SAW for the non destructive evaluation of thermal barrier coatings

Surface acoustic waves (SAW) velocity measurements were used to characterise zirconia thermal barrier coatings (TBC's). The thickness of air plasma-splayed (APS) TBC's deposited on metallic plates ranged from 200 mu m to 500 mu m. A SAW probe prototype based on J.D. Achenbach's ultrasonic bridge was designed using two SAW transducers. The probe generated and detected both SAW and P-waves in the coatings. The coupling between the probe and the TBC surface was dry and damageless. The performance of the SAW probe was assessed with samples of known isotropic materials. The accuracy on measured velocities was better than 0.5%. Due to the high porosity content of the TBC's and surface roughness, SAW attenuation was high and velocity dispersion occurred. A wave propagation model assuming an isotropic material was required. The SAW velocities in TBC's appear to be much lower than the one in compact zirconia, confirming the influence of inclusions on the elastic constants.