Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 289 of 331

testing TESTING COATINGS FOR ABRASION AND WEAR ROBERT D. ATHEY ATHEY TECHNOLOGIES, EL CERRITO, CALIF. The coating on a substrate, be it decorative or protective, is required to with- stand a certain amount of physical damage. The damage may come during use from scratching due to blown gravel or sand, abrasives in cleansers, or the simple rubbing by adventitious contact by a hard object. In any case, the ap- pearance or the protective performance is no longer as likely to be acceptable. Customers like to specify survival against these detrimental influences; to do so, they may choose to perform lab tests to predict adequate performance. Hardness tests are used to make predictions. HARDNESS TESTS Hardness tests normally are classified into three categories (shown in Table I with ASTM standards where applicable). The three categories are indenters, rockers, and scratchers. We have included some abrasion-resistance test methods among the scratchers, although these stresses may be more complex than the sim- ple scratchers. The indenters come mainly from the metal hardness testers. The Pfund, Vick- ers, and Knoop testers use a pointed pyramidal probe, and the image of the in- dentation generated under a set weight is mathematically related to a hardness number. Supplier literature and standard references provide more information.1 A recent book describes several ways the Knoop and Vickers testers may be used and interpreted.2 A slightly different indentation device — the Barcol tester — has a conical point, and indicates the resistance to spring tension within the device (very like the Shore rubber hardness testers). The Barcol tester is mainly used in the plastics industry,3 but occasionally finds usage in plastic coatings that line gasoline storage tanks. There are cautions in using it, however. The surface should be smooth and flat, and the coating should ad here well to the substrate. Only count the highest of the readings taken. Rocker-type hardness testers essentially assess the damping of the rolling 288

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