Nondestructive evaluation of multilayered paint films in ambient atmosphere using terahertz reflection spectroscopy

Highlights

• The reliability of layered material evaluation with terahertz (THz) waves is easily compromised.

• We suggest a THz signal decomposition scheme to analyze angled THz reflection measurements in ambient atmosphere.

• The extracted spectrum from each decomposed THz signal enables individual identification of each constituent layer.

Abstract

The reliability of material characterization with terahertz (THz) waves is typically compromised by irrelevant signals present in the THz measurements. These are well observed in the investigation of layered materials in humid environments, causing difficulty in accurate analyses. In this article, we suggest a realistically employable method of analyzing angled THz reflection measurements in ambient atmosphere for qualitative monitoring and evaluation of multilayered paint films. In particular, we present successive decomposition of THz sample signals for individual identification of each constituent layer in the samples. It is shown that the extracted spectrum of each paint layer presents sufficient information for effective monitoring of the multilayered structures. In this process, THz spectra of the marine paints EH2350, Interthane 989, BEA777, and ENA300 are reported for the first time. Our experimental findings show promise in highly effective nondestructive inspection of layered structures for real-world evaluation applications.

Introduction

Pulsed terahertz (THz) broadband systems have become remarkably important tools for the investigation of numerous materials over frequencies from a few hundred GHz to several THz. Significant developments in THz science [1], [2], [3] have enhanced the performance of material characterization and triggered extensive applications in security [4], medicine [5], semiconductors [6], quality control [7], [8], etc. In particular, an area that would considerably benefit from these advancements is nondestructive evaluation of various materials in industrial applications [9], [10], [11], [12]. However, the inspection results are generally affected by various artifacts present in the THz measurements, such as electronic noise, signal fluctuation from atmospheric water vapor absorption [13], and echo signals arising from internal reflections from the sample or system components [14]. These complications are frequently risen in analyses of layered materials in ambient atmosphere, hence, causing hardship in accurate characterization.

In the last decade, several authors proposed insightful approaches to overcome these obstacles. The temporal peak analysis [8] and the inverse electromagnetic problem [15], [16], [17] are most commonly used, but they require several reflections of the THz pulse and are typically applicable only to homogeneous, low absorption, and single-layered materials. THz signal manipulation techniques, such as the removal of multiple echo signals [18], [19], [20] and signal reconstruction from atmospheric attenuation [21], [22] are greatly competent in gaining fluctuation-free THz signals but face complex signal processing procedures and inevitably possess the possibility of data alteration. The recent dispersion model-based methods [23], [24] provide extremely accurate material analyses, but sometimes require prior knowledge about the sample or undergo necessary parameterization schemes. Therefore, these methods are mostly valid only in limited situations and still remains relatively unused in real-world applications.

In our earlier report [25], we have introduced a simple and intuitive model-based approach that can be universally applied for the decomposition of THz transmission signals. The method showed distinctive advantages in that it did not require any prior information of the sample under investigation or incorporate removal/reconstruction techniques of any kind. Here, we extend the findings from our previous report and present qualitative monitoring and evaluation of materials with layered structures. Particularly, we suggest a realistically employable method of analyzing angled reflection measurements, the most feasible way of nondestructively testing various samples. The article encompasses the investigation of multilayered paint films, verification of the THz signal decomposition algorithm in uncontrolled temperature and humidity settings, and THz reflection time-domain spectroscopy. A brief description and background of the proposed method is given, followed by experimental results and comprehensive performance assessment.