【作者】 赵扬；

【导师】 李喜孟； 林莉；

【作者基本信息】 大连理工大学 ， 材料学， 2010， 博士

【摘要】 表面涂层质量和性能的超声无损检测与表征技术研究,对于表面涂层技术的应用与发展具有重要意义。目前,研究工作遇到的难点问题集中于两个方面：(1)涂层厚度较小(几十至几百微米),致使涂层前后表面的超声反射回波发生混叠；(2)对于非均匀涂层,其内部随机分布着一定数量的孔隙和微裂纹,超声波在该结构中传播时发生了频散。本研究针对难点问题(1)：根据超声波在三层介质中的传播模型,利用涂层前后表面反射回波混叠所产生的干涉现象,提取涂层的声压反射系数幅度谱和相位谱,来分别表征涂层的厚度和密度。进而引入反演技术,实现了涂层纵波声速和厚度的同时测定。与此同时,声压反射系数幅度谱幅值和谐振频率出现规律等特征,受涂层与其相邻介质声阻抗匹配关系变化的影响十分敏感,据此可用于检测涂层界面处的缺陷,如脱粘及TGO (thermal grown oxide)的存在。针对难点问题(2)：建立了非均匀涂层物理模拟模型,进而分别借助于超声检测数值模拟技术和实验测试,研究非均匀涂层结构对声波传播规律的影响。发现窄脉冲超声信号在非均匀涂层中传播时,声波发生了频散现象。据此对均匀涂层的反射系数相位谱与幅度谱表达式进行了修正,并引入谐振频率偏离系数来定量描述涂层中的频散程度,研究成果为非均匀涂层的超声无损检测与表征提供了理论依据。依据建立的均匀及非均匀涂层的超声表征原理,分别对均匀的铝质涂层、环氧树脂涂层和非均匀的TBC (thermal barrier coating)进行了超声检测与表征,主要结果如下：1.利用声压反射系数幅度谱,测量了聚氯乙烯基体上铝质涂层的厚度(404-425μm),所得结果与金相观察结果之间绝对误差范围为±7μm,相对误差范围为±2%,满足工程需要。利用声压反射系数相位谱极值与涂层密度之间的函数关系,对铝质涂层的密度进行了测量,测量结果与阿基米德法测量结果的最大相对误差为5.8%。2.以声压反射系数幅度谱为目标函数,利用高斯-牛顿法对环氧树脂涂层的纵波声速和厚度两个未知参数同时反演,其结果与参考值之间的最大相对误差分别为2.67%和2.35%,均满足工程需要。3.利用涂层及与其相邻介质声阻抗的匹配关系对涂层声压反射系数幅度谱的影响,对铝质涂层/环氧树脂粘结层/有机玻璃基体试样,测定其脱粘及未脱粘状态下的声压反射系数幅度谱,谱线极值变化规律与理论预测结果相符。4.以GH33为基体材料,利用EB-PVD (electron beam physical vapor deposition)法依次沉积了NiCoCrAlY粘结层和YSZ涂层,从而获得了TBC试样。将TBC试样在1050℃×1 h条件下经15次循环氧化后,进行超声检测。由于氧化后YSZ涂层与NiCoCrAlY粘结层界面生成了约3μm的TGO(主要成分为Al2O3),因而改变了涂层与相邻介质之间的声阻抗匹配关系,使得氧化前后涂层声压反射系数幅度谱极值出现的规律发生了改变,实验结果与理论预测相符。5.针对分别由等离子喷涂法及EB-PVD法制备的非均匀涂层,基于随机介质理论和统计学方法,提出了非均匀涂层物理模拟模型,进而利用时域有限差分法进行了超声传播的数值模拟。数值模拟结果发现,随着孔隙率的增加,涂层的非均匀程度增加,导致声波的谐振频率偏离系数增大。在推导出均匀涂层的声压反射系数幅度谱与相位谱表达式的基础上,将式中的超声纵波声速和衰减系数修正为随频率变化的函数形式,创建了非均匀涂层的声压反射系数幅度谱与相位谱表达式。6.利用声压反射系数相位谱,对1Cr18Ni9Ti基体上等离子喷涂的YSZ涂层厚度(256-330μm)进行了超声测量,测量结果与金相观察结果之间的绝对误差范围为±10μm,相对误差范围为±3%,满足工程测试需要。7.利用声压反射系数相位谱的相位极值与涂层密度之间的理论关系,对1Cr18Ni9Ti基体上等离子喷涂的Cr2O3涂层(50μm)的密度进行了定性分析,结果表明Cr2O3涂层存在一定程度的非均匀性,超声测试结果与SEM分析结果相符。同理对GH33基体上的EB-PVD法制备的YSZ涂层(150μm)的密度进行了测量,超声法与阿基米德法的测量结果之间的最大相对误差为4.59%。8.借助于HIPIB (High-intensity pulsed ion beam)技术,对EB-PVD法制备的TBC试样进行不同束流密度辐照的表面改性处理,继而对辐照和未辐照试样分别进行了超声测试。结果发现,未辐照、100 A/cm2辐照及300 A/cm2辐照后涂层的谐振频率偏离系数范围依次为0.80-0.85、0.85-0.88和0.91-0.93,且声衰减系数依次减小。结果说明,HIPIB辐照后涂层的致密度增大、均匀性得到改善,且300 A/cm2辐照后涂层的致密度增大程度及均匀性改善程度最大,涂层SEM观察结果与超声表征结果相符。更多还原

【Abstract】 The investigation on the ultrasonic testing and characterization of surface coatings is significant to the development and application of the coatings technique. At present, there are two major problems on the immersion ultrasonic pulse echo method used to test and characterize the properties of the coatings. (1) The coating is so thin (10-8-10-7m) that the ultrasonic echoes reflected from the front and back surfaces of the coating overlap. (2) For the inhomogeneous coating, ultrasonic wave occurs complex scattering and dispersion when it propagates in such kinds of coating containing the random pores and microcracks.To overcome the problem mentioned above, the systematical work is carried out from the views of the theoretical model, signal processing, numerical simulation and experimental tests. For the problem (1), a water/coating/substrate model is established for the homogeneous coating. According to the interference caused by the reflection waves from the interfaces of water/coating and coating/substrate, the thickness and density of the coatings are determined by the ultrasonic reflection coefficient amplitude and phase spectrum (URCAS & URCPS). Then, the velocity and thickness are determined simultaneously by introducing the inversion technique. Meanwhile, the variation of URCAS extreme is very sensitive to the changes of acoustic impedance match between the coating and its adjacent medium, which can be used to test flaws of the interface, such as disbonding and the existence of TGO (thermally grown oxide).For the problem (2), the influence of inhomogeneous microstructure on propagation of ultrasonic wave is investigated from the view of experiment and numerical simulation. It is found that dispersion phenomenon occurs when the ultrasonic signals transmits in the inhomogeneous coating. Accordingly, the constant forms of longitudinal velocity and attenuation coefficient in the expressions of URCAS and URCPS of homogeneous coating are modified as the frequency dependent form, so the expressions of URCAS and URCPS of inhomogeneous coating are obtained. Then the resonant frequency deviation coefficient (RFDC) is introduced to quantify the degree of dispersion, which provides the basis on the ultrasonic characterization of the inhomogeneous coating.Based on the model and the principle above, ultrasonic testing and characterization of the homogeneous aluminum coating, epoxy coating and inhomogeneous TBC are carried out in this work. The major results are as following:1. The thickness of aluminum coating on PVC substrate (404-425μm) is measured by URCAS, and range of absolute error between ultrasonic method and metallographic observations is±7μm, the range of relative error±2%. The density of aluminum coating is determined by the relation of URCPS extreme and coating density, and the relative error maximum of measurement results between ultrasonic and Archimedes method is 5.8%.2. The ultrasonic longitudinal velocity, density and thickness of homogeneous epoxy coating (277μm) on the aluminum substrate are characterized by the combination of URCAS and inverse method. The relative errors between the velocity and thickness obtained by two parameters inversion based on Gauss-Newton method and "reference value" are 2.67% and 2.35%, respectively.3. The specimens of the aluminum layer/epoxy bonding layer/glass substrate system are tested according to the influence of acoustic impedance match relation between the coating and its adjacent medium on URCAS. Both of the experimental URCAS of disbonding and perfect specimen are obtained, and it is found that the regularity of the experimental URCAS is consistent with the theoretical prediction.4. Ultrasonic testing is carried out on the TBC system (YSZ coating/NiCoCrAlY bonding layer/GH33 substrate) before and after aging under the conditions of 1050℃×1 h after 15 cycles. TGO (main component:Al2O3) with thickness of about 3μm generates between the YSZ coating and NiCoCrAlY bond layer due to the oxidation, which changes acoustic impedance match relations between the coating and its adjacent medium. This makes the law of URCAS extreme occurring of the coating before and after aging reverse. The experimental results agree well with the theoretical prediction, which shows the present method can be used to test TGO in the TBC system.5. The inhomogeneous coating physical simulation model (ICPSM) is proposed to describe the inhomogeneous coating prepared by plasma sprayed and electron beam physical vapor deposition (EB-PVD) based on the random medium theory and statics method. Then the simulation on the ultrasonic propagation in the ICPSM is fulfilled by the FDTD (Finite-difference time-domain) method. It is found that RFDC increase with the increase of coating porosity, which shows that the dispersion degree increases with the increase of inhomogeneous degree of the coating. The constant forms of velocity and attenuation coefficient in the expressions of URCAS and URCPS of homogeneous coating are modified as the frequency dependent form. Then the expressions of URCAS and URCPS of inhomogeneous coating are obtained first, which are particularly suitable to ultrasonic characterization of inhomogeneous coating.6. The thickness of YSZ coating (256-330μm) on 1Cr18Ni9Ti substrate is measured by URCAS, and the range of absolute error between ultrasonic method and metallographic observations is±10μm, the range of relative error±3%. 7. The density of Cr2O3 coating is determined by the relation of URCPS extreme and coating density. It is shown the microstructure of the Cr2O3 coating is inhomogeneous, and the ultrasonic measurement results agree well with SEM observation. Similarly, the density of YSZ coating the prepared by EB-PVD method is determined by the relation of URCPS extreme and coating density, and the relative error maximum of measurement results between ultrasonic and Archimedes method is 4.59%.8. The ranges of RFDC of as-deposited TBC and irradiated TBC by high-intensity pulsed ion beam (HIPIB) with beam densities 100 and 300 A/cm2 are determined by URCPS, and they are 0.80-0.85,0.85-0.88 and 0.91-0.93, respectively. Further, the attenuation coefficients of three kinds of coating are obtained by numerical fitting technique. The attenuation coefficient decreases with increase of beam density. These results indicate that the coating density and uniformity have increased after irradiation and those of coating after 300 A/cm2 irradiation increases most significantly. The ultrasonic results are in agreement with the SEM observation.更多还原