Most electronic handheld coating thickness gauges utilize a magnetic induction principle for measuring non-magnetic coatings applied to a ferrous metal, such as steel, or an eddy current method for measuring non-conductive coatings over a non-ferrous substrate, such as aluminum. Those who measure coating thickness should be aware of several factors that will influence the reading: curvature, edge effect, substrate thickness, surface roughness, permeability, and conductivity. An “adjustment” to the probe for changes in these variables will improve accuracy. Advancements in technology have provided “smart probes” with built-in memory chips that indefinitely store variables such as geometry, material characteristics, and so on. This provides a higher degree of accuracy. These advancements in probes, along with a new generation of coating thickness gauges, provide excellent process control capabilities.

Practical considerations, as well as tolerance requirements, determine which type of coating thickness probe will be most appropriate. For starters, the surface area of the part will determine whether an integrated probe unit or a separate probe unit will be able to take the reading. Units pictured in Figure 2 read the thickness values with an integrated probe. Note that two LCD displays are present. This is ideal for viewing the measured data from all angles. Powder-coated parts like lawn and garden furniture, aircraft, bridges, ships, and water tanks are typical examples of applications for which an integrated probe gauge can be used.

Other applications, such as measuring on small components, require units with a separate probe. Coating thickness gauges such as the one in Figure 3 operate with interchangeable probes. These interchangeable probes are capable of addressing the aforementioned measuring influencing factors. Adjustment using a one-point or two-point procedure can improve accuracy and be accomplished by the user if they have an uncoated surface as well as a shim or shims with the target thickness range. This is cumbersome, however, if an uncoated sample is not available. It is not common for uncoated samples to be available in the case of incoming inspection departments.

As previously mentioned, “smart probes” store the adjustment data in a memory chip and contain all the data necessary to make a measurement. The user simply recalls the measurement application, and the system is ready to measure immediately instead of having to once again use shims and an uncoated substrate to make an adjustment.

Technological advancements such as the unit pictured in Figure 3 store application settings. The unit uses software based on Windows CE, which enables intuitive operation. The color touch screen display allows users to set up the parameters depending on their particular measurement challenges or report requirements. For example, Figure 4 shows four examples of the type of display available. They include a statistical display, histogram display, specification limits display, or simply a display with the coating thickness value. Soft keys on the the display can be chosen and arranged to reduce the possibility of errors occurring. Likewise, the specific statistical data required either by block or in the final report can be configured via a drag and drop option shown in Fig. 5.

One of the most commonly recurring influences on coating thickness is curvature. A part with a curvature can cause erroneous readings without proper adjustment of the gauge and probe. The influence of the curvature varies with the make and type of the instrument but becomes more pronounced as the radius of curvature decreases. Figure 6 illustrates the effect of curvature. Newer-generation probes incorporate a curvature compensation method that is ideal for measuring paint, lacquer, plastic, and anodized coatings commonly used for automobiles, appliances, and so on. In addition to the challenge of curved surfaces, anodized parts typically are measured when slightly wet and acidic. High-quality probes are protected from the corrosive acidic contents of the anodized layer.

Edge effect is defined by ASTM B244 as being sensitive to abrupt changes in the surface contour of the test specimen. Therefore, measurements made too near an edge or inside corner will not be valid unless the instrument is specifically adjusted for such a measurement. In order to have probe access to an edge or inside area, probes need to be designed for such geometrical shapes. Figure 7 is a practical example of how edge effect could influence coating thickness values. Like curvature, edge effect can be adjusted, and the measurement characteristics can be stored in “smart probes.”

ASTM D 7091 “Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic Nonconductive Coatings Applied to Non-Ferrous Metals” is an excellent resource. This practice discusses surface roughness and base metal reading (BMR) on the uncoated substrate using a coating thickness gauge. As noted in the document, “The BMR is the measured effect of substrate roughness on a coating thickness gauge that is caused by the manufacturing process (for example, castings) or surface profile (roughness)-producing operations (for example, power tool cleaning, abrasive blast cleaning, etc.).” A double-tip measurement probe like the one pictured in Figure 8 offers higher repeatability when measuring rough surfaces. The spring-loaded measuring system also allows for exact positioning and constant pressure when measuring soft coatings. Figure 8B illustrates what likely occurs to probes measuring over rough surfaces and the importance of taking multiple readings. Soft coatings present an equally challenging task. Figure 8C illustrates the problems that occur when the coating is indented by the probe that damages the coating and provides an inaccurate reading. Certain probes are especially suited for measuring soft coatings commonly used in screen printing material or plastic industries.

Substrate thickness, permeability, and conductivity, as previously discussed, are all factors that influence the measurement and can be adjusted with “smart probes” and gauges that offer application settings.

Duplex Coatings

Thus far, the selection of probes and influences of common coating thickness applications have been presented. There are, however, many challenges that require specialized technology, including the measurement of “duplex coatings.” Types of duplex coating include:

1. Hot-dip galvanized steel components with a zinc coating thicker than 80 microns, distinctive zinc-iron diffusion zones, and a paint coating with a typical thickness of more than 85 microns. These coatings are primarily found in steel structures.
2. Other duplex coatings contain zinc thicknesses of between zero and 10 microns deposited galvanically or in a dip bath, and paint thicknesses of up to 150 microns. These duplex coatings are primarily found in the automotive market.
3. Coating systems on piping (e.g, brake lines), metal sheets for building facades, articles such as shopping carts or household appliances with zinc thicknesses of up to about 30 microns, and an organic coating of paint or plastic with thicknesses of typically less than 200 microns.
4. Coatings consisting of an organic layer and a zinc-iron or zinc-nickel alloy layer.

Using a magnetic inductive principle, the user can measure the total thickness of both the zinc and the paint thickness. However, in many cases, it is important to know both thicknesses. Figure 9 illustrates a duplex coating.

In automotive manufacturing, the zinc coating is already applied by the sheet metal supplier either by galvanic deposition or in a zinc dip bath. If the zinc coating thickness is uniform, the thickness of the subsequently applied paint coating could be measured using a conventional magnetic induction coating thickness measurement instrument. One would simply have to deduct a constant value from the actual reading.

The uniformity of the zinc thickness is typically recorded on a supplied sheet. When the body parts are formed, flowing or even scraping off the zinc coating may occur in the areas of severe bending. This may result in thickness variation between 3 and 9 microns, occasionally removing the coating altogether.

Similar situations are encountered when repairing a body area that has coating defects due to sanding and subsequent repainting of the defective area. In these cases, the zinc coating may be sanded away as well, leading to an apparent reduction in the paint thickness if the aforementioned conventional paint coating thickness measurement system is used. This is not only problematic for the inspection of the finished painted body but also particularly critical in the quality monitoring of a cataphoretic paint, because that thickness is typically only about 20 microns. A defect in the coating thickness measurement of 5 or 6 microns with reduced zinc coating will exceed the tolerance limits.

Certain probes are ideally suited for measuring the paint thickness independent of the zinc thickness. In order to reduce vehicle weight, the use of aluminum is becoming increasingly popular for non-safety relevant components of the body. Therefore, the measuring instrument is also equipped with a conventional eddy current channel to measure the paint thickness on these parts according to a standard. Without any action by the operator, who may not even know which parts are made of steel and which parts are made of aluminum, the instrument automatically selects the required measuring method—duplex or eddy current. These gauges simultaneously record the paint thickness data in the same application in such a way that a simple evaluation of the paint distribution is possible regardless of the type of sheet metal.

In general, several questions can be asked in determining which coating thickness gauges and probes should be used for particular measurement tasks. They include the following:

1. What is the substrate in which the coating will be applied? Ferrous, non-ferrous , both, or other?
2. What is the part’s geometry? Will this require an integrated or separate probe?
3. What is the measurement range?
4. Is it a special application such as a duplex coating, or is it a coating, such as anodizing, that could affect a normal probe?

In every application the user should make it a practice to place the probe perpendicular to the surface to be measured. Figure 10 illustrates the proper positioning of a coating thickness gauge probe. There are additional devices available to assist with the proper positioning of thickness gauge probes, such as the stand in Figure 11. This will ensure the highest level of accuracy and reproduceability of the measured value.

In conclusion, technical data regarding the measurement range, measurement accuracy, measurement precision, and related considerations should be reviewed when selecting a probe. These data are usually available on the manufacturer’s technical sheets and performance specifications. It is likely that a sample uncoated part with a similar shape and material properties will not always be available for adjustment purposes. Therefore, having a unit with “smart probe” technology and application setting storage will save time and improve measurement accuracy and precision.

Bio

Paul Lomax is the marketing director for Windsor, Conn.–based Fischer Technology, Inc. For more information, contact him via e-mail.