微波检测热障涂层孔隙率的可行性研究

由于热障涂层具有隔热性好、抗氧化、耐腐蚀等优点,已被广泛应用于先进发动机热端部件上,孔隙率是影响热障涂层热障效果的主要因素之一。对热障涂层孔隙率的微波无损检测进行了仿真研究,并进行了实验验证。仿真中检测结果表明,当涂层厚度为工作波长的1/4时,微波检测孔隙率具有较高的检测灵敏度。在18~26.5 GHz工作频率内能对厚度为600~900μm的热障涂层孔隙率进行检测;在26.5~40 GHz工作频率内能对厚度为350~550μm的热障涂层孔隙率进行检测;在40~50 GHz工作频率范围内能对300~350μm的热障涂层孔隙率进行检测,因此可以根据实际需要选择不同的工作频率段对热障涂层的孔隙率进行微波无损检测。在26.5~40 GHz工作频率内对厚度为400μm的热障涂层孔隙率进行实验检测,实验结果和理论结果基本吻合。     

反射系数法微波检测热障涂层厚度的实验研究

为了保证热障涂层的隔热效果,对热障涂层的厚度进行了微波无损检测.检测中探头选用R281A带法兰矩形波导探头,工作频率范围为26.5 ～ 40 GHz.仿真计算利用CST微波工作室(computer simulation technology-microwave studio)软件进行.实验结果表明,在26.5～40 GHz工作频率范围内,反射系数的相位差能良好地表征热障涂层的厚度.并且在高频率段时可以达到相位差变化l°,热障涂层的厚度变化约8μm的分辨率.在28.5～29.1 GHz工作频率范围内,反射系数的幅值也能表征热障涂层的厚度.实验结果与仿真结果基本吻合.     

同轴探头微波检测热障涂层的参数敏感性研究

为实现低频率段对热障涂层(Thermal barrier coating,TBC)的微波无损检测,利用同轴探头在0~17 GHz工作频率范围内对热障涂层的厚度、孔隙率和金属基体表面的裂缝进行微波无损检测理论仿真,对检测中的参数敏感性进行研究,从而提高检测的灵敏度,防止漏检现象的产生。采用CST微波工作室对检测中同轴探头的尺寸和热障涂层的厚度等参数的检测敏感性进行理论计算。热障涂层厚度检测结果显示,同轴探头越小时检测灵敏度越高,并且对应的相位差零点处的工作频率越高;热障涂层孔隙率检测结果表明热障涂层的厚度越大时检测灵敏度越高;金属基体表面裂缝的检测结果表明扫描步长小于外导体内半径时可以防止漏检。由研究结果可知,利用合适尺寸的同轴探头可以实现在低频率段对热障涂层的微波无损检测。     

EB-PVD热障涂层TGO三维结构分析

热生长氧化物(TGO)的厚度及形貌是影响热障涂层热应力的关键因素。首先开展了电子束物理气相沉积(EB-PVD)热障涂层1 050℃恒温氧化实验,采用聚焦离子束-扫描电子显微镜(FIB-SEM)实现了EB-PVD热障涂层三维成像,获得了576幅热障涂层高温氧化48 h后的SEM图像;其次,通过EB-PVD热障涂层三维结构的分割与提取,得到了真实TGO三维结构和柱间孔隙三维分布,实现了TGO层的厚度测量;最后建立了基于TGO三维结构和柱间孔隙三维分布的EB-PVD热障涂层有限元热应力分析模型,分析了真实TGO三维结构和柱间孔隙三维分布对EB-PVD热障涂层热应力的影响。结果表明,TGO层的平均厚度为2.37μm,有限元模型中最大拉应力为403 MPa,最大压应力为-282 MPa,最大拉应力与最大压应力均出现在陶瓷层中的柱间孔隙处。     

等离子喷涂热障涂层中应力分布的有限元模拟

采用热弹性有限元方法研究了热载荷条件下由数值模拟方法生成的热障涂层冷却至室温后的应力分布,分析了热生长氧化物(TGO)层及其厚度对热障涂层应力分布的影响,并与文献的试验结果进行了对比。结果表明:不含TGO层的热障涂层,陶瓷层和粘结层的凸起处为拉应力,凹陷处为压应力;TGO层形成后,陶瓷层凸起处表现为压应力,凹陷处表现为拉应力;界面附近的陶瓷层在TGO层达到一定厚度时出现应力反转现象,且该现象的出现随着界面粗糙度的增大而延迟;模拟预测的应力与文献报道的结果相近,证明了模拟结果的正确性。     

热障涂层折射率与厚度的太赫兹无损检测

精确提取陶瓷层（Top coat,TC）与热生长氧化层（Thermally grown oxide,TGO）层在太赫兹频段的折射率是进行热障涂层（Thermal barrier coatings,TBCs）太赫兹无损检测研究的重要条件。由于对涂层样品只能采取反射式测量,所以首先比较了反射式与传统透射式测量条件下提取样品太赫兹光学参数及厚度的结果,随后利用反射式太赫兹时域脉冲成像系统提取等离子体喷涂的8YSZ热障涂层（TBCs）中TC层与TGO层的折射率,并依据所提取折射率进一步对TC层的厚度分布进行测量及成像。试验结果表明在材料中衰减较小的有效频段下反射式测量同样可以精确提取样品的折射率以及厚度,反射式测量TC层的平均折射率为5.23,TGO层的折射率为2.91,TGO的主要成分α-Al₂O₃的折射率为2.85。TBCs样品中TC层的平均厚度为257 μm,从TC层厚度的太赫兹图像中可观察到TC与粘结层（Bond coat,BC）界面的不均匀程度。反射式太赫兹无损检测可精确提取TBCs中TC与TGO的折射率和厚度,这对于TBCs中裂纹和气泡等缺陷的识别以及TGO生长太赫兹检测具有重要意义。     

热障涂层系统中TGO应力生成的数值模拟

采用有限元分析软件ABAQUS对热障涂层系统中TGO的应力生成进行了数值模拟.基于实验测得的TGO生成厚度和材料性能参数,研究了热氧化循环载荷中TGO应力产生的机理和影响因素,验证了材料属性转化方法模拟TGO生成过程的有效性.结果表明,热膨胀系数差异和TGO高温生长是导致TGO应力产生的主要原因,TGO厚度的增加导致了外层TGO中拉伸应力的产生,随着厚度的增加该应力逐渐增大.当TGO达到某一厚度时,该应力保持恒定.     

含抗氧化夹层热障涂层界面应力特性的研究

通过喷涂等工艺技术,在热障涂层粘结层和陶瓷层之间预制Al2O3夹层,能有效抑制热障涂层高温不规则氧化生长及其所导致的失效.但是,预制的Al2O3夹层又会导致热障涂层界面应力场重新分布.文中针对热障涂层界面三维形貌特征,利用弹性理论中的空心球模型,考虑陶瓷层(ZrO2层)、夹层(Al2O3层)、氧化层(thermally grown oxide, TGO)及粘结层(bondcoat, BC),建立三维四层球模型的理论分析,并推演出相应的应力解析式.文中通过凹凸两种界面形貌下抗氧化夹层的算例分析,表明以上分析方法能有效模拟夹层对热障涂层界面应力分布规律的影响.     

超音速喷涂粘结层对热障涂层抗氧化性能的影响

采用超音速喷涂工艺在Ni基高温合金GH99上制备NiCoCrAlY粘结层,研究超音速火焰喷涂工艺对热障涂层抗氧化性能的影响.结果表明:等离子喷涂粘结层表面存在着大量的尖角和钩状的突出颗粒,而超音速喷涂粘结层表面较为平整,内部黑色氧化物较少.超音速喷涂涂层在氧化15 min后表面生成一层较为致密的颗粒状氧化物.超音速喷涂试样在前40 min氧化增重速率快于等离子喷涂试样,此后变得缓慢,而等离子喷涂试样氧化速率依然较高.超音速喷涂的涂层在氧化300 h后热生长氧化物(Thermally grown oxide, TGO)出现分层,TGO内层组织致密,TGO外层组织存在大量的孔洞和间隙.超音速喷涂工艺使热生长氧化物瞬态氧化阶段时间缩短,稳态氧化阶段时间延长,减少了氧化缺陷的产生,有效地提高了涂层抗高温氧化性能.     

重型燃气轮机透平叶片热障涂层的高温氧化寿命预测

采用大气等离子喷涂技术在DZ411合金基体上制备ZrO₂-8Y₂O₃(质量分数/%)热障涂层,研究了热障涂层的高温氧化性能和氧化后的截面微观形貌,建立了高温氧化寿命预测模型并进行了试验验证。结果表明：在940,1 030℃下氧化时,热障涂层的氧化质量增加Δm和热生长氧化物层厚度均呈抛物线增长规律;热障涂层在1 030℃下的Δm增大速率较940℃下高;建立的热障涂层高温氧化寿命预测模型预测的热生长氧化物层厚度值与试验结果相比在±2倍的分散带内,说明建立的寿命模型及分析方法能较好地预测热障涂层的高温氧化寿命。     

Time of flight improved thermally grown oxide thickness measurement with terahertz spectroscopy

As a nondestructive testing technique, terahertz time-domain spectroscopy technology is commonly used to measure the thickness of ceramic coat in thermal barrier coatings (TBCs). However, the invisibility of ceramic/thermally grown oxide (TGO) reflective wave leads to the measurement failure of natural growth TGO whose thickness is below 10 μm in TBCs. To detect and monitor TGO in the emergence stage, a time of flight (TOF) improved TGO thickness measurement method is proposed. A simulative investigation on propagation characteristics of terahertz shows the linear relationship between TGO thickness and phase shift of feature wave. The accurate TOF increment could be acquired from wavelet soft threshold and cross-correlation function with negative effect reduction of environmental noise and system oscillation. Thus, the TGO thickness could be obtained efficiently from the TOF increment of the monitor area with different heating times. The averaged error of 1.61 μm in experimental results demonstrates the highly accurate and robust measurement of the proposed method, making it attractive for condition monitoring and life prediction of TBCs.     

Hyperbolic-cosine waveguide tapers and oversize rectangular waveguide for reduced broadband insertion loss in W-band electron paramagnetic resonance spectroscopy

The two-way insertion loss of a 1 m length of waveguide was reduced by nearly 5 dB over a 4% bandwidth at W-band (94 GHz) for an electron paramagnetic resonance (EPR) spectrometer relative to WR10 waveguide. The waveguide has an oversize section of commercially available rectangular WR28 and a novel pair of tapers that vary in cross section with axial position according to a hyperbolic-cosine (HC) function. The tapers connect conventional rectangular WR10 waveguide to the WR28. For minimum loss, the main mode electric field is parallel to the long side of the WR28. Using mode coupling theory, the position of maximum flare (inflection point) in the taper was optimized with respect to the coupling to higher order modes and the reflection of the main mode. The optimum inflection point position is about one-tenth of the taper length from the small end of the taper. Reflection and coupling were reduced by about 20 dB relative to a pyramidal (linear) taper of the same length. Comb-like dips in the transmission coefficient produced by resonances of the higher order modes in the oversize section were about 0.03 dB. Specially designed high-precision, adjustable WR28 flanges with alignment to about 5 μm were required to keep higher order mode amplitudes arising from the flanges comparable to those from the HC tapers. Minimum return loss was about 30 dB. This paper provides a foundation for further optimization, if needed. Methods are not specific to EPR or the microwave frequency band.     

一种基于SIW的阶梯式太赫兹宽带传输线

提出了一种适用于太赫兹频段、长距离传输的阶梯式宽带传输线.将2个基片集成波导(substrate integrated waveguide,SIW)分别置于一个矩形波导两端并分别呈直角转接,整个结构组成一级阶梯形状以实现波导口的空间位置平移.通过在矩形波导两端使用耦合窗的方法完成空气与基片介质之间无模式转换的转接.所提...     

波导内飞灰灰位与折射率的微波测量方法

为了进一步提高微波波导法对飞灰含碳量检测的准确性,提出了一种利用微波在波导内不同介质分界面的反射作用,通过时域变换找到微波到达不同介质分界面的时间,再结合微波在波导内不同介质中的传输速度,计算得到波导内飞灰的灰位及相对群折射率的方法.本文使用规格为7.9 mm×15.8 mm×100mm的矩形波导,在12.4～18 G...     

Noise floor and dynamic range analysis of a microwave attenuation measurement receiver from 50 MHz to 26.5 GHz

The noise floor and dynamic range of a proposed voltage-ratio based microwave attenuation measurement receiver are analysed. The thermal noise and phase noise effect on attenuation measurement is studied. For frequencies higher than 1 GHz, the receiver employs a lock-in amplifier with a coherent frequency reference to minimize the phase noise effect and achieve very wide dynamic range of attenuation measurement. The equivalent noise floor at the receiver mixer output port is found to be around −172 dBm. The dynamic range of the receiver is 181-175 dB from 50 MHz to 26.5 GHz. Measurement results of a 0-170 dB synthesized step attenuator at 26 GHz is given to verify the performance of the proposed receiver.     

Microwave Nondestructive Detection of Corrosion Under Thin Paint and Primer in Aluminum Panels

Detection of corrosion, under paint and primer, in various metallic structural components, particularly when used in moist and salty environments is an important practical concern. Moreover, nondestructive testing techniques that do not require paint removal are desired. Near-field microwave nondestructive inspection techniques, employing open-ended rectangular waveguide probes, have shown tremendous potential for detecting and evaluating the presence of corrosion under paint in steel substrates. The objective of this investigation has been to investigate the potential of these techniques for detecting corrosion under paint and primer in aluminum substrates. To accomplish this goal, an electromagnetic formulation, simulating detection of corrosion in layered structure using open-ended rectangular waveguide probes, was used to gain an insight into the functionality of measurement parameters such as the frequency of operation and standoff distance. In conjunction with this simulation, the dielectric properties of paint, primer, real and chemically produced aluminum oxide were measured in a wide range of microwave frequencies (2.6–18 GHz). The results showed that the dielectric properties of paint, primer and aluminum oxide are very similar to each other. Subsequently, the theoretical simulation was conducted in a wide frequency band (8.2–40 GHz). The overall result of the simulation effort was that higher frequencies and standoff distances of a few mm are more optimal for detecting thin corrosion layers under paint. Two specially prepared aluminum panels with induced areas of corrosion and surface pitting were produced as well. Using these panels and several phase sensitive measurement systems, experiments were conducted producing 2-D images of various areas of these panels. Images were produced at different standoff distances and at frequencies of 9, 11.725, 24.1, and 33.5 GHz. The overall results of the experimental investigation were extremely promising when detecting the thin regions of corrosion in these panels. This paper presents the approach and results of this investigation.     

The effect of TBC morphology and aging on the erosion rate of EB-PVD TBCs

Since thermal barrier coatings (TBCs) have been used in gas turbines most of the research conducted on them has involved the bond coat and the growth of the thermally grown oxide (TGO) as failure of the bond coat and the TGO were considered to be the primary causes of failure. Erosion of TBCs has been considered as a secondary problem and as such received less attention. Most of the initial work on the erosion of TBCs covered the effects of velocity and impact angle on the erosion rates of both plasma sprayed (PS) and electron beam physical vapour deposited (EB-PVD) TBCs and compared the differences between the two deposition systems. It must be noted that most of the tests were conducted on coatings in the as received condition. This paper aims at expanding the understanding of the erosion of EB-PVD TBCs by examining the effects of TBC morphology, column diameter, column inclination angle and the effects of aging and sintering on the erosion rates of EB-PVD TBCs. The paper also looks at how erosion rate changes as the coating is eroded through to the bond coat.The paper also looks at the mechanisms of foreign object damage of EB-PVD TBCs under a range of different impact conditions. The different damage mechanisms have been identified and related to the size and impact velocity of the impacting particles. The effect of temperature on the plasticity and hence the mechanisms are also discussed, while mapping is used to set the boundary limits for the different types of damage mechanisms that have been identified. It was found that at temperatures above 800 °C the coatings can accommodate a large degree of plastic deformation, while at room temperature there is a greater degree of cracking, for similar types of FOD impact. The 800 °C is not necessarily a limit, but the temperature at which the coatings were tested, and the limiting temperature could in fact be significantly lower. It was found that, all else being equal, erosion rate decreases with a decrease in the column diameter, while aging results in an increase in the erosion rate, dependent on the aging temperature and time. A decrease in the inclination angle of the columns with respect to the substrate increases the erosion rate, when the inclination angle is less than 60° the erosion rate increases catastrophically. These effects are all discussed and explained in terms of erosion mechanisms and mechanical properties in the paper.