“…A G peak appeared at 1563 cm −1 in the transfer layers of PCD (see Fig. 2 ), revealing the possibility of surface graphitization during sliding contact 12 , 21 , 39 , 40 . Figure 2 Raman spectra of transfer layers (TL) formed on the counterfaces after sliding tests (22% RH) conducted on graphene and PCD.…”
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (E
b) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (E
B) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.
“…A G peak appeared at 1563 cm −1 in the transfer layers of PCD (see Fig. 2 ), revealing the possibility of surface graphitization during sliding contact 12 , 21 , 39 , 40 . Figure 2 Raman spectra of transfer layers (TL) formed on the counterfaces after sliding tests (22% RH) conducted on graphene and PCD.…”
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (E
b) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (E
B) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.
“…In recent years, some researchers have begun to explore the possible combined effects of doping elements and adding transition layers, so as to effectively improve the adhesion between the DLC coatings and Al alloys [ 22 , 23 , 24 ]. However, the effect of the transition layer thickness on the structure, mechanical and adhesion properties is still unclear, which is of great significance for the engineering application of the DLC coating on the Al alloy.…”
Multilayers of Ti doped diamond-like carbon (Ti-DLC) coatings were deposited on aluminum alloys by filtered cathodic vacuum arc (FCVA) technology using C2H2 as a reactive gas. The effect of different Ti transition layer thicknesses on the structure, mechanical and adhesion properties of the coatings, was investigated by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), nanoindentation and a scratch tester. The results showed that the Ti transition layer could improve interfacial transition between the coating and the substrate, which was beneficial in obtaining excellent adhesion of the coatings. The Ti transition layer thickness had no significant influence on the composition and structure of the coatings, whereas it affected the distortion of the sp2-C bond angle and length. Nanoindentation and scratch test results indicated that the mechanical and adhesion properties of the Ti-DLC coatings depended on the Ti transition layer thickness. The Ti transition layer proved favorable in decreasing the residual compressive stress of the coating. As the Ti transition layer thickness increased, the hardness value of the coating gradually decreased. However, its elastic modulus and adhesion exhibited an initial decrease followed by an increasing fluctuation. Among them, the Ti-DLC coating with a Ti transition layer thickness of 1.1 μm exhibited superior mechanical properties.
“…Previously, lots of experimental studies on the tribological properties of DLC films have been carried out by investigating the effects of intrinsic properties, i.e., hydrogen concentration, hardness, metal/nonmetal doping, and so on [8][9][10][11]. However, the tribological properties of DLC films depend on not only the intrinsic parameters, but also many extrinsic factors, such as sliding velocity, normal load, environment atmosphere, and properties of friction interfaces [12][13][14][15].…”
The purpose of the present work is to probe the friction mechanism of hydrogenated diamond-like carbon (H-DLC) film in air by varying sliding velocity (25-1000 mm/s). Friction tests of Al 2 O 3 ball against H-DLC film were conducted with a rotational ball-on-disk tribometer. As the sliding velocity increases, both the friction coefficient and the surface wear of H-DLC film decrease, reach the minimum values, and then increase in the high sliding velocity region. Based on the observed results, three main friction mechanisms of H-DLC film-namely graphitization mechanism, transfer layer mechanism, and passivation mechanism-are discussed. Raman analysis indicates that the graphitization of worn surface on the H-DLC film has a negligible contribution to the variation of the friction coefficient and the surface wear. The origin of the sliding velocity dependence is due to the synergistic interaction between the graphitized transfer layer formation and the surface passivation. The present study will not only enrich the understanding of friction mechanism of H-DLC films in air, but will also help to promote their practical engineering applications.
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