Characterisation of Colmonoy Coatings using Lock-In Thermography

Hard facing alloys like Colmonoy are overlaid on nuclear components to provide corrosion, wear and galling resistance under high temperature service conditions. Typical reactor components on which overlays are provided include grid plate, diverse safety rod drive mechanism etc. The thickness of the overlay ranges from 0.5-3 mm. Typical defects likely during deposition include lack of bonding between substrate and overlay, cracks and porosities. Conventionally ultrasonic, LPT and radiography has been used for the process qualification and on the actual components. It is well established that thermal imaging can be used for detection of detects especially lack of bonding and materials characterization. Conventionally active techniques such as pulsed thermal imaging have been employed. This paper highlights the successful application of lock-in thermography for characterization of colmonoy coatings. The main advantage of lock in technique is that it is unaffected by emissivity variations and also defect depth can be estimated.