What constitutes “failure” of a coating depends on the application. In optics, any scratch, even one that does not result in full coating delamination, can cause undesirable diffractions. Scratches that deform the coating may affect finish sheen, even if invisible to the naked eye. These types of failures are known as cohesive coating failures. Adhesive failures involve loss of material contact and subsequent exposure of substrate. In the case of protective coatings and traces designed to conduct electricity, small amounts of cohesive damage over the lifetime of a product may fit within the tolerances of the performance, while adhesive coating failures will likely be quickly followed by loss of function. Typically it is assumed that the resistance to scratch damage is the desirable consumer feature when speaking of coatings (unless the purpose of it is to come off, e.g. scratch-off lottery tickets).
Whether aesthetic or functional, once the concerns of a product development or QC engineer are correlated with microscopic scratch phenomena, the challenge becomes to explore how changing conditions in the manufacturing process can push these phenomena to occur at ever higher normal loads. An instrumented scratch tester is the ideal tool for this job.