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Hard facing alloys like Colmonoy and Stellite are overlaid on nuclear components to provide corrosion and wear resistance under high temperature service conditions. For nuclear applications especially for components used inside the reactor, Colmonoy overlays are preferred over Stellite. A Silver-420 system with pulsed and lock in thermography was used for the study. It can be concluded that lock in thermography is a sensitive technique for detection of debonds in colmonoy overlays. The main advantage as compared to other active thermal imaging techniques is that emissivity and surface variations don't manifest and the phase angle variation is a clear indicator of the defective regions. Compared to ultrasonics, the main advantage of lock in thermal imaging is that it is a whole field technique and an entire area can be covered in a single step. The examination time is also quite small. No surface preparation or cleaning is required. It gives a visual image that can be evaluated and the nature and size of the defects determined. However, the technique is limited by the material thickness that can be examined.