Failure mode maps in the thin film scratch adhesion test

The scratch test has been used to assess thin coating adhesion for some time now. 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 coating detachment 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 particularly for tribological applications. In this paper the load regimes in which the main adhesion-related failure modes (spallation and buckling) occur as a function of coating thickness will be presented for thermally grown oxide and sputtered nitride coatings. The origin of the observed failure modes and the use of the scratch test to assess coating/substrate adhesion in a more quantitative fashion is discussed in the light of these observations. (C) 1997 Elsevier Science Ltd.