Effects of indenter radius on the critical load in scratch testing

The scratch adhesion test appears to be a simple rapid method to assess qualitatively the adhesion of hard tribological coatings. Despite the widespread use of the technique, it is difficult to express adhesion in a quantitative way because the critical load is influenced by many factors such as substrate hardness, film thickness, interface bonding, and by several parameters related to the test condition. This paper considers the indenter radius effects on the scratch adhesion of TiN and CrN coatings deposited on metal substrates with different hardness by arc evaporation. The critical load varies with R-m, where R is indenter radius. The index m for the present coatings slightly increased with substrate hardness from 1.16 to 1.20 for 2.2 mum TiN coatings and from 1.11 to 1.24 for 2.8 mum CrN coatings. The critical scratch depth for coating failure depends on only the extent of sample's deformation, not the substrate materials. The critical load also increased proportionally to the product of the scratch depth and the composite hardness at the critical load. These results suggest that the failure mode of the present coatings is mainly dominated by cohesive strength of deposited films. The scratch adhesion was analyzed from the point of view of the composite hardness of the coating-substrate system. The calculated critical load showed good agreement with the measured critical load of coated materials. The indenter radius effects on the scratch adhesion related to the plastic properties of the indicated coated materials corresponding to the scratch depth at the critical load. (C) 2002 Elsevier Science B.V All rights reserved.