Résumé
The aim of this experimental work is to provide a non destructive evaluation tool for quality control on paint films used for industrial purpose after deposition and in operation. The most sensitive conditions have to be found for ultrasonic inspection and the measured acoustic parameters have also to be linked to the state of the complex chemical structure of polymers. Ultrasonic reflectometry is tested to investigate the thermal ageing of epoxy coatings. As function of frequency and incident angle, the most sensitive modes have been identified from simulation and validated by experiments using an ultrasonic reflection goniometer system working around 5 MHz. Scanning Acoustic Microscopy has been complementary used to check the bonding quality and the layer homogeneity. The effect of local variations of the coating thickness on the sensitivity of this acoustic reflection method is presented as well as acoustic beam diffraction limitation. Next, these 100µm thick paint films on steel plates have been submitted to thermal ageing tests up to 110°C for several weeks. Evolutions of the acoustic angular reflection coefficient versus exposure time are presented with the corresponding IR spectra variations to compare the chemical coating oxidation kinetics with the sensitivity of ultrasounds to elastic properties.