Scratch resistance of superhard carbon coatings - a new approach to failure and adhesion evaluation

Scratch testing was methodically studied by examining the adhesion of tetrahedral amorphous carbon coatings (ta-C). Usually, critical loads LC are used as a parameter to characterize the adhesion strength of coatings in scratch testing. However, the comparability of the critical loads for different materials and coating thicknesses is limited. The critical loads result from different coating failure mechanisms that require a careful investigation with respect to plastic substrate deformation and coating thickness. In this work, the different failure modes of ta-C coatings occurred in strong dependence on the geometrical proportions (indenter radius and coating thickness) for a wide range of hardness and Young's modulus: Adhesive failure in the center of the groove was located at cohesive bending cracks for coating thickness of 1 to 3 mm. For increasing coating thickness failure by spallations was found instead of bending cracks. At the edge of the groove outside the scratch-induced substrate plasticity, wing-shaped delaminations were observed for all test and coating configurations. A new method was introduced that quantifies the adhesive failure by means of segmentation of an optical image. The relative area of delamination was found to correlate with calculated shear stresses at the interface plane outside the heavily deformed scratch track. Furthermore, it was possible to distinguish the effect of two different plasma pretreatments on adhesion regardless of the coating thickness. Thus, this new method expanded the information gained from the standard scratch test and allows a better interpretation of coating adhesion than the LC-evaluation.