Abstract

Nondestructive methods previously developed for the detection of flaws, such as delaminations, or for the evaluation of the elastic properties of materials, could be judiciously adapted in order to get a better assessment of plasma-sprayed coatings. Laser ultrasonics is a nondestructive evaluation method which relies on both optics and ultrasonics. A short-pulse laser generates an ultrasonic wave into the material to be inspected, and a long-pulse laser, coupled to an interferometer, detects the resulting ultrasonic displacement. Laser ultrasonics is a remote sensing method and therefore could be used for the monitoring of hot plasma-sprayed coatings during the deposition process. In this work, experiments were performed on samples composed of ZrO2 sprayed under different conditions onto thick copper substrates. The samples were first probed by conventional ultrasonic transducers and then by a non contact laser-ultrasonic scanning system. The two series of measurements agreed well. These experiments showed that, after calibration, the coating thickness could be measured during the deposition process, with a relatively good accuracy, by laser ultrasonics. The laser-ultrasonic scanning system also revealed non uniformity of more than 10% in the coating thickness of the tested samples. This thickness variation is possibly caused by a temperature gradient induced in the coatings during spraying.

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