Dry Coupling Ultrasonic High Frequency (10–100 MHz) Sensors for Detection of Surface and Tribological Properties at a Submicrometric Scale

Abstract

Acoustic plane sensors adequately pressed against materials are suitable to measure elastic constants from flight-time measurement, without any coupling fluid for high temperature, with longitudinal and transverse acoustic waves in the megacycle range. Soft delay lines are generally used to match the sample roughness and to make mechanical play correction easier. At the opposite, our sensors use a hard delay line with a mirror-polished end surface. An experimental set-up is presented to perform acoustic reflection measurement in various contact conditions by increasing the applied mechanical load. The frequency dependence of this parameter is also measured in the 10- to 100-MHz range. Reproducibility tests are presented to validate this experimental set-up, but the main results concern the surprising ability of this technique to detect surface property modifications limited to thickness less than 1 μm. Indeed, surface modification induced by different solvents on glass substrates has been detected by this means. This technique has also been used to detect surface property modifications of lixiviated glasses. In this case, atomic force microscopy and inductively coupled plasma analyses have demonstrated that the earlier stage of the surface damage had been detected whereas the thickness altered by ionic diffusion was less than 100 nm with almost no roughness variation. Similarly, tests on mechanically scratched glasses have shown that samples with an average roughness, respectively, of 4 and 120 nm were easily identified from their reflection coefficient versus load curves. Moreover, the pressure dependence of the acoustic reflection is used to estimate the contact stiffness and the contact area between the sensor and the material as a function of the applied compressive stress for contact, adhesion, and friction investigations.