Thermal wave imaging and ultrasonic characterization of defects in plasma sprayed coatings

One of the primary concerns in thermal spray coatings is bond integrity, as disbonds at the coating/substrate interface could cause premature failure of a component. Current quality control test practices use empirical destructive mechanical test methods that do not provide direct correlation between test results and coating performance. Nondestructive evaluation (NDE) techniques used for bulk materials are not readily adaptable for the evaluation and characterization of coatings. This paper reports on the use of thermal wave imaging and ultrasonic immersion C-scan to characterize artificial disbonds in plasma spray coatings on steel substrates. This work demonstrates the capability of these techniques in characterizing total disbonds and presence of “kissing bonds” (where the interface surfaces are in such close contact that they give a appearance of a good bond). The effects of ultrasonic frequency on the C-scan images were also studied. A critical evaluation of both thermal wave imaging and ultrasonic techniques is presented along with their relative advantages and disadvantages.