Effect of contact geometry on the failure modes of thin coatings in the scratch adhesion test

To use the scratch adhesion test for assessing the adhesion of a hard, thin coating to its substrate, it must be ensured that the failure event represents the loss of adhesion, since many coatings fail by non-adhesive modes in this test. This study demonstrates, theoretically and experimentally, that indenter-induced bending stress is very likely to cause cohesive failures, thus, the adhesion of a coating to its substrate cannot be assessed in many scratch adhesion tests. To induce adhesive failure but suppress cohesive failure in the scratch adhesion test, the compressive coating stress should be high but the bending-induced stress should be low. This can be achieved by using an indenter with large radius and a high normal load. To assess coatings with good adhesion to the substrate, the radius of the most commonly used Rockwell C indenter may not be large enough.