ABSTRACT
Scratch adhesion testing consists in displacing the coated specimen at constant velocity under a stylus which applies a stepwise [1] or countinuously [2] increasing normal load to the coating surface. With increasing load, the coating/substrate deformation generates stresses which, at a given load, result in permanent damage, such as chipping of the coating (cohesive failure) or flaking (adhesion failure). The smallest load leading to unacceptable damage is the 'critical' load. Various coating failure criteria have been used to define the critical load, such as initiation of cracking, chipping of
the coating, spalling, initiation of flaking [3], or complete removal of the coating from the scratch track [1]. In essence in each case the criterion adopted for characterizing the failure of the coating should depend on the nature and extent of the damage that can possibly be tolerated for that particular application. Thus, the presence of a small number of microcracks may be acceptable in a protective coating which is anodic (sacrificial) with respect to the substrate [4]. In this case the critical load should not be identified with the cohesive failure load, Lc, which is the minimum load at which crack initiation occurs within the coating, but rather with the adhesion failure load, LA, which is the minimum load at which the crack reaches the coating/substrate interface, causing detachment of the coating and exposure of the substrate. On the other hand, the presence of a single microcrack may be catastrophic when the protective coating is cathodic (noble) with respect to the substrate and the cohesive failure load should then be taken as the critical load.
