Microstructure, chemical bonds, and friction properties of nanocrystalline diamond films deposited in two different plasma media

Nanocrystalline diamond films with the properties dependent on the composition of the gaseous medium have been prepared using the microwave plasma enhanced chemical vapor deposition (MPECVD) method. A nanocrystalline film formed in the Ar/CH$_4$ plasma is characterized by a high crystallinity factor, a small grain size, a large fraction of $sp^2$-amorphous carbon, and, consequently, by an increase in the hardness and elastic modulus. The low value of the friction coefficient of this film is associated with the small grain size and large fraction of the $sp^2$-amorphous carbon boundary phase that ensures an easy sliding. The contact angle of the film is small (hydrophilic properties) in the case when the plasma consists of an Ar/CH$_4$ mixture. It has been shown that the wetting properties of the films are provided by a thin layer of carboxyl and hydroxyl functional groups passivating the dangling bonds at the surface that are responsible for the boundary lubrication mechanism. It has also been found that the friction coefficient of these films is inversely proportional to the contact pressure dependent on the diameter of the sliding counterbody ball.