Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 309 of 331

Fig. 4. Ultrasonic gauge can measure the thickness of coatings on non- metallic substrates. tains an ultrasonic transducer that sends a pulse through the coat- ing (see Fig. 4). The pulse reflects back from the substrate to the transducer and is converted into a high-fre- quency electrical signal. The echo wave form is then digitized and ana- lyzed to determine coating thickness. In some circum- stances, individual layers in a multilayer system can be measured. Couplant is typically used between the probe and the surface to be measured. Typical tolerance is ±3%. The manufacturer's instructions should be carefully fol- lowed for most accurate results. Standard methods for the application and per- formance of this test are available in ASTM D 6132. MICROMETER Micrometers are sometimes used to check coating thickness. They have the ad- vantage of measuring any coating/substrate combination but the disadvantage of requiring access to the bare substrate. The requirement to touch both the sur- face of the coating and the under side 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 be- tween 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 de- signed 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, pre- cise 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 in- terpret the results. Adjusting the measurement reticule to a jagged or indistinct interface is a source of inaccuracy, particularly between different operators; how- ever, 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 308

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