基于功率谱分析的表面涂层厚度超声无损测量方法

The Method of Ultrasonic Thickness Measurement for Surface Coatings Materials Based on Spectral Analysis

针对厚度小于1 mm的薄膜或涂层材料,研究了一种超声测厚信号处理方法.当超声波在两层或多层介质中传播时,不同界面的回波信号会相互叠加并发生干涉.当超声波在厚度L、声速ν、声阻抗Z2的介质(介于声阻抗分别为Z1和Z3的两介质之间)中传播,由于界面回波的叠加与干涉,回波信号的功率谱在频率为f0=ν /4L的奇数或偶数倍处出现周期性极值点.因此,当声速ν已知,待测试样的厚度L可由关系式L=ν /2△f 来求得.其中,△f是相邻两极值的频率间隔.根据该原理,采用水浸聚焦脉冲回波超声检测方法,对镍基高温合金基体上的ZrO2热喷涂涂层(270～400 μm)进行了厚度测试,试验结果与金相法测试结果相符.

The signal processing method of ultrasonic thickness measurement has been investigated for the film or coating materials less than 1mm. When ultrasonic waves are reflected at different interfaces, they interfere with each other. Provided there is a medium with thickness L, sound velocityν, acoustic impedance Z2 between two mediums with acoustic impedances Z1 and Z3, and depending on the interference of echoes from different interfaces, the interference phenomena is observed in power spectrum: the periodic extremums appear at frequencies of odd or integral multiples of the fundamental frequency f0 equal toν/4L. So, when the value ofνis known, the medium thickness L can be calculated by relation L=ν/2△f, where △f is intervalbetween periodic extremums frequencies. Based on this principle, pulsed-echo method was used to test the thickness of sprayed ZrO2 coatings (270~400 μm) on Ni-based superalloy by spectral analysis technique, the experimental results accorded with those of cross-section measurement.