Most methods used to measure the thickness of thin liquid or solid surface films and coatings need access to the coated surface. In this work reflected ultrasonic pulses were used to measure a coating thickness from a solid back face. Piezoelectric transducers on the solid back face emitted ultrasound waves and received the waves that bounced off the front face. The magnitude of the reflected wave was dependent on the film thickness at the front face. Most pulse-echo ultrasonic approaches use the time-of-flight through the surface layer to determine its thickness. However, as the film becomes thinner, the reflected echoes overlap and there is often an acoustic mismatch between the solid and the surface film that reduces the signal strength. In this work, we propose the use of an ultrasonic continuously repeated chirp longitudinal wave to amplify the effect of the surface film. Multiple reflections interfere within the solid to form a superimposed standing wave whose amplitude spectrum is highly dependent on the surface film thickness thus overcoming the acoustic mismatch problem. Two bare 10 MHz piezoelectric elements were bonded to a 10 mm thick aluminium solid in a pitch-catch arrangement such that one continuously sends repeating chirp ultrasound waves and the other acts as the receiver. The transmitter was set to send a repeating chirp wave of 4 ms duration corresponding to the bandwidth of the transducer in order to maximise signal amplitude. The incident and reflected waves constructively and destructively interfere to form a superimposed standing wave within the solid. The solid/surface film to solid/air boundary condition frequency spectra ratio showed the film resonant frequency modes as minima. Using this technique epoxy coatings ranging from 70 μm to 350 μm were measured and showed a good correlation with independent measurements using a surface profilometer.

中文翻译：

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使用叠加超声波驻波非接触式测量表面膜的厚度

大多数用于测量液体或固体表面薄膜和涂层厚度的方法都需要接近涂层表面。在这项工作中，反射超声波脉冲用于测量固体背面的涂层厚度。固体背面的压电换能器发射超声波并接收从正面反弹的波。反射波的大小取决于正面的薄膜厚度。大多数脉冲回波超声方法使用通过表面层的飞行时间来确定其厚度。然而，随着薄膜变薄，反射回波重叠，固体和表面薄膜之间通常存在声学不匹配，这会降低信号强度。在这项工作中，我们建议使用超声波连续重复的啁啾纵波来放大表面膜的效果。固体内的多次反射干涉形成叠加的驻波，其振幅谱高度依赖于表面膜厚度，从而克服了声学失配问题。两个裸露的 10 MHz 压电元件以一发一收的方式连接到 10 毫米厚的铝固体上，这样一个连续发送重复的啁啾超声波，另一个充当接收器。发射器设置为发送持续时间为 4 ms 的重复啁啾波，对应于换能器的带宽，以最大化信号幅度。入射波和反射波相长和相消干涉，在固体内形成叠加的驻波。固体/表面膜与固体/空气边界条件频谱比显示膜共振频率模式为最小值。使用该技术测量了 70 μm 至 350 μm 的环氧树脂涂层，并显示出与使用表面轮廓仪进行独立测量的良好相关性。