FAILURE MODES IN SCRATCH ADHESION TESTING

The scratch test has been used to assess coating adhesion for some time. In this test, a diamond indenter is drawn across the coated surface under an increasing load (either stepwise or continuous) until at some load, termed the critical load L(c), a well-defined failure event occurs; if this failure event represents the loss of coating-substrate adhesion then the critical load can be used as a qualitative measure of coating-substrate adhesion. However, it is well known that a range of possible failure modes can occur and only some of these are dependent on adhesion; other failure modes which depend on plastic deformation and fracture within the coating, rather than any adhesive failure at the coating-substrate interface, may be just as useful in the assessment of coating quality for tribological applications. In this study, titanium nitride coatings have been deposited onto a range of different substrates from soft nickel to a hard cemented carbide and the failure modes which occur during scratch testing have been identified. Failures fall into two general groups, depending on whether the substrate behaves in a brittle or ductile manner during the scratch test. Similar failure modes are observed for titanium nitride coatings produced by several deposition technologies, although there is some variation in the appearance of specific failures which is process dependent. Many of the same failure modes are also observed for other coating materials, such as ZrN or hard carbon, but several different types of failure are also observed. The generation of acoustic emission during the test is related to the occurrence of these different failure modes. The origin of the failure modes and the use of the scratch test to assess coating-substrate adhesion are discussed in the light of these observations.