Non-destructive evaluation of uneven coating thickness based on active long pulse thermography

Non-destructive detection of coating thickness is important after coating deposition and during service. The present work is to establish a measurement method for fast inspection, large area detection of coating thickness with an active long pulse thermography. The measurement method was theoretically analyzed based on the equation of 1D heat transfer in the depth direction, accordingly coating thickness can be quantitively evaluated by recording the temperature decay curve in the cooling stage and computing the time at the minimum of its 2nd derivative. Then, the proposed method was experimentally validated by uniform coating specimen with different thicknesses. Furthermore, the thickness measurement method was successfully applied to measure thickness of an uneven coating specimen within a relative error of 5% with sufficient sampling rate. Finally, some key techniques were discussed for accurate measurements including thermal excitation, sampling rate of thermography, thickness ratio of coating and substrate, etc. The results verify that active long pulse thermography is a powerful tool for non-contact and full-field measurement of coating thickness with merits of safe and easy to implement.