A variety of recognized methods can be used to determine the thickness of organic coatings. The method employed in a specific situation is most often determined by the type of coating and substrate, the thickness range of the coating, the size and shape of the part, and economics. Commonly used measuring techniques are nondestructive dry film methods such as magnetic, eddy current, ultrasonic or micrometer measurement; destructive dry-film methods such as cross-sectioning or gravimetric (mass) measurement; and wet-film measurement.
Magnetic film gages are used to nondestructively measure the thickness of a nonmagnetic coating on ferrous metal substrates. Most coatings on steel and iron are measured this way. Testing with magnetic gages is sensitive to surface roughness, curvature, substrate thickness, and the composition of the substrate. Magnetic gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction.
There are many sources of thickness standards but it is best to ensure they are traceable to a national measurement institute such as NIST.
A regular check against these standards verifies the gage is operating properly.
When readings do not meet the accuracy specification of the gage, the gage must be adjusted or repaired and then calibrated again.
These gages use a permanent magnet, a calibrated spring, and a graduated scale. The attractive force between the magnet and the ferromagnetic substrate works to pull the two together. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. Coating thickness is determined by measuring this pull-off force—the weaker the force, the thicker the coating.
Commonly offered as pencil-type or rollback dial models, magnetic pull-off gages are rugged, simple, inexpensive, portable, and usually require no calibration adjustment. They are the instrument of choice when relatively few readings per day are required.
Pencil-type models use a magnet that is mounted to a helical spring that works perpendicularly to the coated surface. Most pencil-type pull-off gages have large magnets and are designed to work in only one or two positions, which partially compensate for gravity. A more accurate version is available, which has a tiny, precise magnet to measure on small, hot, or hard-to-reach surfaces. A triple indicator ensures accurate measurements when the gage is pointed down, up, or horizontally with a tolerance of ±10%.
Rollback dial models are the most common form of magnetic pull-off gage. A magnet is attached to one end of a pivoting balanced arm and connected to a calibrated hairspring. By rotating the dial with a finger, the spring increases the force on the magnet and pulls it from the surface. These gages are easy to use and have a balanced arm that allows them to work in any position, independent of gravity. They are safe in explosive environments and are commonly used by painting contractors and small powder coating operations. Typical tolerance is ±5%.
Magnetic and Electromagnetic Induction
Magnetic induction instruments use a permanent magnet as the source of the magnetic field. A Hall-effect generator or magneto-resistor is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic induction instruments use an alternating magnetic field. A soft, ferromagnetic rod wound with a coil of fine wire is used to produce a magnetic field. A second coil of wire is used to detect changes in magnetic flux.
These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it nears the surface of a ferrous substrate. The magnitude of the flux density at the probe surface is inversely related to the distance from the substrate. By measuring flux density the coating thickness can be determined. Typical tolerance is ±1%.
Electronic magnetic gages come in many shapes and sizes. They commonly use a constant-pressure probe to provide consistent readings that are not influenced by different operators. Readings are usually shown on a liquid crystal display (LCD). Some electronic gages offer additional utility by accepting interchangeable probes for applications related to coating application: characterization of abrasive blast cleaned surfaces, monitoring and recording environmental conditions, measuring the effects of corrosion on a substrate, and others.
In contrast with magnetic pull-off instruments, most electronic gages have options to store measurement results, perform instant analysis of readings, and output results to a computer for further examination by means of various software solutions. Typically supplied by the gage manufacturer, software for reporting and analysis of stored thickness readings can be installed on a computer or accessed via the Internet (cloud-based). Recent hardware and software innovations permit gage operators to directly integrate their smart devices (mobile phone, tablet, etc.) with the coating thickness measurement, analysis and reporting process.
The gage manufacturer’s instructions should be carefully followed for most accurate measurement results. Standard test methods are available in ASTM D7091, ISO 2178 and SSPC-PA 2.
Eddy current techniques are used to nondestructively measure the thickness of nonconductive coatings on nonferrous metal substrates. A coil of fine wire conducting a high-frequency alternating current (above 1 MHz) is used to set up an alternating magnetic field at the surface of the instrument’s probe. When the probe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on the surface. The substrate characteristics and the distance of the probe from the substrate (the coating thickness) affect the magnitude of the eddy currents. The eddy currents create an opposing electromagnetic field that can be sensed by the exciting coil or by a second, adjacent coil.
Eddy current coating thickness gages look and operate like electronic magnetic gages in that they commonly use a constant-pressure probe and display results on an LCD. They can also have options to store measurement results or perform instant analysis of readings and output to a printer or computer for further examination. The typical tolerance is ±1%. Testing is sensitive to surface roughness, curvature, substrate thickness, type of metal substrate and distance from an edge.
Standard methods for the application and performance of this test are available in ASTM B244, ASTM D7091 and ISO 2360.
It is now common for gages to incorporate both magnetic and eddy current principles into one unit. Some simplify the task of measuring most coatings over any metal by switching automatically from one principle of operation to the other, depending upon the substrate. These combination units are popular with painters and powder coaters.
The ultrasonic pulse-echo technique of ultrasonic gages is used to measure the thickness of coatings on nonmetal substrates (plastic, wood, etc.) without damaging the coating.
The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating. The pulse reflects back from the substrate to the transducer and is converted into a high frequency electrical signal. The echo waveform is digitized and analyzed to determine coating thickness. In some circumstances, individual layers in a multi-layer system can be measured.
Typical tolerance for this device is ±3%. Standard methods for the application and performance of this test are available in ASTM D6132 and ISO 2808.
Micrometers are sometimes used to check coating thickness. They have the advantage of measuring any coating/substrate combination but the disadvantage of requiring access to the bare substrate. The requirement to touch both the surface of the coating and the underside of the substrate can be limiting and they are often not sensitive enough to measure thin coatings.
Two measurements must be taken: one with the coating in place and the other without. The difference between the two readings, the height variation, is taken to be the coating thickness. On rough surfaces, micrometers measure coating thickness above the highest peak.
One destructive technique is to cut the coated part in a cross section and measure the film thickness by viewing the cut microscopically. It can also be determined by making a geometrically-designed incision through the dry-film coating and viewing cross sections with a scaled microscope. A special cutting tool is used to make a small, precise V-groove through the coating and into the substrate. Gages are available that come complete with cutting tips and an illuminated scaled magnifier.
While the principles of this destructive method are easy to understand, there are opportunities for measuring error. It takes skill to prepare the sample and interpret the results. Adjusting the measurement reticule to a jagged or indistinct interface may create inaccuracy, particularly between different operators. This method is used when inexpensive, nondestructive methods are not possible, or as a way of confirming nondestructive results. ASTM D4138 outlines a standard method for this measurement system.
By measuring the mass and area of the coating, thickness can be determined. The simplest method is to weigh the part before and after coating. Once the mass and area have been determined, the thickness is calculated using the following equation:
T = m x 10
A x d
Where T is the thickness in micrometers, m is the mass of the coating in milligrams, A is the area tested in square centimeters, and d is the density in grams per cubic centimeter.
It is difficult to relate the mass of the coating to thickness when the substrate is rough or the coating uneven. Laboratories are best equipped to handle this time-consuming and often destructive method.
MEASUREMENT BEFORE CURE
Wet-film thickness gages help determine how much material to apply wet to achieve a specified dry-film thickness provided that the percent of solids by volume is known. They measure all types of wet organic coatings, such as paint, varnish, and lacquer on flat or curved smooth surfaces.
Measuring wet film thickness during application identifies the need for immediate correction and adjustment by the applicator. Correction of the film after it has dried or chemically cured requires costly extra labor time, may lead to contamination of the film, and may introduce problems of adhesion and integrity of the coating system.
The equations for determining the correct wet-film thickness (WFT), both with and without thinner, are as follows:
WFT = desired dry film thickness
% of solids by volume
WFT = desired dry film thickness /
% of solids by volume
100% + % of thinner added
Four types of gages are used including notch, lens, eccentric roller and needle micrometer. Each has its own operating procedure. Notch gages, also called step or comb gages, are most common.
Notch gages should be used on smooth surfaces, free from irregularities and should be positioned along the length, not the width, of curved surfaces. Using a wet-film gage on quick-drying coatings will yield inaccurate measurements. ASTM D4414 outlines a standard method for this measurement.
Powder coatings can be measured prior to curing with a simple handheld comb or an ultrasonic gage. The uncured powder film comb works much the same way as wet film gage. The comb is dragged through the powder film and the thickness lies between the highest numbered tooth which made a mark and has powder clinging to it, and the next highest tooth which left no mark and has no powder clinging to it. These gages are relatively inexpensive with accuracy of ±5µm. They are only suitable as a guide since the cured film may be different after flow. Marks left by the gage may affect the characteristics of the cured film.
An ultrasonic device can be used nondestructively on uncured powder on smooth metallic surfaces to predict the thickness of the cured film. The probe is positioned a short distance from the surface to be measured and a reading is displayed on the LCD of the device. Measurement uncertainty is ±5µm. A standard method for the application and performance of these powder tests is available in ASTM D7378.
Coating thickness gages are calibrated to known thickness standards. There are many sources of thickness standards but it is best to ensure they are traceable to a national measurement institute such as NIST (National Institute of Standards and Technology). A regular check against these standards verifies the gage is operating properly. When readings do not meet the accuracy specification of the gage, the gage must be adjusted or repaired and then calibrated again.
The primary purpose of coating thickness measurement is to control cost while ensuring adequate coverage and should be a routine event for all coaters. Selecting the correct gage depends on the thickness range of the coating, the shape and type of substrate, the cost of the gage, and the degree of accuracy required.