Ultrasonic characterisation of thermal barrier coatings

The elastic constants of zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) have been studied. Flat, self-standing and waterproofed samples were immersed in water between two 15 MHz transducers and irradiated at varying incidence. A theoretical transfer function was calculated for an unlimited isotropic plate with plane waves and the experimental transfer function was best-fitted. The average P and S waves velocities are 2560 m/s and 1710 m/s respectively and have variations lower than 3% over a wide refraction angle range (0-60 deg.). The corresponding average Young's modulus E is 33 Gpa. Thus, these TBCs may be well modelled by an isotropic material. Such a low E, compared to compact zirconia (241 Gpa), agrees with the high void content (15%) and with the presence of a microcrack network in APS TBCs. These results prove the feasibility of NDE of porous TBCs using ultrasonic SAW.