Understanding the Role of N-Incorporation on the Structure and Properties of Si-DLC Film and the Tribological Mechanism

Si/N-DLC films were deposited to analyze and explore the role of N-incorporation on the structure, mechanical properties and tribological behaviors of Si-DLC films and the low friction and wear mechanism of Si/N-DLC films by controlling the flow of N₂ precursors, which taken advantage of plane cathode plasma enhanced chemical vapor deposition (PC-PECVD) technology. The results showed that the surface and cross-sectional morphologies of as-deposited Si/N-DLC film were uniform and compact, and the microscopic defects and cracks were not observed. No stratification was observed between the substrate and the transition layer, and between the transition layer and Si/N-DLC film, exhibiting an excellent adhesion. The introduction of N element induced the formation of ring sp² carbon structure in DLC films, resulting in the increase of sp²-C content in Si-DLC films, in other words, N-incorporation leaded to the graphitization transition of Si-DLC structure, meaning that the structure of Si-DLC films was more orderly. Besides, the reduction of sp³ fraction caused by N-incorporation increased the hardness and elastic modulus of the films. Interestingly, with the enhancement of N content, the toughness of Si-DLC films and the adhesion with the substrate exhibited a trend of first increasing and then decreasing. The incorporation of a appropriate amount of N element (50 sccm) improved significantly the toughness and the adhesion (>20 N). More importantly, N-incorporation effectively reduced the friction coefficient of Si-DLC film and improved the wear resistance. Compared with Si-DLC film, Si/N-DLC films generally showed lower friction coefficient and wear rate, which decreased firstly and the increase with the improvement of N content. Si/N-DLC film deposited with the N₂ flow of 75 sccm showed the lowest friction coefficient (0.039 5) and wear rate 1.63×10⁻⁷ mm³/(N·m), which were reduced by about 26% and 45% compared with the Si-DLC film (0 sccm), respectively. The friction mechanism was that the formation of graphite-like carbon (GLC) transfer film composed of C, Si and O element, resulting in the transfer of friction interface from GCr15/DLC to the GLC transfer film /DLC. In addition, the decrease of the friction coefficient depended on the graphitization degree and hydrogen content of the friction interface. On the one hand, the sliding interface with high graphitization degree was easy to shear, resulting in low interface friction; On the other hand, there was a repulsive force between hydrogen on the surface of GLC transfer film and hydrogen on the surface of Si/N- DLC film, and it was enhanced with the increase of interface hydrogen content, which promoted the sliding of friction interface and further reduced the friction coefficient of Si/N-DLC film system. The wear behaviors were limited to the film toughness and the ability to resist elastoplastic deformation. The brittle fracture notch formed inside the wear track of Si/N-DLC films with low N-incorporation content, which was attributed to the lower film toughness. It caused large area damage of the transfer film, and aggravated the adhesive wear. In addition, the optimal service range (contact pressure and maximum sliding linear velocity) of Si/N-DLC films with low friction (≤0.05) was determined. The relevant results provided reference for structure and property regulation and engineering application of Si/N-DLC films.