A coating is applied for one of two reasons―decoration or protection. In either case, there are specifications to be met and tests to show how well the coating performs in meeting those specifications. Most coating tests are physical assessments that imply a performance in use, but some are appearance tests as well. The ideal test is one that exactly replicates some aspect of the decorative or protective function. Unfortunately, most tests are hurry-up approximations and may not directly measure actual performance. The ideal test would measure exactly some parameter that directly relates to end use; however, because a physical test applies a complex stress to the coating, the coating may fail owing to only one or another of the actual pure stresses. For instance, a coating may mar in an impact test (one type of failure); it may scratch (another type of failure); or it may chip off (loss of adhesion, yet another type of failure). Each type of failure needs to be reported differently, and each says something about the coating formulation, processing or substrate.
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