZ, 8GdSZ, and 8YSZ, respectively. The XRD and Raman results confirmed the presence of tetragonal ZrO2 when the dried precipitates were calcined in the temperature range from 600 to 1000°C for 2 h. The crystallite size increased with increasing calcination temperature. The activation energies were calculated as 12.39, 12.45, and 16.59 kJ/mol for 8SmSZ, 8GdSZ, and 8YSZ respectively.     

La2Zr2O7/YSZ梯度热障涂层的热冲击失效行为

隔热性能是热障涂层对基体热保护功能最直接的衡量标准。制备La₂Zr₂O₇(LZO)/ YSZ 梯度热障涂层(DCL-TBCs)，在1 300 ℃的顶部涂层表面温度下与500 μm厚的YSZ TBC进行热梯度循环试验，评估它们的寿命。分析试验结果表明，当厚度为100 μm和200 μm的YSZ涂层被LZO涂层替代时，DCL-TBCs的寿命会比500 μm厚的YSZ TBC高出一倍多。证明DCL-TBCs技术可在较高温度下有效延长涂层的使用寿命。     

超声速等离子喷涂氧化锆粒子时的加热熔化特性分析

应用格林函数法求解超声速等离子喷涂(SAPS)过程中氧化钇稳定二氧化锆(YSZ)粒子的加热熔化时间问题,得到了一个解析解;对SAPS过程中粒子熔化时间和熔化界面推移情况进行了数值模拟,所得结果与实验结果相符.熔化过程的模拟结果显示:粒子的初始温度对熔化速率有一定的影响,喷涂前预热粒子可以加快它的熔化;减小粒子半径可以缩短粒子的熔化时间,提高粒子的扁平化程度.在SAPS过程中,通过过筛控制粒子半径有助于获得结构精细、致密的典型细密柱晶组织.     

La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> TBCs toughened by Pt particles prepared by cathode plasma electrolytic deposition

La₂Zr₂O₇ thermal barrier coatings (TBCs) with dispersed Pt particles were prepared by cathode plasma electrolytic deposition (CPED) with ceramic balls added to the cathode region. Compared with the conventional CPED, when ceramic balls are used in the cathode region, the plasma discharge ignition current density decreases approximately 62-fold and the stable plasma discharges occur at the whole cathode surface. Such TBCs with a thickness of 100 μm exhibit a crack-free surface and are composed of pyrochlore-structured La₂Zr₂O₇. Cyclic oxidation, scratching, and thermal insulation capability tests show that such TBCs not only exhibit high resistance to oxidation and spallation but also provide good thermal insulation. These beneficial effects are attributed to the excellent properties of TBCs, such as good thermal insulation because of low thermal conductivity, high-temperature oxidation resistance because of low-oxygen diffusion rate, and good mechanical properties because of the toughening effect of Pt particles.     

Phase transformation and crystal growth behavior of 8mol% (SmO<Subscript>1.5</Subscript>, GdO<Subscript>1.5</Subscript>, and YO<Subscript>1.5</Subscript>) stabilized ZrO<Subscript>2</Subscript> powders

Nanocrystalline powders of ZrO₂-8mol%SmO_1.5(8SmSZ), ZrO₂-8mol%GdO_1.5 (8GdSZ), and ZrO₂-8mol%YO_1.5(8YSZ) were prepared by a simple reverse-coprecipitation technique. Differential thermal analysis/thermogravimetry (DTA/TG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) were used to study the phase transformation and crystal growth behavior. The DTA results showed that the ZrO₂ freeze-dried precipitates crystallized at 529, 465, and 467℃ in the case of 8SmSZ, 8GdSZ, and 8YSZ, respectively. The XRD and Raman results confirmed the presence of tetragonal ZrO₂ when the dried precipitates were calcined in the temperature range from 600 to 1000℃ for 2 h. The crystallite size increased with increasing calcination temperature. The activation energies were calculated as 12.39, 12.45, and 16.59 kJ/mol for 8SmSZ, 8GdSZ, and 8YSZ respectively.     

基于AE技术与HHT的环锭纱拉伸断裂过程研究

在常规纱线拉伸强力仪上采用自主搭建的声发射检测系统,分别采集涤纶长丝和环锭纱在拉伸破坏过程中的声频,利用Matlab软件编制Hilbert-Huang transform(HHT)程序对采集的信号进行分析与处理,以获取纱线拉伸过程中各破坏源的时频特征。分析结果表明,AE信号经HHT可识别涤纶环锭纱拉伸过程所涉及的纤维断裂和纤维滑移2种模式,其特征频率分别为30,20kHz和6,3kHz,本研究为纱线拉伸断裂机理的分析和进一步的延伸研究提供一种有效方法。     

Mechanism of brittle-ductile transition of a glass-ceramic rigid substrate

The hardness, elastic modulus, and scratch resistance of a glass-ceramic rigid substrate were measured by nanoindentation and nanoscratch, and the fracture toughness was measured by indentation using a Vickers indenter. The results show that the hardness and elastic modulus at a peak indentation depth of 200 nm are 9.04 and 94.70 GPa, respectively. These values reflect the properties of the glass-ceramic rigid substrate. The fracture toughness value of the glass-ceramic rigid substrate is 2.63 MPa?m1/2. The material removal mechanisms are seen to be directly related to normal force on the tip. The critical load and scratch depth estimated from the scratch depth profile after scratching and the friction profile are 268.60 mN and 335.10 nm, respectively. If the load and scratch depth are under the critical values, the glass-ceramic rigid substrate will undergo plastic flow rather than fracture. The formula of critical depth of cut described by Bifnao et al. is modified based on the difference of critical scratch depth     

Characterizing Acoustic Sources in Pressure Vessels

IntroductionThe challenge of acoustic emission testing is theinverse- source problem,i. e.,to find thecharacteristics of an acoustic emission(AE) sourceby analyzing the AE signals,including thelocation,nature,and severity for pressure vesseltesting. Linear,planar,and 3 - dimensional sourcelocation techniques based on time differences areable to find active defects[1] ,but are unable tonondestructively evaluate the vessel condition.Theaim of AE signal processing is to obtain a completedescrip…     

Effect of Al2O3 and Y2O3 on the corrosion behavior of ZrO2-benzotriazole nanostructured coatings applied on AA2024 via a sol-gel method

zirconia-based nanostructured coatings were deposited on AA2024 to improve the corrosion resistance properties. Three different nanostructured coatings, namely, zirconia-benzotriazole, zirconia-alumina-benzotriazole, and zirconia-yttria-benzotriazole, were applied on AA2024 via a sol-gel method using the dip-coating technique. Next, the coatings were annealed at 150℃ after each dipping period. The phases and morphologies of the coatings were investigated using grazing incidence X-ray diffraction (GIXRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The corrosion properties were evaluated using electrochemical methods, including polarization and electrochemical impedance techniques in 3.5wt% NaCl solution. The obtained results confirm the formation of homogeneous and crack free zirconia-benzotriazole-based nanostructured coatings. The average roughness values for zirconia-benzotriazole, zirconia-alumina-benzotriazole, and zirconia-yttria-benzotriazole nanostructured coatings were 30, 8, and 6 nm, respectively. The presence of alumina as a stabilizer on zirconia coating was found to have a beneficial impact on the stability of the corrosion resistance for different immersion times. In fact, the addition of alumina resulted in the dominance of the healing behavior in competition with the corrosion process of zirconia-benzotriazole nanostructured coating.     

钢包耳轴活性缺陷声发射信号特征与定位分析

钢包耳轴是整个钢包结构的关键部位之一，耳轴根部在长期的高温吊拉作业中会产生活性疲劳裂纹.针对常规超声法难以对耳轴根部焊缝进行无损检测的问题，提出基于声发射(Acoustic emission, AE)平面定位技术对钢包耳轴根部焊缝活性缺陷进行检测的方法.对原始AE数据进行滤波处理并结合小波包变换对信号进行分析.实验结果表明该方法能够较准确定位焊缝活性缺陷的位置.经过小波包对AE信号进行3层分解与重构后，活性缺陷AE信号频带主要分布在62.5~125 kHz，其能量占总能量的81.25 %，符合一般裂纹扩展的频率特征.     

噪声环境下基于小波熵的声发射识别

针对声发射技术在旋转机械故障检测中的强噪声干扰问题,提出了一种基于小波熵的声发射检测算法.该算法首先给定一个合理的阈值.对声发射信号进行小波分解.然后进行分帧处理,使信号在较短的时间间隔内保持特性基本不变,从而求出每一帧信号的小波熵.通过比较每一帧信号的小波熵值与阈值的大小,判断该信号为声发射帧还是噪声帧.为了检验算法的检测效果,在转子实验台上获得碰摩声发射信号.并在测试数据上叠加不同信噪比的高斯白噪声和非平稳噪声,进行声发射识别.实验结果表明:该算法具有较高的识别正确率;在低信噪比环境下,通过调整阈值的可调参数可以有效提高识别的正确率.     

基于HHT和SVM的纤维拉伸断裂声发射信号的特征提取及分类研究

为了研究纤维拉伸断裂声发射信号的特征提取及分类方法,采用声发射技术采集了芳纶1313和阻燃黏胶2种纤维的拉伸断裂的声发射信号。通过小波变换,对采集的2种纤维的声发射信号进行消噪预处理以去除部分噪声,应用希尔伯特-黄变换对2种纤维去噪后的信号进行特征频率的提取,运用最小二乘支持向量机(LSSVM)对2种纤维的特征频率进行分类识别。结果表明:小波去噪方法可以去除信号的部分噪声;希尔伯特时频谱可以一定程度上反映2种纤维材料在时间维度上的断裂情况,边际谱上可以提取2种纤维材料声发射信号的特征频率;LSSVM能够对2种纤维材料拉伸断裂的特征频率分类识别,芳纶1313的识别率为40%,阻燃黏胶的识别率为80%,总的识别率为60%。     

Effects of surface modification on thermal cycling lifetime of thermal barrier coatings with HVOF NiCrAlY bond coat

NiCrAlY bond coat was prepared by HVOF (high-velocity oxygen fuel) spray on nickel-based superalloy. Surface treatments like grit-blasting, shot-peening and vacuum treatment methods were carried out in order to study the effects of surface modification on thermal cycling lifetime of TBCs. The surface-modified TBCs exhibited better thermal shock resistance. Failure of TBCs with the as-sprayed bond coat occurred within the top coat and at the interface between spinels and the top coat, while that of after shot-peening, grit-blasting and vacuum treatment occurred mainly within the top coat. TGO (thermally grown oxide) formed on as-sprayed bond coat was composed of a Ni(Al,Cr)2O4 spinels outer layer and a Al2O3 inner layer. But, a continuous and uniform Al2O3 formed after surface modification . Formation of the mixed oxides (spinels) on the as-sprayed bond coat accelerated the failure of TBCs.     

Effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 thermal barrier coatings

LaMgAl11O19 thermal barrier coatings (TBCs) were applied to carbon steels with a NiCoCrAlY bond coat by plasma spraying. The effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 TBCs were investigated in 3.5wt% NaCl solution using polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray diffrac-tion (XRD). The results show that a large number of cracks are found in the LaMgAl11O19 TBCs after the samples are heat-treated, including some through-thickness cracks. The corrosion forms of the as-sprayed and heat-treated TBCs are uniform corrosion and pitting corrosion, respectively. The as-sprayed TBCs exhibit three EIS time constants after being immersed for less than 7 d, and then a new time constant ap-pears because of steel substrate corrosion. When the immersion time is increased to 56 d, a Warburg impedance (W) component appears in the EIS data. The EIS data for the heat-treated TBCs exhibit only two time constants after the samples are immersed for less than 14 d, and a new time constant appears when the immersion time is increased further. The heat treatment reduces the corrosion resistance of carbon steel coated with LaMgAl11O19 TBCs. The corrosion products are primarilyγ-FeOOH and Fe3O4.