Effects of normal load on nanotribological properties of sputtered carbon nitride films

The nanotribological properties of amorphous carbon nitride (CN_x) films of ∼380 nm thickness were investigated, in the normal (contact) load range of 2–20 mN, using a Berkovich diamond indenter. The amorphous CN_x films tested in this work were grown on Si(100) substrates by reactive sputtering and energetic ion bombardment during deposition (IBD). The dependence of the friction behavior of the CN_x films on normal load (NL) was investigated in terms of nanomechanical properties, deformation mode and Atomic Force Microscopy (AFM) images of scratched surfaces, and the intensity of IBD. In films sputtered without IBD, the increase of the normal load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it remained constant. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and ploughing mechanisms contributed to the friction behavior in the intermediate and high-load ranges. Elastic and plastic deformation (PD) and delamination of the amorphous CN_x films occurred, depending on the normal-load ranges. On the other hand, films sputtered with high-energy IBD showed a load-dependent transition in both the scratch and the friction responses. Nanoscratching below 5 mN showed mainly elastic behavior of the film, 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 and AFM images showed no evidence of film failure. The increased load-carrying capacity, higher hardness and elastic response obtained with films grown with high-energy IBD, and the dominant friction mechanism at each load range illustrate the normal load dependence of the nanotribological properties of the sputtered CN_x films.