The Applications And Importance Of Measuring Coating Thickness

Coating thickness measurement is a major concern for both finishers and for customers who receive finished components. Defective coatings can easily corrode. As a result, product liability requires quality control of coatings and often documentation of measurements in an electronic format. Conflicts can occur if a manufacturer and a customer of a finished product use different methods or types of instruments to determine coating thickness.

The magnetic induction method of measurement is used on non-magnetic coatings over ferrous substrates and magnetic coatings over non-magnetic substrates. This method of coating thickness measurement involves the placement of a probe onto the part to be measured. When the probe has been positioned, the distance between the probe tip that is in contact with the surface and the base substrate is measured.

Inside the probe is a coil that produces a changing magnetic field. When the probe is placed against the substrate, the magnetic flux density of the field is altered. This change in the magnetic inductance is measured by a secondary coil. The output is transferred from the secondary coil to a microprocessor where it can be viewed as a coating thickness measurement on the digital display.

The magnetic induction method is fast and it can be used with either a bench-top or hand-held coating thickness gauge. It is also non-destructive, relatively low-cost, easy to operate, accurate, repeatable and measurements can be instantaneously displayed. Common applications for this test method include either liquid or powder coatings. It works well on plated finishes such as chrome, zinc and cadmium as well as on phosphate over steel or iron substrates.

Eddy-current thickness measurement evaluates non-conductive coatings on non-ferrous conductive substrates, non-ferrous conductive coatings on non-conductive substrates and some non-ferrous metal coatings on non-ferrous metals. It is reasonably similar to the magnetic induction method and can use some of the same probe designs. The advantages of the eddy-current coating thickness measurement method are similar to those of the magnetic induction method, since it is also low cost, easy to operate, accurate and repeatable and provides instantaneous measurement with a digital display.

Eddy-current measurement also uses a probe that contains a coil. The probe/coil is driven by a high-frequency oscillator which generates an alternating high-frequency field. When this field is moved near a metallic conductor, eddy currents are generated in the conductive material. This results in an impedance change of the probe coil. The distance between the probe and the conductive substrate material establishes the amount of the impedance change. The coating thickness is therefore determined by the impedance change in the form of a digital reading. Some conventional applications for eddy-current coating thickness measurement include liquid or powder coating over aluminum or non-magnetic stainless steel as well as anodize over aluminum.

Many coating thickness test instruments combine both the magnetic induction and the eddy-current methods allowing the user to perform multiple measurement tasks without switching gages. Users can also choose between basic gages that just provide a value on the digital display or gages that store measurements and provide statistical information. This data can include the average, standard deviation and high/low values of the measurements.

Another important selection criterion depends on the shape of the part to be measured. Since all parts cannot be measured using a gage with a built-in probe, some coating thickness measurement units offer a probe on a cable to provide more flexibility while working on multiple part configurations. Many units with separate probes also offer the ability to exchange probes should the application change. For example, measuring coating thickness on the inner diameter of a tube may require a 90-degree probe, while measurements on a flat surface are best performed with a zero-degree probe. Factors such as curvature, edge effect, surface roughness, substrate thickness, permeability and conductivity all influence the measurement and can all be managed through proper calibration.

Other methods for obtaining coating thickness measurement include the Coulometric, beta backscatter and X-ray fluorescence techniques. These are especially useful in automotive industry applications.