The scratch tester equipment uses a small diamond stylus to test the adhesion of coatings. The diamond with a radius of 200µm is scratched across the coated surface of a substrate at a constant velocity whilst a load is applied with a constant loading rate. The load on the diamond causes stresses to be increased at the interface between the coating and the substrate that can result in delamination or chipping of the coating to occur. The load at which the coating first delaminates is called the critical load.
Coating failure can be detected using a number of different methods such as a load cell that measures the change in friction, coustic emission or simply looking a the scratch channel under a microscope.