Multi-layer paint coating stack-ups (MPCS) on military platforms have required significantly more maintenance than conventional platforms. As an example, this maintenance results in a 14-month period to strip and recoat B2 aircraft every seven years. Compared to conventional aircraft coatings, the removal and reapplication of MPCS requires large quantities of Hazardous Air Pollutants and Volatile Organic Compounds thereby generating a significant hazardous waste stream in the process: roughly 80,000 lbs of hazardous waste per aircraft, along with 400 gallons of chemical stripper. The frequency and extent of such depot maintenance could be reduced with accurate knowledge of MPCS durability. Using periodic in situ non-destructive monitoring of the MPCS on each unique aircraft, aircraft MPCS could be refurbished on an ‘as needed only’ basis (i.e., condition-based maintenance) thereby minimizing the hazardous waste stream which is generated by removing and reapplying MPCS. In addition, unexpected failures of MPCS in the field have proven that the fundamental degradation mechanisms MPCS have not been well understood.

The long-term objective of this project was to develop and demonstrate a proof-of-concept for TD-THz reflection spectroscopy as a standardized in situ non-destructive test methodology to accurately assess the durability of MPCS.

The Time-Domain Terahertz (TD-THz) method of MPCS measurement was a non-contact electromagnetic technique analogous to pulsed-ultrasound with the added capability of spectroscopic characterization. The TD-THz sensor emits a near-single cycle electromagnetic pulse with a bandwidth from 0.1 to 3 THz. This extremely wide bandwidth pulse was focused on the coating, and echo pulses are generated from each interface (air-coating, layer-layer, coating-substrate). The TD-THz method was able to penetrate the whole MPCS and sample the properties of each layer. Because the individual layer reflections were typically overlapping in time, the dielectric and magnetic properties of individual coating layers and substrates as well as their thicknesses could be extracted using an appropriate multilayer model transfer function which describes the multiple reflections within a paint layer stack. Unlike ultrasound, however, no contact was required of the emitter with the coating. While reflection TD-THz spectroscopy was conceptually similar to ultrasound methods to determine the subsurface dimensions, it has the added benefit of measuring spectral information which can be used to sense degradation in the MPCS.

The first goal of this work was to perform accelerated aging lifecycle sequences on a series of test coupons with different MPCS. The coupons were tested during aging using standard testing methods such as electrochemical, FTIR spectroscopy, SEM, Raman Scattering, cross-sectional thickness, durometer and density measurements to ascertain, quantify, and understand the breakdown mechanisms of the coatings. The second goal was to test the same samples using TD-THz techniques and develop the testing and data analysis methods which are sensitive to the degradation of MPCS. The final goal was to correlate the ‘standard’ diagnostic tests with the TD-THz tests and develop a statistical function which predicts key MPCS figures of merit from the non-destructive in-situ TD-THz measurements.

The TD-THz method was applied to MPCS prior to and during accelerated aging on a series of test coupons to determine if the TD-THz method can be utilized as a non-destructive evaluation technique to sense degradation of the MPCS. The coupons were also examined during aging using ATR (attenuated total reflectance)-FTIR spectroscopy, Raman scattering spectroscopy, and Scanning Electron Microscopy (SEM) to ascertain, quantify, and understand the breakdown mechanisms of the coatings. The results suggested that the degradation mechanism begins in the top coat layer. In this layer, 254 nm UV illumination in combination with the presence of moisture worked partially with oxides as catalysts to decompose the polymer matrix thereby creating porosity in the top coat layer. Since the catalytic effect was partial, loss of the oxides by chemical reaction could also occur. As the topcoat layer became more porous, it allowed water vapor to permeate the topcoat layer and interact with the rain erosion layer via breakdown of the polymer matrix in the rain erosion layer. The presence of the salt accelerated the pitting degradation. 

The use of TD-THz NDE to detect the presence of degradation has mixed results. In the case of initial defects in the MPCS, THz radiation could detect the presence of the defects prior to advancing degradation. For blisters which form in the coating after degradation, TD-THz NDE could detect the presence of the formation of the blisters due to the increased time delay between pulses which reflect from the front paint layer of the blister and the back substrate.

Other than the ‘bubble’ defects (an initial defect in the coating) or a blister defect (which is induced by aging), there was not convincing evidence that TD-THz NDE frequency domain analysis as implemented can detect the onset of the MPCS degradation in regions of ‘good quality’ coatings (ie. regions where blisters do not form). The absolute deviation in the imaginary refractive index with aging was larger than the deviation in the real index with aging which is suggestive that the imaginary refractive index could potentially be a good figure of merit, but the noise in the measured values makes a conclusive statement difficult. In the near future, the analysis could be improved using a multilayer model for the reflectivity in the time domain. Using that model, which was originally developed to determine the thickness and refractive index of drying paint layers, should exhibit several improvements over the frequency domain analysis described in this report.

At this point, with the frequency domain analysis presented in this report, development and deployment of a TD-THz evaluation system for MPCS was premature. More research and improved data analysis will be performed to clearly demonstrate the capabilities of the technique.

The long-term benefit of this work to Department of Defense (DoD) is the development of a standardized in situ non-destructive test methodology to accurately assess the durability of MPCS systems. There are three current gaps in the understanding and characterization of MPCS. The first gap is the lack of a fundamental understanding of the coatings’ degradation mechanisms resulting in failure of MPCS prior to the end of their predicted life. The second gap is a lack of standardized testing methodologies which need to be adopted by both the DoD and OEMs. The third gap is the diagnostic instrumentation which could characterize the MPCS as they age in the field to determine a go/no go decision on depot maintenance. An improved understanding of the degradation methods coupled with appropriate standardized accelerated aging methodologies and diagnostic non-destructive instrumentation to detect degradation of the coating will result in a realistic understanding of the service life of MPCS. If the coating health state were measured periodically in-situ on each unique aircraft using a non-destructive method, aircraft MPCS could be refurbished on an ‘as needed only’ basis thereby minimizing the hazardous waste stream which is generated by removing and reapplying MPCS.