Metal Finishing Guide Book


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MICROMETER 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 Fig. 4. Ultrasonic gauge can measure the thickness of coatings on nonmethe under side of tallic substrates. 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. 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 it cross sectionally with a scaled microscope. A special cutting tool is used to make a small, precise V-groove through the coating and into the substrate. Gauges are available that come complete with cutting tips and 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 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. 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. 550

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