What noncontact methods are used to measure cured or uncured adhesive bond primer thicknesses?
The Defense Systems Information Analysis Center (DSIAC) received a technical inquiry requesting information on available options for noncontact thickness measurements of a mostly transparent adhesive primer. The inquirer specified that it would be preferable if the solution were handheld, but a desktop device was also acceptable. DSIAC subject matter experts from Texas Research Institute Austin drew upon their extensive experience in noncontacting and nondestructive measurements to provide answers and insight on this topic. Two methods, terahertz wave imaging and ellipsometry, are described briefly before a list of commercial options for bond thickness measurements.
Defense Systems Information Analysis Center (DSIAC) subject matter experts (SMEs) from Texas Research Institute Austin (TRI Austin), including author Doyle T. Motes, P.E., drew upon their extensive experience in noncontacting and nondestructive measurements to provide insight on the inquiry topic.
2.0 SME Response
The most common suggestion was to use eddy current (EC) or ultrasonic testing (UT) methods. However, the tolerances on typical EC or UT instruments available in a nondestructive evaluation lab and conventional UT and EC thickness gauges have an accuracy that is approximately one tenth of what is required by the inquirer (i.e., it is only capable of resolving a coating thickness of 0.001 inch or 1 mil). It was suggested that the dimpling from the UT instrument may be eliminated with hydrophilic plastic. In addition, for an ultrasonic measurement, the frequencies likely need to be very high for the sound waves to interact substantially with the coating; however, this depends on the properties of the coating. A noncontacting measurement is possible with ultrasonics, but it requires an immersion or a squirter system (which may not be appropriate for an uncured bond). Terahertz (THz) wave imaging and ellipsometry were identified as potential options to address the problem.
2.1 THz Wave Imaging
THz imaging has been used in a pulse-echo c-scan configuration to map variations in the peak amplitude of the echo off the metal surface. The variations occur when the attenuation from the coating affects the signal [1, 2]. This configuration is very similar to UT signal analysis, but the microwave signals do not penetrate the metallic substrate. Gating is used for signal analysis to determine the peak amplitude, although the data acquisition must be capable of sub pico-second resolution (as shown in Figure 1). The reflection-mode THz inspection system identified in Figure 1 is used for performing inspections on the National Aeronautics and Space Administration’s Space Shuttle external tank thermal protection system .
Figure 1: Basic Schematic of a Reflection-Mode THz Inspection System (Source: NASA).
In ellipsometry, the measured signal is the change in polarization as the incident radiation (which is transmitted in a known and characterized state) interacts with the material structure of interest (causing some of the light to be reflected, absorbed, scattered, or transmitted), as shown in Figure 2. The polarization change is quantified by the amplitude ratio and phase difference. The signal depends on the thickness as well as the material properties; therefore, ellipsometry can be a universal tool for contact-free determination of thickness and optical constants of various films .
Upon analyzing the change in the light’s polarization, the ellipsometry method can yield information on layers that are thinner than the wavelength of the probing light itself, even down to a single atomic layer. Ellipsometry can probe the complex refractive index or dielectric function tensor, giving access to fundamental physical parameters. It is commonly used to characterize film thicknesses for single layers or complex multilayer stacks ranging from a few angstroms or tenths of a nanometer to several micrometers, with an excellent accuracy .
3.0 Commercially Available Options
There are several commercially available options for performing coating thickness measurements, which have various degrees of precision. Some of the options may not be suitable for the inquirer’s applications, but the following are immediately available to commercial customers:
- QNix’s coating thickness gauges .
- DeFelsko’s paint thickness measurement .
- Elcometer’s coating thickness gauge .
- PCE Instruments’ coating thickness gauge .
Other resources available to researchers interested in determining coating thicknesses can be found online; the article “Quality 101: Understanding Coating Thickness Measurement Test Methods” by P. Lomax may provide additional information . Nondestructive measurement methods endorsed by the American Society for Testing and Materials (ASTM) are identified in the document Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals .
 Roth, D. J., J. P. Seebo, and W. P. Winfree. “Simultaneous Noncontact Precision Imaging of Microstructural and Thickness Variation in Dielectric Materials Using Terahertz Imaging.” National Aeronautics and Space Administration, https://ntrs.nasa.gov/archive/nasa/ casi.ntrs.nasa.gov/20080015440.pdf, March 2008.
 Roth, D. J., L. M. Cosgriff, B. Harder, D. Zhu, and R. E. Martin. “Self-Calibrating Terahertz Technique for Measuring Coating Thickness.” Materials Evaluation, vol. 73, no. 9, pp. 1205─1213, 1 September 2015.
 Tompkins, H. G., and E. A. Irene (editors). Handbook of Ellipsometry. Norwich, New York: William Andrew Publishing, 2005.
 Bondaz, A., L. Kitzinger, and C. Defranoux. “Review of Characterization of ZnO by Spectroscopic Ellipsometry.” ECS Transactions, vol. 16, no. 12, pp. 33-40, https://iopscience.iop.org/article/10.1149/1.2985841/pdf, 2008.
 QNix. “Coating Thickness Gauges for Non-Destructive Measurement – the QNix Series.” https://www.q-nix.com/en/coating-thickness-gauges/?gclid=EAIaIQobChMIh9Dr1uzV4QI Vk8pkCh1_NgGlEAAYBCAAEgKZWfD_BwE, accessed 14 April 2019.
 DeFelsko. “Paint Thickness Measurement – Drywall.” https://www.defelsko.com/resources/ paint-thickness-measurement-drywall, accessed 14 April 2019.
 Elcometer. “What Is a Coating Thickness Gauge?” https://www.elcometer.com/en/coating-thickness-gauge.html, accessed 14 April 2019.
 PCE Instruments. “Coating Thickness Gauge.” https://www.pce-instruments.com/us/ measuring-instruments/test-meters/coating-thickness-gauge-kat_40109.htm, accessed 14 April 2019.
 Lomax, P. “Quality 101: Understanding Coating Thickness Measurement Test Methods.” Quality Magazine, https://www.qualitymag.com/articles/87956-quality-101–understand-coating-thickness-measurement-test-methods, 7 October 2011.
 ASTM International. Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals. ASTM D7091-13, West Conshohocken, PA, 2013.