Analysis of laser-induced vibrations to detect non-adhesive regions of coatings

A method is presented to find areas in coatings, which show no adhesion to the substrate. The combination of a Q-switched Nd : YAG laser as a non-destructive excitation tool and a laser-doppler-vibrometer as a contact-free measuring device was found to be useful for gaining information about qualitative adhesion conditions. We made use of two different mechanisms to excite the samples: direct irradiation generates thermoelastic waves, whereas ignition of a breakdown in air leads to shock wave generation and indirect excitation of the sample. We give evidence that the vibrations of areas that show no adhesion to the substrate correlate with membrane vibrations. In addition we show how the Mirage effect becomes visible in our experimental setup.