Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films

Abstract

Tribological behaviors of hydrogenated amorphous carbon (a-C:H) films under single asperity contact are investigated by molecular dynamics (MD) simulations. Hydrogen concentration and normal load are found to play essential roles in the frictional behavior of a-C:H films. With low hydrogen concentration, the a-C:H film shows high adhesion and friction even at very low normal loads (1.75 nN). The sp 3 -to-sp 2 rehybridization is observed in the a-C:H films with all studied hydrogen concentrations, which is greatly enhanced with increasing normal load. At high normal loads, formation of nanocrystalline graphene-like lamellar structures is observed locally, usually beneath the instantaneous contact area, acting as a lubricating agent, demonstrating local graphitization due to combined effects of the compression and shear process. With high hydrogen concentration, the friction shows a linear increase along with the normal load and higher load bearing capability. Hydrogen atoms accumulate on the film surface during the sliding process, reducing friction significantly.Hydrogen passivation becomes more obvious with higher hydrogen content especially at low normal loads, where a hydrogen-rich monolayer is found to attach onto the asperity, functioning as a lubricating transfer layer. This work shows the combined effects of structural transformation and surface passivation on the frictional behaviors of a-C:H films, and may shed light on the well-known but not well-understood superlubricity mechanism of a-C:H films.

Advanced spectroscopic analyses on a:C-H materials: Revisiting the EELS characterization and its coupling with multi-wavelength Raman spectroscopy

“…Recently there is a renewed interest for these materials as they exhibits a superlow friction coefficient and long wear life in ultra-high vacuum which makes them one of the most promising solid lubricant coating candidate for aerospace applications [6][7][8][9][10]. The origin of the superlow friction is still under investigation but seems to be closely related to the passivation of the surface by the hydrogen and to the sp 3 to sp 2 rehybridization [8][9][10][11]. The proper characterization of the hydrogen content together with the sp 2 fraction is thus of particular importance.…”

Crack-tip plasticity and intrinsic toughening in nano-sized brittle amorphous carbon

“…Under this circumstance, the spatial distribution of sp 2 /sp 3 ratio can serve as an experimental tracer for the local plastic deformation in DLC materials because the carbon orbital rehybridization from sp 3 to sp 2 may concurrently occur with this atomic shearing process (Liu and Meletis, 1997;Voevodin et al, 1996) -or we can equivalently consider the orbital rehybridization as one of the specific atomic shearing mechanisms in DLC. In most amorphous carbon materials, both sp 2 and sp 3 atomic structures co-exist (Kowalczyk et al, 2012;Marks et al, 1996;Robertson and O'reilly, 1987;Theye and Paret, 2002) and the externally-applied high shear stresses assist transitions between them (Bouchet et al, 2015;Chen et al, 2015;Gao et al, 2002;Kunze et al, 2014;Li et al, 2019;Ma et al, 2014;Pastewka et al, 2011;Romero et al, 2014;Sanchez-Lopez et al, 2003). This dependence of rehybridization on the shear stress magnitude enables us to experimentally assess the distribution of local plastic deformation of DLC by using sp 2 /sp 3 ratio as an indicator for the amount of local plastic strain.…”

Microtribological Properties of DLC Films Prepared by Different Methods

“…In addition, they also tested the stick-slip phenomenon when the prove tip was slipped over the graphite and mica surface. Since then, numerous studies [16][17][18] have distinguished the characteristics of microtribology from macrofriction. Nonetheless, the microtribology mechanism remains poorly understood.…”