The long-term objective of this Phase II research effort was to develop and demonstrate a proof-of-concept for terahertz time-domain (THz-TD) reflection spectroscopy as a standardized in-situ non-destructive evaluation (NDE) methodology to accurately assess the durability of aircraft paint coating systems. The key technical question to be answered by this research was “Does THz-TD NDE methodology have sufficient sensitivity to measure thickness, complex refractive index, and/or reflectivity changes for early detection of failures due to corrosion in multilayer paint structures?”

The THz-TD sensor emits a near-single cycle electromagnetic pulse with a bandwidth from 0.1 to 3 THz. This extremely wide bandwidth pulse is focused on the coating and echo pulses are generated from each interface (air-coating, layer-layer, coating-substrate). The THz-TD method is able to penetrate the whole coating stack and extract the frequency dependent dielectric permittivity as well as thickness of each individual paint layer. In addition, the roughness of the substrate – which indicates the presence of corrosion under the paint layers – may be determined.

The THz-TD method is applied to multilayer coatings prior to and during accelerated salt-fog/humidity cycling aging on a series of multilayer paint test coupons. The results show that the corrosion of the metallic layer through the edges of the sample is a much faster process than the comparably slow degradation through the ultra-violet-rain erosion layers. The presence of coating degradation and corrosion can be detected using TD-THz NDE through a number of mechanisms. Prior to blistering of the paint stack surface (ie. prior to visible evidence of corrosion on the coating surface), the corrosion of the substrate layer can be detected by a reduction in the amplitude of the terahertz pulse which reflects from the substrate layer. Corrosion causes a roughening of the substrate surface which results in a reduction of the reflected pulse amplitude. Detachment of the paint layer stack (blistering) from the substrate may be sensed either by a reduction in the measured terahertz reflectivity or by the time-of-flight of reflected terahertz pulses which directly probe the topology of the paint stack outer surface.

SERDP is interested in technological approaches for improving early detection of failures in multilayer paint stacks. The THz NDE method can test multiple layer stacks as they are employed on the aircraft and provide, if needed, three-dimensional reconstructed images of the layers. As the paint layers corrode/degrade, the goal is to detect failures as early as possible in the paint layers. Based on the findings in this research, THz-TD NDE is capable of detecting corrosion below the coating surface before it becomes apparent on the outermost layer. The long-term goal of the research is to develop THz-TD as a standardized test methodology that can accurately assess the durability of paint layer coatings thereby reducing the environmental impact of maintenance over the lifecycle of Department of Defense weapon and vehicle system platforms. As an end goal, by testing the viability of coating material systems and only replacing coatings which are near failure, the volume of hazardous waste and air emissions which are required to strip and reapply the paint coating system can be greatly reduced resulting in a reduced lifecycle cost.