Abstract
Friction was measured on a single molybdenum disulfide (MoS2) nanotube and on a single MoS2 nano-onion for the first time. We used atomic force microscopy (AFM) operating in ultra-high vacuum at room temperature. The average coefficient of friction between the AFM tip and MoS2 nanotubes was found considerably below the corresponding values obtained from an air-cleaved MoS2 single crystal or graphite. We revealed a nontrivial dependency of friction on interaction strength between the nanotube and the underlying substrate. Friction on detached or weakly supported nanotubes by the substrate was several times smaller (0.023 ± 0.005) than that on well-supported nanotubes (0.08 ± 0.02). We propose an explanation of a quarter of a century old phenomena of higher friction found for intracrystalline (0.06) than for intercrystalline slip (0.025) in MoS2. Friction test on a single MoS2 nano-onion revealed a combined gliding-rolling process.PACS, 62.20, 61.46.Fg, 68.37 Ps
Highlights
Inorganic solid lubricant molybdenum disulfide (MoS2) is a known lubricant, which has been applied extensively for decades
We report on the first tribo-testing performed on a single MoS2 nanotube and on a single MoS2 nano-onion using atomic force microscopy (AFM) in ultra-high vacuum (UHV) at room temperature
The counterparts in the experiments consisted of the AFM tip and the top surface of nanotubes
Summary
Inorganic solid lubricant molybdenum disulfide (MoS2) is a known lubricant, which has been applied extensively for decades. Tribological properties, and anti-corrosive mechanisms have been studied deeply [1]. The easy mutual gliding of MoS2 layers along (001) basal planes and its surface inertness allow its low friction performance. Ultra-low friction coefficients as low as 0.003 between MoS2 flakes and MoS2 surfaces have been reported [2] and explained by an easy shear of basal planes of the crystal structure parallel to the sliding direction in accordance with the Amontons-Coulomb law. Edges of layered crystals with high hardness are prone to oxidation which reduces the efficiency of lubrication, especially in humid environment. Thin flakes with a high active surface and with relatively low number of unsaturated bonds at edges are preferable. The lubrication mechanism of MoS2 nanosheets, 50-nm thick, prepared
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