The Terahertz Time Domain Reflection Spectroscopy (THz-TDS) method of paint layer diagnostics is a non-contact
electromagnetic technique analogous to pulsed-ultrasound with the added capability of spectroscopic characterization.
The THz-TDS sensor emits a near-single cycle electromagnetic pulse with a bandwidth from 0.1 to 3 THz. This wide
bandwidth pulse is focused on the coating, and echo pulses are generated from each interface (air-coating, layer-layer,
coating-substrate). In this paper, the THz-TDS method is applied to specialty aircraft coatings. The THz-TDS method is
able to penetrate the whole coating stack and sample the properties of each layer. Because the reflected pulses from
individual layers typically overlap in time, the complex permittivity function and thickness of each layer is determined
by a best fit of the measured reflection (either in time or frequency domain) to a layered model of the paint. The THz-
TDS method is applied to specialty coatings prior to and during accelerated aging on a series of test coupons. The
coupons are 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. In addition, the same samples are characterized using THz-TDS techniques to determine if
the THz-TDS method can be utilized as a non-destructive evaluation technique to sense degradation of the coatings. Our
results suggest that the degradation mechanism begins in the top coat layer. In this layer, 254 nm UV illumination in
combination with the presence of moisture works partially with oxides as catalysts to decompose the polymer matrix
thereby creating porosity in the top coat layer. Since the catalytic effect is partial, loss of the oxides by chemical reaction
can also occur. As the topcoat layer becomes more porous, it allows water vapor to permeate the topcoat layer and
interact with the rain erosion layer via carbonization of the polymer matrix in the rain erosion layer. The presence of the
salt accelerates the pitting degradation. The goal of this paper is to determine if THz-TDS can be used to sense
degradation of the coating.
Carley R. Nicoletti, Laura Cramer, Alan Fletcher, David Zimdars, Zafar Iqbal, and John F. Federici, "Non-destructive evaluation of specialty coating degradation using terahertz time-domain spectroscopy," Proc. SPIE 10206, Disruptive Technologies in Sensors and Sensor Systems, 102060W (Presented at SPIE Defense + Security: April 12, 2017; Published: 2 May 2017); https://doi.org/10.1117/12.2258027.
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