Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol’s three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule’s complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.
Large-scale quantum molecular dynamics of water-lubricated diamond (111) surfaces in sliding contact reveals multiple friction regimes. While water starvation causes amorphization of the tribological interface, small H_{2}O traces are sufficient to preserve crystallinity. This can result in high friction due to cold welding via ether groups or in ultralow friction due to aromatic surface passivation triggered by tribo-induced Pandey reconstruction. At higher water coverage, Grotthuss-type diffusion and H_{2}O dissociation yield dense H/OH surface passivation leading to another ultralow friction regime.
Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.
A stable
passivation of surface dangling bonds underlies the outstanding
friction properties of diamond and diamond-like carbon (DLC) coatings
in boundary lubrication. While hydrogen is the simplest termination
of a carbon dangling bond, fluorine can also be used as a monoatomic
termination, providing an even higher chemical stability. However,
whether and under which conditions a substitution of hydrogen with
fluorine can be beneficial to friction is still an open question.
Moreover, which of the chemical differences between C–H and
C–F bonds are responsible for the change in friction has not
been unequivocally understood yet. In order to shed light on this
problem, we develop a density functional theory-based, nonreactive
force field that describes the relevant properties of hydrogen- and
fluorine-terminated diamond and DLC tribological interfaces. Molecular
dynamics and nudged elastic band simulations reveal that the frictional
stress at such interfaces correlates with the corrugation of the contact
potential energy, thus ruling out a significant role of the mass of
the terminating species on friction. Furthermore, the corrugation
of the contact potential energy is almost exclusively determined by
steric factors, while electrostatic interactions only play a minor
role. In particular, friction between atomically flat diamond surfaces
is controlled by the density of terminations, by the C–H and
C–F bond lengths, and by the H and F atomic radii. For sliding
DLC/DLC interfaces, the intrinsic atomic-scale surface roughness plays
an additional role. While surface fluorination decreases the friction
of incommensurate diamond contacts, it can negatively affect the friction
performance of carbon surfaces that are disordered and not atomically
flat. This work provides a general framework to understand the impact
of chemical structure of surfaces on friction and to generate design
rules for optimally terminated low-friction systems.
We have developed a crystal growth simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD) method and applied it to plasma-enhanced chemical vapor deposition (PECVD) processes for silicon thin-film growth via SiH 3 radicals on hydrogen-terminated Si(001). We successfully simulated the abstraction of a surface hydrogen atom by irradiated SiH 3 radical and the formation of a dangling bond on the hydrogen-terminated Si(001) surface. SiH 3 radical was subsequently adsorbed on this dangling bond. When these processes were repeated, the thin film grew. Thus, a detailed mechanism was successfully found for the chemical reaction and electron transfer dynamics of silicon thin film growth by PECVD.
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