A comparative study of the nanoscratching behavior of amorphous carbon films grown under various deposition conditions

Two types of amorphous carbon (a-C) films: one series with a layered structure (sequence of soft/hard carbon layers) resulting in thick, stable and sp3 rich films, and another series of monolithic films rich in sp2, were deposited on Si substrates by magnetron sputtering and were investigated with respect to their nanoscratching behavior. The main purposes of this work are to investigate the scratch performance of a-C films developed in layered structure and to compare this with the films rich in sp2 content. Ultra-low load scratch tests were performed in the load range from 2 to 20 mN to explore the deformation mechanisms of such thin films and their adhesion to the substrate. Below 5 mN, the scratches on layered structure films exhibit an almost full elastic response while on films rich in sp2 content show massive brittle fragmentation. At 20 mN, the scratches on the former films reveal some areas which are completely elastic and some areas which were typical of a buckle failure. These areas may correspond to hard and soft carbon layers, respectively. The coefficient of friction varied from ∼0.2 to ∼0.3, depending on the maximum normal load and being lower for films of the first type. Scratch testing results have shown that layered structured films are resistant to plastic deformation during contact and, together with the stability of the thick layered films, support their potential for protective overcoats.