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
The magnetic technique is used to nondestructively measure the thickness of nonmagnetic coatings on ferrous substrates. Most coatings on steel and iron are measured this way. Gages use one of two principles of operation: magnetic pull-off or magnetic and electromagnetic induction.
• Magnetic pull-off
These gages consist of a permanent magnet, a calibrated spring and a graduated scale. The attractive force between the magnet and the magnetized steel 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. Testing is sensitive to surface roughness, curvature, substrate thickness and alloy content.
Magnetic pull-off gages are rugged, simple, inexpensive, portable and usually do not require any calibration adjustment. They are the instrument of choice when a low number of readings per day are required. Pull-off gages are typically classified as either pencil-type or roll-back dial models.
Pencil-type models mount a magnet to a helical spring. The spring acts perpendicularly to the surface to pull the magnet off. Most 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%.
Roll-back dial models are the most common form of magnetic pull-off gage. A magnet is attached to one end of a pivoting balanced arm. This assembly is 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
These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it is brought near steel. The magnitude of the flux density at the probe surface is directly related to the distance from the steel substrate. By measuring flux density, the coating thickness can be determined.
Magnetic induction instruments use a permanent magnet as the source of the magnetic field. A Hall-effect generator, which is also known as a 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.
Electronic magnetic gages come in all shapes and sizes and commonly use a constant-pressure probe to remove operator influence and display results on a liquid crystal display (LCD). Optionally, they can store measurement results, perform instant analysis of readings on their displays, and output results to a printer or computer for further examination. Typical tolerance is ±1%.
The manufacturer's in-structions should be carefully followed for the most accurate results. Standard methods for the application and performance of this test are available in ASTM D1186, ISO 2178 and ISO 2808.
Eddy current
Eddy current is used to nondestructively measure the thickness of nonconductive coatings on nonferrous metal substrates, such as paint on aluminum and acrylic on copper. Eddy current inspection is based on the principles of electromagnetic induction, and therefore has many similarities to the electromagnetic induction test method. A coil of fine wire conducting a high-frequency alternating current (above 1 megahertz) 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 sets up eddy currents on the surface. The magnitude of the eddy current is related to substrate characteristics and the distance, or coating thickness, from the probe. The eddy currents create their own opposing electromagnetic field that can be sensed by exciting the coil or by a second, adjacent coil.
Eddy current coating thickness gages look and operate like electronic magnetic gages. They are used to measure coating thickness on all metals except steel and iron. They commonly use a constant-pressure probe and display results on an LCD. Optionally, they can store measurement results or do 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.
The gage manufacturer's instructions should be carefully followed for the most accurate results. Standard methods for the application and performance of this test are available in ASTM B 244, ASTM D 1400, ISO 2360 and ISO 2808.
It is now common for gages to incorporate both magnetic and eddy current principles into a single unit. Some simplify the task of measuring most coatings over any metal by switching automatically from one principle of operation to the other, depending on the substrate. These combination units are popular with painters and powder coaters.
Ultrasonic
The ultrasonic pulse-echo technique is used to nondestructively measure the thickness of various coatings on nonmetal substrates. Applications include paint on plastic, lacquer on wood and epoxy on concrete.
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 wave form is then digitized and analyzed to determine coating thickness. In some circumstances, individual layers in a multi-layer system can be measured.
A couplant is typically used between the probe and the surface to be measured. Typical tolerance is ±3%. Standard methods for the application and performance of this test are available in ASTM D6132.
Micrometer
Micrometers are sometimes used to check coating thickness. They have the advantage of measuring any coating and 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 micrometers 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.
Cross-sectioning
Coating thickness can be measured by cutting the coated part and 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, opportunities exist for measuring error. It takes skill to prepare the sample and interpret the results. Adjusting the measurement reticule to a jagged or indistinct interface is a source of inaccuracy, particularly between different operators, however, direct observation of these conditions is sometimes informative. This method is used when inexpensive, nondestructive methods are not possible, or as a way of confirming nondestructive results. ASTM D 4138 outlines a standard method for this measurement system.
Gravimetric
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. After 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.
Wet-film measurement
Wet-film thickness gages help determine how much wet material to apply 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 labor, 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:
Without thinner:
WFT = desired dry film thickness
% of solids by volume
With thinner:
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.
The gage should be used on smooth surfaces, free from irregularities and should be used along the length, not the width, of curved surfaces. Using a wet-film gage on quick-drying coatings such as inorganic zinc and vinyl will yield inaccurate measurements. ASTM D4414 outlines a standard method for measurement of wet-film thickness by notch gages.
Thickness standards
Coating thickness gages are calibrated to known thickness standards. There are many sources of thickness standards but care must be taken to ensure that they are traceable to a national measurement institute such as National Institute of Standards and Technology (NIST). It is also important to verify that the standards are at least four times as accurate as the gage they will be used to calibrate. A regular check against these standards verifies the gage is operating correctly. When readings do not meet the accuracy specification of the gage, the gage must be adjusted or repaired and then calibrated again. Q
TECH tips
• A variety of recognized methods can be used to determine the thickness of organic coatings.• The method employed is often determined by the coating and substrate, the thickness range of the coating, the size and shape of the part, and economics.
• Techniques include: the magnetic technique; eddy current; ultrasonic; micrometers; cross sectioning; gravimetric techniques; wet-film thickness gages; and thickness standards.
