Modern automobile finishes consist of four or five layers of distinctly different materials, each of which must be applied evenly for rust protection as well as good looks. According to researchers in Germany, ultrashort terahertz pulses can be used to measure the thicknesses of these layers. [Image: iStock]
It’s tricky enough to apply an evenly thick coating of paint to a wall. Try applying multiple coats of different liquids to curved surfaces with an industrial robot, and “evenness” becomes a much harder quantity to measure.
Now a Germany-based team of researchers has modeled a method for using time-of-flight measurements of reflected terahertz pulses to measure the thickness of the coating layers applied to new automobiles (Appl. Phys. Lett., doi:10.1063/1.4955407). The team believes that the method could allow for greater precision and better quality control in this important industrial process.
Modern automobiles leave the factory with as many as five layers of “paint”: a zinc phosphate undercoat for protection against corrosion, an “electrocoat” applied under high voltage, a primer, a colored base coat and a clear coat. The complexity of the coatings limits conventional measurement methods such as ultrasound and magnetic gauges.
Commercially available pulsed terahertz systems can penetrate a wide variety of dielectric materials, including those in car paint, but their minimum thickness resolution exceeds the depth of the multiple automotive paint coatings (between 5 μm and 60 μm, depending on the material and manufacturer). An additional challenge for interpreting the reflection of terahertz pulses off painted surfaces is that the undercoats are not always completely dry when additional layers are applied, causing a partial merging of adjacent layers.
The researchers, led by physicist René Beigang of the University of Kaiserslauten, developed an advanced numerical regression algorithm that calculates the optical parameters of all paint layers simultaneously. The team then tested their model by beaming terahertz radiation on several types of automobile-part samples and measuring the reflections with a compact fiber-coupled spectrometer. Comparison of the spectrographic results and cross-sectional micrographs showed good agreement with the theoretical model; the terahertz-reflection method could resolve layers as thin as 5 μm.
The Kaiserslauten team’s technique works for all types of paint substrates that make up cars’ body parts: metals, carbon-fiber-reinforced polymers and dielectrics.