Measurement of coating thickness with eddy current method

Manuel Norbert Loos, summer semester 2011

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Using eddy current methods, it is possible to measure the coating thickness of metal substrates (designation of painting surface in surface technology). In most cases, the thickness of paint coatings and other sealed coatings are measured.

History

The original investigations into eddy current technology can be traced back to Friedrich Förster (1908 – 1999), a German physicist. He primarily worked on material research into metals. In doing so, he was able to prove the influence of the Earth's magnetic field on metals. Using these findings, he was able to substantiate his explanations of eddy current models. These are mathematically and physically very demanding and thus will be simplified in what follows. ^[1]

Physical fundamentals of eddy current technology

This chapter was moved here.

Measuring principle

Fundamentally, a distinction is made between two different measuring tasks whose measuring principles are explained below:

• Measuring the thickness of a electrically conductive coating on a non-conductive base material.
• Measuring the thickness of non-conductive coating on a conductive base material.

If an electrically conductive coating is measured, then a calibration curve needs to be initially determined. If the coating thickness is null, then the receiver signal also has to be null as the substrate is not conductive and thus causes no changes in the magnetic field. Coatings with increasing thickness are then applied and the signal on the receiver increases; this increase depends on the electrical conductivity as well as the relative permeability of the material applied. The strength of the receiver signal applied over the coating thickness results in the calibration curve. Using this, non-destructive thickness measurements are then possible.

If measuring a non-conductive coating on a conductive substrate, the base signal is measured without the applied coating. Further receiver signals are then measured for different coating thicknesses which results in a calibration curve. The measurement engineering fundamentals for changing signals are distance loci. They indicate the course of the operating point of receiver coil voltage in the impedance plane when the distance between the receiver coil and the test surface is changed. If the distance between the surface and the receiver coil are changed or different coating thicknesses are measured, then the geometrical ratios of the test set-up change. This is linked to a change in the spatial distribution of the magnetic field strength and the eddy current density in the test piece and thus to the signal to the receiver coil. In distance loci, the imaginary ratio of the receiver coil voltage with workpiece $//<![CDATA[ \begin{array}{l}U E\end{array} //]]>$ and of the receiver coil voltage without workpiece $//<![CDATA[ \begin{array}{l}U E L\end{array} //]]>$ are plotted over the real ratio of both values. The different loci are dependent on the frequency of the testing alternating current. ^[1]

However, distance loci curves are not represented in modern measuring equipment currently used, merely the coating thicknesses already calculated. Thus, thicknesses are constantly measured indirectly.

Application

At present, the practice of measuring coating thickness is dealt with by the following German standards:

• E DIN EN ISO 2808:2004: Determination of film thickness
• DIN EN ISO 2360 Non-conductive coatings on non-magnetic electrically conductive base metals - Measurement of coating thickness - Amplitude-sensitive eddy-current method

The standard mentioned first deals with all measuring methods to be used, whereas the standard 2360 deals with eddy current methods in particular. To offset measuring errors and to compensate for interfering effects in the process of vehicle manufacture, the manufacturers of coating thickness measuring equipment using eddy currents especially provide combined equipment that measures using both eddy current and magnetic induction. It should also be noted that these measurements are not part of an automated process at present, but random tests are carried out manually within the framework of quality assurance. Furthermore, the coating thicknesses are carried out during the introduction of new colours on the test sheets and bodywork out of the process in order to set and optimise the application of coatings (from a discussion with Michael Stoll, Planner Paint Coating Technologies at AUDI AG and Member of the Committee for Standardisation).

Automobile producers do not carry out any calibration of the equipment in the proper meaning of the word. This is due to the fact that the automobile industry constantly uses the same sheet thickness. For this reason, it suffices to monitor the machines each week. It is measured by the control sheet measured by the German Federal Institute for Materials Research and Testing (BAM). Provided that the right thickness set out by BAM is delivered, the machine can continue to be used. If the measurement is incorrect, the machine is sent to the manufacturer for calibration.

Represented in the images is a laboratory instrument that is primarily is used on test panels for paint applications. There are also hand-held instruments that can be used directly on the production line.

The coating thickness is indicated directly on the monitor of the test instrument. The measurement values are saved in a continuous chart. The sensing head is placed orthogonally for measuring the sheet and the operating button on the probe is then pressed down, thus triggering the measurement.

Critical Appraisal

Generally, testing coating thickness using an eddy current is regarded as a very reliable method of measuring paint coating thickness on sheets in vehicle manufacture. To a large extent, temperature influences can be ruled out. Measuring inaccuracies can thus occur as other eddy currents are primarily measured due to material defects, even if the coating thickness remains the same. However, as the sheets to be used are continuously tested during the deep drawing process of components and body construction, this interference factor can also be ruled out to a large extent. In addition, this measuring method is assured by using same sheet thickness, as previously mentioned, as well as always using the same materials (substrates) with the same composition. The drawback is that selected measurements in the sense of random testing are only possible with the aid of laboratory and hand-held machines. However, this suffices for the quality assurance of layer structures in automobile manufacturing, especially due to the high safety that results from the multi-layered coating structure on cars (priming coat, filler coat, clear coat finish).

In general, it can be said that measuring accuracy has become very precise through the use of microscopic testing using eddy currents. Thus, resolution (in a layer) of up to $//<![CDATA[ \begin{array}{l}10 \mu m\end{array} //]]>$ can be achieved on the surface. A measuring accuracy of $//<![CDATA[ \begin{array}{l}10 nm\end{array} //]]>$ is possible in the case of coating thickness information. ^[2] The specification of the paint coating thickness in vehicle manufacturing is on the whole usually $//<![CDATA[ \begin{array}{l}\mu m\end{array} //]]>$ so that a measuring accuracy in the nano-range by all means suffices for quality assurance purposes.

Literature

• E DIN EN ISO 2808:2004: Bestimmung der Schichtdicke. September 2004.
• DIN EN ISO 2360: Non-conductive coatings on non-magnetic electrically conductive base metals - Measurement of coating thickness - Amplitude-sensitive eddy-current method. April 2004.
• Stegemann, Dieter: Der Einsatz von Wirbelströmen für die Zerstörungsfreie Werkstoffprüfung. 1. Auflage. DVS Media. Berlin 2010.
• Szielasko, Klaus; Kopp, Melanie; Tschuncky, Ralf; Lugin, Sergey; Altpeter, Iris: Barkhausenrausch- und Wirbelstrom-Mikroskopie zur ortsaufgelösten Charakterisierung von dünnen Schichten. DACH-Jahrestagung. Salzburg 2004.

References

1. Stegemann, Dieter: Der Einsatz von Wirbelströmen für die Zerstörungsfreie Werkstoffprüfung. 1. Auflage. DVS Media. Berlin 2010.
2. Szielasko, Klaus; Kopp, Melanie; Tschuncky, Ralf; Lugin, Sergey; Altpeter, Iris: Barkhausenrausch- und Wirbelstrom-Mikroskopie zur ortsaufgelösten Charakterisierung von dünnen Schichten. DACH-Jahrestagung. Salzburg 2004.