Quantitative Thermal Diffusivity Imaging of Disbonds in Thermal Protective Coatings Using Inductive Heating D

Problems of adherence of thermal protective coatings to aerospace structures have become important with their use on vehicles such as the space shuttle and for jet engine turbine blades. In these structures a dielectric coating which acts as a thermal barrier. is bonded to a conductive substrate. Failure of these coatings to adhere to the substrate can result in catastrophic failure of the vehicle. Thermal diffusivity is a parameter that can be used to assess the degree of adherence of coatings to a substrate. In this work. an inductive heating technique for making therrral diffusivity images of disbands between thermal protective coatings and their substrates is described. Any flaw in the bonding of the coating and the substrate appears as an area of lowered values in the diffusivity image. The advantages of the inductive heating approach lie in its ability to heat the conductive substrate without directly heating the dielectric coating. The substrate itself then, acts as the heat source, increasing the sensitivity of the thermal images to disbands when compared to the case when heat must travel through the coating. to the substrate. and then back to the coating surface to be imaged. This heating technique also has the advantage that it can be non-contacting. and can be used on the fro!lt face of structures, whose interiors are inaccessable. Results are given for a series of samples with fabricated disbands. for a range of coating thicknesses.