Quality Measurement: Coating Thickness Gages: A Fast Evolution
It may not be as compelling as Charles Darwin's theory of evolution, but it can be said that the coating thickness testing discipline is constantly changing, with one evolutionary line of products followed by another. The result? Diversity in the marketplace is constantly increasing, with survivors discovering a world full of application opportunities.
"As a discipline, coating thickness testing to combat corrosion has been around for years," says Aivars Freidenfelds, vice president at ElektroPhysik (Arlington Heights, IL). "But in today's ever-competitive world market, manufacturers increasingly must walk the fine line of scrutinizing materials costs while adhering to strict quality standards. Requirements for target coating thickness have been established to help minimize the amount of coating needed during coating operations."
Today's thickness gages also are more simplified, and are far more advanced and sensitive than in years past, Freidenfelds adds. "While there are conditions to be aware of to ensure accurate measuring, they no longer are used solely to prevent corrosion. Today, the technology has grown to the point where a variety of instruments are available for multiple coatings on steel, for example, and for insulating coatings on nonferrous metals, including anodizing or paint on aluminum and zinc die casting."
Understanding what equipment is available and how to use it is critical to ensuring process control, product quality and cost control. Depending on the type of coating, its thickness range, the substrate material, and the size and shape of the part, operators can turn to any number of measuring techniques.
While there are companies that provide many solutions, here is what some of the experts have to say about a few of today's widely used or new technologies: magnetic, eddy current and ultrasonic coating thickness measurement.
Magnetic gages are widely used to measure electroplating, nickel coatings, phosphating, paint, plastic and coatings such as enamel on steel. Based on the Magnetic Attraction principle, they allow for the quick, precise, nondestructive measurement of non-magnetic coatings applied over ferrous substrates, Freidenfelds says.
"The attractive force is related to the distance between a permanent magnet and a steel substrate," he says. "The distance represents the thickness of the coating to be measured. The magnet is lifted from the surface by means of a spring connected to the magnet arm. The force needed to lift the magnet is directly related to the angle of rotation of the torsion spring."
ElektroPhysik has been supplying magnetic coating thickness gages for more than 40 years. "We provide both a MikroTest manual version and, more recently, an automatic version that turns the dial automatically, and holds and locks the reading when the magnetic lifts off of the surface at the point where the thickness is determined," Freidenfelds says. The gage involves simple set-up, requires no power supply or calibration, and measurements can be taken at any angle.
Depending on the application, the MikroTest automatic gage can provide balanced measuring at horizontal, vertical or overhead angles. "The gage also allows for measurements on curved surfaces, and precise measurements in tubes and on round parts," he says.
"Of course, beyond measurement techniques based on magnetic attraction, you have eddy current, which applies to nonferrous metals and provides for nondestructive testing," Freidenfelds says. ElektroPhysik offers another product, the eXacto coating thickness gage, which is based on both magnetic induction and eddy current principles, and provides menu systems driven by operator need. "Like Windows XP, it is a menu system that is user-friendly-anyone can use it for applications in non-magnetic coatings such as paint, enamel and chrome on steel, and insulating coatings such as paint and anodizing on non-ferrous metals. It also includes a compact design for one-hand operation; an internal or external probe, dual-version FN that automatically adjusts to the correct substrate metal; high precision measurement; statistical evaluation and infrared interface; and calibration foils and standards that are stored safely in gage housing," Freidenfelds says.
Eddy current technique
The most common use of eddy current measurement is for non-conductive coatings over non-ferrous metals such as anodize or paint over aluminum. This technique allows for an accurate reading with even very thin materials through elimination of the base effect.
Tom Leone, senior vice president at OICM (Elk Grove Village, IL) explains how eddy current measurement works: "When a conductive material is subjected to an AC magnetic field from a probe, eddy currents occur in the material in proportion to frequency of the drive signal and conductivity of the material. The induced eddy currents generate an opposing magnetic field that alters the circuit reactance and the output voltage of the probe. The change in output voltage is used to calculate the coating thickness."
OICM gages use eddy current technology for measuring metallic coatings over ferrous substrates. Unlike the more common method, magnetic induction, eddy current technology is not limited by geometry, and it eliminates the need for calibration on the bare part.
"At one time, the only way to measure metallic coatings over ferrous substrates was with magnetic induction," Leone says. "With our CMI200 Series gages, we've incorporated the eddy principle for metallic coatings into the technology, providing for more reliable readings when dealing with small components such as fasteners in the automotive or appliance industries. This is because the method is not sensitive to the base or geometry of the part, and it enables operators to perform an accurate measurement when reading coatings such as zinc or nickel over steel."
The technology also comes with a scanning option that allows operators to continuously measure over large areas with one swift measurement. The feature calculates an average measurement, and it provides for an accurate reading for thickness over uneven or textured substrates, thus eliminating the need to take multiple readings. Some gages include a detachable probe on a long, flexible cable, affording greater accessibility when measuring hard-to-reach surfaces. The adaptability of the instrument provides for maximum flexibility for measuring a variety of coating thicknesses.
Ten years ago, the first handheld instrument appeared that was designed to measure coating thickness ultrasonically. "The way ultrasonics works is a vibration is sent into the coating by a probe, or transducer, with the assistance of a couplant applied to the surface," explains David Beamish, general manager for DeFelsko Corp. (Ogdensburg, New York).
Many industries today use this technology in their quality programs. "There are many principles and benefits that an ultrasonic approach bring to thickness coating measurement," Beamish says. "For one, coating thickness gages that use an ultrasonic principle support or replace destructive measurement techniques for concrete, wood and plastics applications. They can replace time-
consuming, destructive tests that would have required patching.
"The vibration travels through the coating until a material with different mechanical properties is encountered, typically the substrate, but perhaps a different coating layer," Beamish says. "The vibration is partially reflected at this interface and propagates back to the transducer. Meanwhile, a portion of the transmitted vibration continues to travel beyond that interface and also experiences further reflections at any material interfaces that are encountered."
Because multiple echoes can occur, the gage is designed to choose the maximum or "loudest" echo from which to calculate a thickness measurement. Instruments that measure individual layers in multi-layer applications also choose the loudest echoes. "The operator simply selects the number of layers to measure and the gage measures the loudest echoes. Softer echoes from coating imperfections and substrate layers are ignored," he says.
For more detailed information about a coating or substrate interface region, ultrasonic instruments with a graphical LCD illustrate substrate roughness by displaying wide "peaks" when the substrate is rough and narrow "peaks" when it is smooth. This type of display helps in measuring coating systems in which the number of layers is unknown. It presents interface information about bond strength and material densities, he adds.
Many industries today use the technology in their quality programs. "There are many principles and benefits that ultrasonics bring to thickness coating thickness measurement," Beamish says. "Coating thickness gages that use an ultrasonic principle are becoming increasingly popular. They support or replace destructive measurement techniques for concrete, wood and plastics applications."
He adds that the ultrasonic pulse-echo technique of ultrasonic gages, as provided by DeFelsko's PosiTector 100 and PosiTector 200, is used to measure the thickness of coatings on nonmetal substrates such as plastic and wood without damaging the coating.
"The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating," Beamish says. "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 the device is ±3%. Standard methods for the application and performance of the test are available in ASTM D 6132, Beamish adds.
Word to the wise
A host of both tried-and-true and new-generation coating thickness measurements are available, including those discussed, as well as others based on the beta backscatter method, micrometer technology, destructive techniques and more.
"Regardless of the method used, operators need to be aware that if they're introducing coating thickness testing for the first time to their operations, while it may be a necessity, the training to support it also needs to be addressed," Freidenfelds says. "This is a key point because, while the devices used to measure a coating thickness have been simplified, without adequate training, erroneous information-measurements far from zero calibration-can result."
To address the issue of training and other issues that can affect measurement results, operators should consult with the American Society for Test and Measurements, which houses specifications, methods and guides for a range of industries, including coating-related topics.
"In addition, many associations exist today that support specific applications, and offer training sessions and documents that provide details about many of the issues related to an application," Freidenfelds says. "Those involved in these associations likely are more than happy to share their findings about a particular issue. This can help avoid some of the challenges of performing any given thickness coating measurement function." Q
Understanding what coating thickness measurement technologies are available and how to use them is critical to ensuring process control, product quality and cost control. Technology examples:
• Magnetic gages are widely used to measure electroplating, nickel coatings, phosphating, paint, plastic and coatings such as enamel epoxy on steel.
• Eddy-current instruments are used for measuring both non-magnetic, metallic coatings over steel, as well as nonconductive coatings over non-ferrous metals.
• Coating thickness gages that use an ultrasonic principle support or replace destructive measurement techniques for concrete, wood and plastics applications.