Evaluation of the coating–substrate adhesion by laser-ultrasonics: Modeling and experiments

Abstract

The aim of the present study is to investigate a laser-ultrasonic method to determine the oxide-coating’s adhesion strength on a metallic substrate. In order to demonstrate this method a completely contactless system is used, combining laser generation and detection of ultrasonic waves. A nanosecond pulsed Nd:YAG laser is used to irradiate the transparent oxide coatings, while the normal displacement of the rear surface of the metallic substrate is detected at the epicenter by a laser heterodyne interferometer. Increasing the laser beam energy, the generated acoustic wave forms are correlated with the thermoelastic regime, the fracture of the coating–substrate interface, and finally the expulsion of the coating. The generation of ultrasonic signals in the thermoelastic regime is described by a two-dimensional model suitable to calculate the in- and out-of-plane components of the mechanical displacement versus time. The values of the so-called practical adhesion found are in agreement with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). This work demonstrates the suitability of this quantitative and contactless test to evaluate the coating–substrate adhesion.