Nondestructive and contactless evaluation of surface coatings and adhesion defects by photothermal radiometry

For more than a double photothermal measurement techniques offer the suitability for a non-destructive and contactless evaluation of coating thicknesses and for the investigation of boundaries between a surface layer and its base material. In the following sections, we want to describe in detail the use of the photothermal radiometric technique (which is the most promising photothermal method for an in-line manufacturing control) on different interfaces embedded in an optically opaque workpiece. Additionally, we will demonstrate the effect of adhesion defects on the photothermal signals, or, vice versa, how the obstructed propagation of thermal waves can be explained by thermal contact resistances when delaminations or disbondings are located between a coating and its substrate. For this purpose, we mainly analyze very accurately the frequency dependence of the photothermal phase signal. Theoretical calculations and some experimental verifications on painted and on galvanically plated metal substrates will be presented. In summary, the photothermal measurement techniques allow to characterize subsurface adhesion defects possibly leading to a failure of a used component. Therefore we suggest that the photothermal instruments are usable for production control.