Nanomechanical and nanotribological properties of carbon based films

Submicrometer-thick hard amorphous carbon (a-C) and carbon nitride (CNx) films were deposited on Si and stainless steel (SS) substrates by rf magnetron sputtering (MS) and reactive MS, respectively. a-C films were synthesized by either sputtering alternating ultrathin layers of soft and hard a-C on Si or by using a Cr interlayer between a-C and SS substrate. The main purpose of this work is to investigate and to compare the nanomechanical, adhesion and nanotribological properties of the CNx and a-C films. Hardness (H) and elastic modulus (E) were obtained from indentation experiments performed with a nanoindenter using the continuous stiffness method. The H of a-C films was found to be equal to ∼25 GPa, and the corresponding values of a single-layer amorphous CNx films were found to be equal to ∼12 GPa. Nanoscratch tests of the films were performed by a Nano Indenter XP system with a lateral force measuring attachment. Ultralow load scratch tests were performed in the load range from 2 to 20 mN. Below 5 mN, nanoscratching showed mainly elastic behavior of the CNx and a-C films, while above 10 mN, a mixed elastic-plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in situ profiling of the scratch trace showed no evidence of film failure. The coefficient of friction was found to vary in the range 0.15-0.3, being lower for a-C films. The higher elastic properties, no permanent damage and higher elastic recovery under the same normal load imply that a-C films grown on both SS and Si substrates sustain higher scratch-induced stresses and can be more effective as protective coating material.