Quantification of Plasma Sprayed Coating Adhesion Using Pulsed Laser Induced Decohesion Technique

Abstract

The aim of the present study is to compare a laser ultrasonic technique with a conventional indentation test for the determination of intrinsic properties and the adhesion of alumina coatings, of different thicknesses (30–350 µm), deposited on stainless steel substrates by atmospheric plasma spraying (APS). For this purpose, a pulsed Nd:YAG laser is used to irradiate the coated specimens, and the ultrasonic waves generated by the laser are recorded at the epicenter using a laser interferometer. In the thermoelastic regime, the good agreement between the experiment and computation allows determination of the longitudinal wave velocity as well as the Young's modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, breakdown at the coating/substrate interface occurs. An analytical model connected the acoustic waveforms with the length of the interfacial cracks and the time delay of the debonding. The critical tensile stress at the interface is calculated using a numerical model, and the practical adhesion defined by the laser technique was compared with indentation test results.