Paint and Coatings Thickness Testing

Touchstone has numerous ways to measure film thickness including the Elcometer Magnetic Gage, Tooke Paint Inspection Gage, Digital Eddy Current Tester, Micrometer Method, and Microscopic-Image Analysis Method.

Elcometer Magnetic Gage ­ The Elcometer magnetic gage is a hand-held gage used to determine the thickness of non-magnetic films applied to magnetic substrates such as steel, iron, magnetic stainless steel, etc.

This type of mechanical gage uses the principle that the attractive force between a permanent magnet and a magnetic metal substrate is inversely proportional to the distance between them.

Tooke Inspection Gage ­ This gage provides for estimating the thickness of a film from the geometry of a V-groove cut in the film by a special tool. With the aid of a X50 illuminated magnifier equipped with a reticle in the eyepiece, the operator measures the lateral distance from the top edge of the cut and the projection of the intersection of the cut and the substrate. Film thickness is then measured by counting scale divisions.

Eddy Current Tester ­ The eddy current method for measuring film thickness is applied to cladings on nonferrous metals. It is based on the effect that a high-frequency alternating current has on an electrically conductive surface causing highly localized current flow or “eddy currents.” These currents generate a high-frequency field. The magnitude of the changes in the field is proportional to the distance from the probe coil to the substrate, that is, to the thickness of the coating.

Micrometer Method for Dry Film Thickness of Organic (Paint) Coatings – ASTM D1005 ­ Test method ASTM D1005 covers the measurement of film thickness of dried films of paint, varnish, lacquer, and related products using hand-held or stationary micrometers. The total thickness of the substrate and coating is measured. The substrate is then re-measured after removing the coating. The coating thickness is the difference between the two measurements. ASTM D1005 is recommended only for thicknesses over 0.5 mil and is accurate to ± 0.1 mil.

Reference: ASTM D1005 – Standard Test Method for Measurement of Dry-Film Thickness of Organic Coatings Using Micrometers

Microscopic Method / Image Analysis ­ This method covers measurement of local thickness of organic films on metal substrates by microscopical examination of cross sections using an optical microscope or a Scanning Electron Microscope (SEM).

Samples are mounted in an appropriate mounting media such as epoxy. The samples and the mounting material are abraided and polished to an almost mirror finish exposing the coating in cross secion. The thickness of films is measured by an image analysis system or by simple measurement with the microscope.

This method has a few advantages. First, using the SEM, very thin coatings can be measured and chemically analyzed during observation in the SEM using the X-ray spectrometer mounted on the SEM. In addition with both the light microscope and SEM techniques, additional information about the coating can be assessed. The coating bond, coating uniformity and coating defects such as porosity and inclusions can be evaluated. In the case of metallic coatings, intermetallics such as the brittle Iron/Zinc layer which forms in galvanized steel can also be analyzed.