Wear and adhesion of thin carbon coatings

The ability of a hard coating to resist delamination in wear applications is found to depend on the substrate crystal orientation. A variety of alloys with and without a carbon coating were tested by microscratching with a diamond stylus. The hardness of the carbon coating was characterized by nanoindentation techniques before the microscratch tests. As the diamond passed from one crystal to another differing amounts of damage were observed for each crystal orientation. The width of the delaminated region was found to be dependent on the extent of the slip line fields in the substrate generated by the scratching. To eliminate the possibility of differing adhesion on different crystal faces, a nickel aluminide intermetallic with the two crystals rotated about the same pole was tested. In the uncoated state when scratched across the grain boundary the slip line fields were extensive for one crystal but limited for the other crystal. Similar fixed-load scratches on the carbon coating produced delamination on the easy slip orientation but only a slight deformation of the coating on the other crystal. The observed failure appears to be due to the deformation of the substrate and not inferior adhesion of the coating to the crystal face. This finding should have important ramifications in the design and failure analysis of systems, such as magnetic recording media, medical implants, etc.