The long-term objective of this project is to develop Terahertz Time-Domain reflection spectroscopy as a standardized in situ non-destructive test methodology to accurately assess the durability of multilayer paint coating systems. The key technical question to be answered by this research is “Does THz-TD NDE methodology have sufficient sensitivity to thickness and complex refractive index changes for early detection of failures due to corrosion in multilayer paint structures?” The successful completion of this one-year project in conjunction with the experience and data from the first SERDP SEED project, WP-2520, will provide the data necessary to reduce the technical risk and uncertainty for a Standard Proposal which will be submitted near the conclusion of this project.

The Terahertz Time-Domain (THz-TD) method of multilayer paint coating measurement is a noncontact electromagnetic technique analogous to pulsed-ultrasound with the added capability of spectroscopic characterization. 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 pulse is able to penetrate the whole coating stack and sample the properties of each layer. Using an analysis code which extracts from the measured reflected pulse the thicknesses and permittivity and permeability of each paint layer, THz-TD non-destructive evaluation may be used to detect changes in the paint layer coatings which result from corrosion/degradation/cracking of the paint layer stack. This one-year project will include the following tasks:

• Task 1: Develop time-domain analysis of reflected waveforms to extract layer thickness and refractive indices.
• Task 2: Measure THz-TD reflectivity on pristine (uncorroded) multilayer samples.
• Task 3: Implement new corrosion protocols and accelerate aging of samples.
• Task 4: Measure THz-TD reflectivity on samples as a function of aging.
• Task 5: Analyze reflected THz-TD waveforms using methods of Task 1.
• Task 6: Compare analysis results as a function of aging. Is there conclusive evidence that degradation of multilayer paint stack leads to measurable change in THz-TD waveform?
• Task 7: Create standardized data interfaces to support digital twins of the paint samples affected by degradation processes.

The benefit of this research program is to develop a non-destructive evaluation method that can test multiple paint layer stacks as they are employed on the aircraft and provide – if needed – 3D 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. 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 (DoD) weapon and vehicle system platforms. As part of this Phase II SEED project, the research will implement new accelerated aging methodologies in conjunction with THz-TD testing. Using these new testing standards, which will eventually be used to establish realistic life and durability expectations for paint coatings, this project will determine how corrosion/degradation is manifested in the THz-TD testing standards. 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. Lockheed Martin Advanced Technologies Laboratories, New Jersey Institute of Technology’s partner in this research, will use the data developed under this contract to proceed further with digital twin development for sustainment of DoD platforms such as the F-35 and F-22 aircrafts.