Kanja J, Mills R, Li X, Brunskill H, Hunter A K, Dwyer-Joyce R S
The Leonardo Centre for Tribology, University of Sheffield, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD, UK.
The Leonardo Centre for Tribology, University of Sheffield, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD, UK.
Ultrasonics. 2021 Feb;110:106291. doi: 10.1016/j.ultras.2020.106291. Epub 2020 Oct 26.
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.
大多数用于测量薄液膜或固体表面薄膜及涂层厚度的方法都需要接触被涂覆表面。在这项工作中，反射超声波脉冲被用于从固体背面测量涂层厚度。固体背面的压电换能器发射超声波，并接收从正面反射回来的波。反射波的幅度取决于正面的薄膜厚度。大多数脉冲回波超声方法利用穿过表面层的飞行时间来确定其厚度。然而，随着薄膜变薄，反射回波会重叠，并且固体与表面薄膜之间常常存在声学失配，这会降低信号强度。在这项工作中，我们提出使用超声连续重复啁啾纵波来增强表面薄膜的效应。多次反射在固体内干涉形成叠加驻波，其幅度谱高度依赖于表面薄膜厚度，从而克服了声学失配问题。两个裸露的10MHz压电元件以一发一收的布置方式粘结到一块10mm厚的铝固体上，使得一个连续发送重复的啁啾超声波，另一个作为接收器。发射器被设置为发送持续时间为4ms的重复啁啾波，该波对应于换能器的带宽，以便使信号幅度最大化。入射波和反射波相长干涉和相消干涉，在固体内形成叠加驻波。固体/表面薄膜到固体/空气边界条件的频谱比显示薄膜共振频率模式为最小值。使用该技术对70μm至350μm的环氧涂层进行了测量，结果表明与使用表面轮廓仪的独立测量结果具有良好的相关性。
