High Temperature Tribological Behaviours of Silicon Doped Diamond-Like Films Produced by Hybrid HiPIMS-MFMS Co-Sputtering

Element doping is an important way to improve the thermal stability and tribological properties of diamond like carbon (DLC) films. In this paper, silicon doped hydrogen-free diamond-like carbon (Si-DLC) films with different Si contents were deposited on AISI 304 stainless steel substrate by a hybrid high-power impulse magnetron sputtering and mid-frequency magnetron sputtering (HiPIMS-MFMS) co-sputtering system. The effects of Si contents on the microstructure, mechanical and tribological properties of DLC films were systematically studied by atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, nano-indentation and UMT-TriboLab friction testing machine. The friction and wear mechanism of Si-DLC films at high temperature atmosphere was emphatically discussed. The results showed that Si atoms were randomly distributed in the amorphous DLC matrix in the form of tetrahedral silicon carbide, which effectively enhanced the toughness of the films. At the same time, the study also found that Si doping made the atomic structure of Si-DLC films changed more from graphite to diamond structure, thus improved the hardness of the films to a certain extent. In addition, the atomic clusters formed during doping increased the surface roughness of the films. The room temperature friction and wear results showed that the tribological properties of the films were improved by the appropriate amount of Si doping. When atom fraction of Si was 15.38%, the friction coefficient and wear rate of Si-DLC film at room temperature were the lowest. The high temperature friction results showed that the addition of silicon significantly enhanced the high temperature resistance and tribological properties of the film. The friction coefficient of Si-DLC film could also maintain a low value (about 0.1) at 300 ℃. It was due to the tetrahedral silicon carbide structure in Si-DLC films could improve the stability of sp³ bond. At the same time, the form of silicon oxide protective layer in Si-DLC film surface during high temperature friction process could slow down the oxidation of Si-DLC films and transition layer, so that the films could maintain a low friction coefficient and wear rate until 300 ℃. In addition, the large number of transfer films observed on the dual sphere was also an important reason for the low friction coefficient of the films. With the further increase of atom fraction of Si to 24.62%, the mechanical properties of Si-DLC films were improved, but the tribological properties of the films were decreased significantly. The main reason was that when Si content in Si-DLC film was too high, the high temperature friction in the air was easy to aggravate the oxidation of the films, thus generating more silicon oxide. The formation of excessive silicon oxide damaged the original structure of Si-DLC film, resulting in a significant degradation in the friction performance the films.