Terahertz-Thickness Measurement Applies Itself to Paint and Coatings

There is a constant struggle in the transportation industry to balance paint and coating requirements against manufacturing costs and fuel economy. If the paint or coating is too thin, then flaking and peeling may occur. Conversely, if the paint is too thick, it contributes unnecessary manufacturing costs and increases vehicle weight, which can detract from fuel economy.

The thickness measurement principle based on terahertz pulse time-of-flight tools. Courtesy of TeTechS Inc.

These factors have made fast, accurate, and cost-effective thickness measurement of coatings, thin films, and paint a critically important challenge to address. In many industries, such as automotive manufacturing, reducing the painting costs associated with material waste, labor, and time is a rational strategy for enhancing overall efficiency. In this context, automation of quality control (i.e., real-time thickness measurement of the paint and coating) can become even more essential to productive manufacturing.

Measure and destroy

Broadly speaking, paint thickness measurement involves either destructive or nondestructive tests. Destructive tests employ invasive procedures that damage the sample and render it unusable. Conversely, nondestructive methods measure the thickness of coatings and thin films without inflicting damage to the sample.

There are various destructive techniques for measuring paint thickness. The more common examples include:

Optical cross-sectioning, which simply relies on cutting the coated part of the sample and investigating the component under an optical or electron microscope suitable for inspecting very thin coatings.

Height measurement, which involves removing the coating on some parts of the sample to measure the height difference with a profilometer.

Gravimetric methods, which calculate the mass of a coating within a given area on the sample based on the established density of the paint and coating material.

Even without sample preparation, destructive methods can be time-consuming. They are not viable solutions for real-time quality control and can impose long waiting times on production lines.

Measure and preserve

Nondestructive methods employ advanced technologies for measuring the thickness of coatings and films without damaging the test sample. Often, optical, electromagnetic, or ultrasonic energy is used to probe the sample and determine the thickness of the coating. Some representative nondestructive measurement methods include:

Permanent magnets and magnetic inductive testing.

Eddy current testing.

Optical testing methods.

Ultrasonic waves.

Terahertz time-of-flight measurement.

Magnetic and electromagnetic inductive methods measure the thickness of electrically nonconductive coatings and films on magnetic and nonmagnetic metallic substrates. These methods are suitable for thickness measurement of coatings and paints on most metals, while eddy current methods are the typical choice for measuring thickness on nonferrous metal surfaces.

Optical methods generally apply to measuring transparent coatings and paints on reflective and transparent substrates in transmission mode. Ultrasonic methods apply ultrasonic vibration to the paint and determine thickness by measuring the travel time of the reflected waves inside the coating material. Ultrasonic thickness gauges have a measurement range of typically 13 to 1000 µm and are ideal for measuring total coating system thicknesses.

Powered by terahertz waves

Terahertz metrology solutions are powered by terahertz waves, which fall between microwave and infrared energy on the electromagnetic spectrum. Terahertz time-of-flight measurement techniques are particularly suited for measuring the thickness of multiple layers of paints or coatings. The thickness of each layer is calculated based on the speed of terahertz waves in the material. Hence, it is a “true thickness” measurement with no ongoing calibration required.

Terahertz time-of-flight measurement provides accuracies >0.00004 in. (~1 μm) and can be used to measure a minimum thickness of 0.0003 in. (8 μm) and a maximum thickness of 1.180 in. (30 mm).

Terahertz waves pass through visually opaque and transparent materials, an attribute that — unlike with optical methods — forgoes the limitations on measuring visibly transparent paints and coatings. In contrast to magnetic and eddy current methods, terahertz time-of-flight measurements are unconstrained by the electrical or magnetic properties of the substrates.

Real-time quality control

Gauges for terahertz thickness measurement employ either air- or fiber-coupled excitation arms. Fiber-coupled gauges, in particular, provide better signal-to-noise ratios and higher dynamic ranges. They are also robust, rigid measurement systems, ideal for harsh industrial applications. They can be mounted on robotic arms to allow real-time automatic quality control of paints, coatings, plastics, and polymers.

Terahertz time-of-flight measurement of a thin coating of paint (82 μm). Courtesy of TeTechS Inc.

Gauges designed for terahertz time-of-flight measurements rely on short-pulse lasers that are able to deliver pulse durations under 100 fs. The sources used typically emit at 790 nm for air-coupled gauges and either 1030 or 1550 nm for fiber-coupled gauges.

Short-pulse 1550-nm lasers are attracting greater interest because they present the greatest potential for achieving compact and cost-effective systems. Adoption of 1550-nm pulsed lasers for terahertz time-of-flight gauges further allows application of a broad range of components that are already well developed for optical telecommunications.

Accurate and fast

To explore the effectiveness of terahertz waves when measuring multiple coating types and configurations, recent experiments applied the technology to thin monolayer paints of various thicknesses that coated the surface of metal substrates. The total thickness of each thin layer of paint was measured using destructive techniques based on profilometers, and nondestructive terahertz time-of-flight techniques using a fiber-coupled gauge. While the measured thickness values obtained from the two methods closely matched, the terahertz time-of-flight method proved to be much faster — performing 50 measurements per second with no sample preparation overhead, compared to much slower height measurement methods using stylus profilometers or contact thickness measurement gauges.

Paint thickness measurement using destructive profilometry-based methods. Courtesy of TeTechS Inc.

Another area in which the terahertz time-of-flight method excelled compared to profilometers was in the measurement of paint on plastics. The automotive industry in particular has dramatically expanded the use of painted plastic over the last few years to enhance product appearance, ensure color matching, and promote the stability of plastic surfaces. The terahertz time-of-flight method can measure the thickness of paint on plastic with excellent accuracy of >1 μm compared to much slower destructive techniques employing stylus profilometry.

In addition to being a nondestructive metrology method, terahertz time-of-flight technology offers a highly accurate real-time measurement tool that can be applied to a wide range of coatings and materials. It performs reliably regardless of the substrate and is compatible with robotic automation arms, making terahertz time-of-flight technology an extremely robust candidate for enabling faster and more cost-effective quality control in the production environment.

Meet the authors

Navid M.S. Jahed, Ph.D., is head of optoelectronics and microfabrication at TeTechS Inc.; email: navid@tetechs.com.

Daryoosh Saeedkia, Ph.D., is CEO and founder of TeTechS Inc.; email: daryoosh@tetechs.com.