Abstract
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.
Highlights
There are many examples of functional thin surface ilm applied to engineering components such as; protective coating layers on automobiles, marine and submarine vessels, solid lubricant coatings on bearings, the oil distribution around a gearbox casing, the oil ilm that forms ahead of an approaching piston liner, diffusion coatings on jet engines, thermoset epoxy resins and ibre-reinforced plastics that coat storage tanks and pipes in chemical plants
Where the surface layer is relatively thick in comparison to the solid to which it is adhered to, usually, the surface layer thickness is inferred from the time delay between the echoes relected off the interface of interest and the surface layer
A signiicant temperature change in both the metal solid and the surface ilm would result in linear thermal expansion which would in turn affect the solid resonance peak locations
Summary
There are many examples of functional thin surface ilm applied to engineering components such as; protective coating layers on automobiles, marine and submarine vessels, solid lubricant coatings on bearings, the oil distribution around a gearbox casing, the oil ilm that forms ahead of an approaching piston liner, diffusion coatings on jet engines, thermoset epoxy resins and ibre-reinforced plastics that coat storage tanks and pipes in chemical plants. When a normal incident wave strikes a surface, some of the energy will be transmitted into the neighbouring component while the remaining energy will be relected towards the wave source. When a bulk ultrasound wave of a known frequency, f strikes a boundary between two materials at normal incidence, relection occurs. If a sound wave is relected at a boundary between an acoustically dense medium to an acoustically denser medium, i.e. the limit z1/z2 → 0, the relected wave has the same amplitude as the incident wave with no phase change. If it is relected vice versa, i.e. z1/z2→∞, Ø=π
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