Research ArticleMATERIALS SCIENCE

Negative friction coefficient in microscale graphite/mica layered heterojunctions

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Science Advances  17 Apr 2020:
Vol. 6, no. 16, eaaz6787
DOI: 10.1126/sciadv.aaz6787
  • Fig. 1 NFC in graphite/mica heterojunctions.

    (A) Schematic diagram of the experimental setup. (B) A typical friction loop with the enclosed area being the energy dissipation ΔE for a displacement D = 1 μm. The friction is defined as Ff = ∆E/2D. (C) The friction Ff shows a negative dependence on normal load FN. The Ff-FN curves in loading and unloading regimes almost overlap with each other. The slopes are defined as COF, which is found to be on the order of −0.01 (−0.012 ± 0.0005 for loading and −0.011 ± 0.0007 for unloading). (D) NFC is found in all the 11 flakes tested. The COF is found to be negative for both loading and unloading with only one exception (flake #6). The sliding velocity is 1 μm/s for all the tests.

  • Fig. 2 NFC at different temperatures and velocities measured experimentally.

    (A) NFC is found for the same flake at temperature ranging from room temperature (28°C) to 150°C. (B) COF as a function of temperature. NFC is found for all the temperatures tested. (C) Friction force under a normal load of 65 μN as a function of temperature (sliding velocity is 0.5, 1, and 5 μm/s, respectively). (D) Friction force under a normal load of 65 μN as a function of sliding velocity (temperature, 28°, 80°, and 150°C). All the error bars in these figures are calculated as the SDs of five independent friction loops.

  • Fig. 3 Friction measured experimentally for the graphite/mica heterojunction treated with RH ~60%.

    (A) The friction dependence on normal load for three samples. The slope from bottom to top is 0.020, −0.011; 0.031, 0.006; and 0.065, −0.112, respectively. (B) Friction of three consecutive measurements for the same sample. The slope from bottom to top is 0.020, −0.011; 0.005, −0.012; and 0.005, −0.018, respectively. The typical fitting error for the slope is 8.55 × 10−4. The error bars for all the data points are too small to be observed. A typical error bar for Ff is 0.153 μN. The sliding displacement is 1 μm, and the sliding velocity is 1 μm/s. The results are measured under ambient conditions (RH of ~22%) at room temperature (~28°C) after treated in high humidity.

  • Fig. 4 MD simulation results of NFC.

    (A) The simulation model of the graphite/mica heterojunction. From the top to bottom, the atoms in cyan, red, gray, blue, brown, green, and pink are carbon, oxygen in water, hydrogen, potassium, oxygen in mica, silicon, and aluminum, respectively. (B) Relationship between the friction stress τf (top) and out-of-plane deformation h (bottom) of the bottom graphene layer and the normal load pressure. The dashed lines are the linear fitting of the data points. The COF fitted is −0.012 ± 0.001. (C) Probability density distribution of oxygen atoms in water molecules along z axis. Full width at half maximum (FWHM) of the oxygen atom under different normal pressure is shown in the inset. (D) Structure factor of the oxygen atoms in water molecules along kx direction, while inset shows the two-dimensional landscape of the structure factor. a.u., arbitrary units.

  • Fig. 5 Load-dependent friction for the graphite/WSe2 heterojunctions measured experimentally.

    The COF for flake #1, flake #2, and flake #3 is 0.0067 (loading), 0.0083 (unloading); 0.0074 (loading), 0.0101 (unloading); and 0.0118 (loading), 0.0165 (unloading). The typical fitting error for COF is 4.57 × 10−4. The sliding displacement is 1 μm, and the sliding velocity is 1 μm/s. The results are measured in dry nitrogen (RH of <5%) at room temperature (~28°C).

Supplementary Materials

  • Supplementary Materials

    Negative friction coefficient in microscale graphite/mica layered heterojunctions

    Bingtong Liu, Jin Wang, Shuji Zhao, Cangyu Qu, Yuan Liu, Liran Ma, Zhihong Zhang, Kaihui Liu, Quanshui Zheng, Ming Ma

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