Measurement of lubricant film thickness using normal incidence ultrasound

The study of the reflection of ultrasonic waves from a solid-solid interface can be used to measure the thickness between two solid interfaces. This paper describes the theoretical analysis and experimental research of the lubricant film thickness measurement through using normal incidence ultrasonic reflection. The proportion of the incident wave which is reflected at the interface was measured as a function of frequency with a single wideband ultrasonic transducer. With the thickness of the lubricant film increasing, the measured reflection coefficient decreases. According to the thickness of the lubricant film, the resonant model or the spring model could be used in the measurement. When the wavelength of the ultrasound is large compared to the thickness of the lubricant film, it is generally not practical to test the frequency, then, the film thickness measurement need to use a spring model.