Superlow Friction of a-C:H Coatings in Vacuum: Passivation Regimes and Structural Characterization of the Sliding Interfaces

Abstract

A combination of atomistic simulations and vacuum tribometry allows atomic-scale insights into the chemical structure of superlubricious hydrogenated diamond-like carbon (a-C:H) interfaces in vacuum. Quantum molecular dynamics shearing simulations provide a structure-property map of the friction regimes that characterize the dry sliding of a-C:H. Shear stresses and structural properties at the sliding interfaces are crucially determined by the hydrogen content CH in the shear zone of the a-C:H coating. Extremely small CH (below 3 at.%) cause cold welding, mechanical mixing and high friction. At intermediate CH (ranging approximately from 3 to 20 at.%), cold welding in combination with mechanical mixing remains the dominant sliding mode, but some a-C:H samples undergo aromatization, resulting in a superlubricious sliding interface. A further increase in CH (above 20 at.%) prevents cold welding completely and changes the superlubricity mechanism from aromatic to hydrogen passivation. The hydrogen-passivated surfaces are composed of short hydrocarbon chains hinting at a tribo-induced oligomerization reaction. In the absence of cold welding, friction strongly correlates with nanoscale roughness, measured by the overlap of colliding protrusions at the sliding interface. Finally, the atomistic friction map is related to reciprocating friction experiments in ultrahigh vacuum. Accompanying X-ray photoelectron and Auger electron spectroscopy (XPS, XAES) analyses elucidate structural changes during vacuum sliding of a hydrogen-rich a-C:H with 36 at.% hydrogen. Initially, the a-C:H is covered by a nanometer-thick hydrogen-depleted surface layer. After a short running-in phase that results in hydrogen accumulation, superlubricity is established. XPS and XAES indicate a non-aromatic 1–2-nm-thick surface layer with polyethylene-like composition in agreement with our simulations.

Superlubricity of Silicon-Based Ceramics Sliding against Hydrogenated Amorphous Carbon in Ultrahigh Vacuum: Mechanisms of Transfer Film Formation

“…In this case, the D parameters are 18.5 and 17.2 eV for the Si 3 N 4 and SiO 2 balls, respectively, i.e., larger than the D parameter recorded outside the wear scar on the a-C:H flat (D = 16.8 eV, Figure 2d). In addition, shoulder peaks around 262 eV, attributed to the presence of polymeric chains −(CH 2 −CH 2 ) n −, 23 are less pronounced inside the wear scars on the ceramic balls than inside and outside the wear scars on the a-C:H(36)-coated flats. These spectroscopic analyses confirm the presence of a carbonaceous transfer film with a thickness of a few nanometers on the ceramic surfaces and suggest sp 3 -to-sp 2 rehybridization and dehydrogenation of the transfer films.…”

“…At first sight, these results could be ascribed solely to the number of hydrogen-passivated carbon dangling bonds, which should increase with increasing C H . However, it should be noted that the passivation of a-C:H surfaces can be effectively achieved also at low C H , for instance by localized aromatization . Both passivation mechanisms require plastic rearrangement of a-C:H during running-in .…”

“…However, it should be noted that the passivation of a-C:H surfaces can be effectively achieved also at low C H , for instance by localized aromatization. 23 Both passivation mechanisms require plastic rearrangement of a-C:H during running-in. 23 Therefore, it is crucial to investigate the effect of C H on the a-C:H plastic shear deformation and on the film transfer process.…”

Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon

“…Therefore, the frequency of thermal barrier crossings is underestimated and most reactions occur via stress-induced instabilities. However, it is evident from previous studies that simulations at higher shear rates still provide useful qualitative insights into mechanochemical reactions in carbon based tribosystems 37 , 38 .…”

A review on diamond-like carbon-based films for space tribology

“…Lugo et al [27] deposited hydrogenated DLC films using a pulsed DC PECVD system and controlled hydrogen content by altering deposition voltage and found films with high-hydrogen content (40 at.-%) have lower COF (0.014) compared with higher COF (0.38) of low-hydrogen film (25.52 ± 0.82 at.-%). Kuwahara et al [28] combined atomistic simulation and vacuum tribometry to reveal atomic-scale structure of the super-lubricated a-C:H surface during friction in vacuum. It was found that hydrogen content on the counterface plays a major role in vacuum friction.…”