Elastic Lubricious Effect of Solidlike Boundary Films in Oil-Starvation Lubrication

Herein, a boundary
lubrication (BL) mechanism based on the carbon-based
tribofilms formed via Ni­(Cu)-catalytic dehydrogenation of hydrocarbon
oils was queried by performing standard ball-on-disk tribotests on
magnetron-sputtered WN, Ni, and W–N–Ni coatings in the
oil-starvation condition. The experimental results indicate that both
the bare GCr15 steel substrate and the sputtered W–N–Ni/MoN–Cu
coatings exhibited comparable fine antiwear performance with friction
coefficients below 0.09; however, no clues regarding the formation
of the effectiveness of the graphite-like tribofilms on the contact
surfaces were found, no matter whether the tested coatings were composited
in amorphous or nanocrystallized structures. An understanding of the
antiwear mechanism was achieved by conducting a nonequilibrium molecular
dynamics (NEMD) simulation of the confined shearing process, taking
1-decene chains as a model of the poly-α-olefin 10 base oil.
It was found that as the normal load increases from 0.14 to 9.2 GPa,
the “sandwiched” sheared oil molecules undergo a liquid-to-solidlike
phase transition at a threshold pressure of approximately 0.54 GPa,
above which the solidified oil film responds as an elastic protective
layer between rigid walls and provides lower friction coefficients
and energy dissipation than those of laminar shear motion. The main
reasons are the elastic compliance of solidlike oil molecules and
the occurrence of boundary slippage on the liquid–wall interface.
The comparison of NEMD simulation among confined sheared boundary
films of nonhydrogenated diamond-like carbon (DLC), hydrogen-terminated
DLC, laminar fluid layers, and solidified oil films further reveals
that it is the flexibility of C–C backbones as well as the
elastic compliance of terminal C–H bonds that play a dominant
role in the low-friction behavior of solidlike oil films in oil-starvation
lubrication. Moreover, the simulation results indicate that the temperature
increment of the confined sheared 1-decene under 0.14–9.2 GPa
loads is generally below 20 K, which is far below the active temperature
of 1-decene’s catalytic dehydrogenation and pyrolysis reaction.
These results support that in a steady state BL with low friction
coefficients at room temperature, the formation of lubricious tribofilms
via catalytic dissociation of alkanes can hardly be triggered due
to the limited temperature increment, even when the catalysts of Ni/Cu
are composited in the sliding surfaces.