APPLICATION OF ULTRASONIC RAYLEIGH-WAVES TO THICKNESS MEASUREMENT OF METALLIC COATINGS

The velocity of Rayleigh waves propagating in a layered medium depends on the frequency of the waves, the thickness of the coating and the properties of both the coating and the substrate materials. Rayleigh waves of various frequencies were generated using a broadband pulse and their velocities were measured as a function of the frequency and compared to the theoretical dispersion curve of the specimen. The thickness of the layer was deduced from comparison between these two curves. A special transducer was designed in order to achieve these measurements. Experiments were carried out on AISI 316L specimens coated with vacuum plasma sprayed (VPS) NiCoCrAlY of various thicknesses (190-330 mu m) and various surface states. A good correlation was obtained between the thicknesses measured by means of ultrasonics and those obtained using photomicrographs of the cross-section of the specimens. The elastic characteristics of the coating and the substrate materials were measured using a transmission method for parallel-faced plates (disbonded samples of coating material). The velocity of the shear and compression waves propagating in various directions is obtained by changing the angle of incidence on the sample surface. A small anisotropy of the coating material was found. Finally, we demonstrated the efficiency of the use of surface waves for the detection of surface breaking cracks and disbonding at the interface between the coating and the substrate.