INFLUENCE OF SUBSTRATE SURFACE-TOPOGRAPHY ON THE CRITICAL NORMAL FORCE IN SCRATCH ADHESION TESTING OF TIN-COATED STEELS

The influence of pre-coating substrate surface topography on the critical normal force obtained during scratch testing of four different TiN-coated tool steels has been studied. Six different surface preparation techniques (three based on grinding, and three on grit blasting) were used. The resulting surface topographies were characterized using scanning electron microscopy and surface profilometry. The critical normal force decreases somewhat with increasing surface roughness for two TiN-coated high speed steels, while no effect of surface roughness is found for coated hot- and cold-work steels. In the latter cases, an eventual influence of surface roughness is screened by the relatively large experimental scatter. The results of the present investigation emphasize the importance of a thorough post-test scratch inspection and that the definition of "critical normal force" is important for the relevance of the scratch adhesion test. An important conclusion is that the critical normal forces frequently quoted as measures of the adhesion between well-adhering coatings and their substrates instead are measures of the ability of the coatings to resist deformation.