Research ArticleAPPLIED PHYSICS

Atomic-scale insights into the interfacial instability of superlubricity in hydrogenated amorphous carbon films

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Science Advances  27 Mar 2020:
Vol. 6, no. 13, eaay1272
DOI: 10.1126/sciadv.aay1272
  • Fig. 1 Structures, compositions, mechanical properties, and frictional performances of a-C:Hs films.

    (A) Atomic composition and basic properties of the as-synthesized various kinds of a-C:Hs films. (B) The low-magnification cross-sectional bright-field (BF)–STEM image shows a typical bilayer structure of the a-C:H:O (designated as ACFO-1) film grown on Si wafer. (C) The high-magnification BF-STEM image and (D) the EELS spectrum image (SI) mapping image reveal that the high-energy ion implantation induced zig-zag interfacial morphology, confirming the excellent bonding adhesion of the interlayer to the Si wafer. (E) High-resolution TEM (HRTEM) image and diffraction pattern demonstrating the amorphous characteristic of the ACFO-1 film. (F) Measured C-K and O-K EELS spectra of the ACFO-1 film. a.u., arbitrary units. (G) Frictional behaviors of self-mated ACF-1, ACFO-1, and ACFO-2 tribo-couples in dry gaseous atmospheres of O2 and N2, for the purpose of evaluating the oxygen effect. (H) Frictional behaviors of a-C:H (ACF-1) and Si-doped a-C:H:Si (ACF-2 to ACF-6) films sliding against bare steel in ambient air, for the purpose of evaluating the humid effect.

  • Fig. 2 Surface morphologies and Raman spectra of the a-C:Hs contact areas after the friction tests in Fig. 1 (G and H).

    (A) Optical images showing the ball wear scars and disc wear tracks for a-C:Hs in different environments. The FIB-slicing cites for preparing nanoscale-thick TEM lamellas and Raman measuring positions are also indicated in the images. (B) Raman spectra showing the structural evolutions of different contact surfaces by evaluating the changes of photoluminescence (PL) background, as well as the position and shape of D- and G-peaks.

  • Fig. 3 Interfacial microstructures and surface chemistry of self-mated a-C:H films after the friction tests in dry N2 and O2 gaseous atmospheres (ACF-1, Fig. 1G).

    (A and B) HRTEM images showing the microstructures of the a-C:H sliding interfaces from the ball wear scars resulted from dry N2 and O2, respectively. (C) TOF-SIMS intensity spectrum revealing the chemical composition of the ball wear scar as a function of profiling depth. A tribo-affected sublayer with a thickness of 2 nm was detected. (D) 3D TOF-SIMS image further confirming the existence of an oxygen-rich (carbon-deficient) near-surface layer. (E) 2D TOF-SIMS images showing the distributions of chemical fragments within the disc wear track and ball wear scar.

  • Fig. 4 Surface morphologies and interfacial microstructures of self-mated a-C:H:O films after the friction tests in dry N2 gaseous atmosphere (ACFO-1 and ACFO-2, Fig. 1G).

    (A and B) 3D atomic force microscopy images showing the ultrasmooth wear scar surfaces of ACFO-1 and ACFO-2, respectively. The insets are the optical images captured during probe scanning. (C to E) HRTEM images showing the bonding structures from different sliding interfaces: (C) ball wear scar for ACFO-1, (D) ball wear scar for ACFO-2, and (E) disc wear track for ACFO-2. (F) BF-STEM image showing the existence of a shallow shear band on top of the bulk a-C:H:O (ACFO-2). The positions for EELS SI mapping and line acquisition are also indicated. (G) EELS Cr-L, C-K, and O-K maps and their composite showing the elemental distribution along the sliding interface as marked in (F). (H) Evolution of EELS C-K and O-K core edge spectra recorded across the shear band as marked in (F). (I) Carbon bonding fractions calculated from the C-K core edges in (H) and the mass density estimated from the plasmon energy in the low-loss spectra (fig. S2).

  • Fig. 5 Microstructures of the tribolayer formed on bare steel ball surface after sliding against a-C:H film in humid air (ACF-1, Fig. 1H).

    (A) Low-magnification BF-STEM image showing a tribolayer with a thickness of 250 nm formed on bare steel ball surface. (B) HAADF-STEM image indicating the tribolayer is mainly composed of three sublayers, marked as (C), (D), and (E). (C to E) High-magnification BF-STEM images clarifying the microstructures of the as-observed three sublayers in (B): a tumor-like nanocluster-distributed amorphous sublayer near the sliding surface (C), an iron oxide nano-crystallites/amorphous domains mixed interlayer in the middle (D) and an iron-based highly oxidized and crystallized bottom layer near the steel ball surface (E). Insets are the corresponding fast Fourier transform (FFT) images for each sublayer. (F) EELS SI maps of C-K, O-K, Fe-L, and their composite image show the elemental distribution across the tribolayer, as marked in (B). (G) EDS elemental distribution across the tribolayer as marked in (B). The simultaneously recorded plasmon energies and the calculated mass densities are also indicated. (H) Evolution of EELS C-K, O-K, and Fe-L core edges acquired point by point across the tribolayer as marked in (B).

  • Fig. 6 Microstructures of the antimoisture tribolayer formed on bare steel ball surface after sliding against a-C:H:Si film in humid air (ACF-6, Fig. 1H).

    (A) Low-magnification HAADF-STEM image showing the retained winding interface of the bare steel ball surface after the superlubricity test. The inset BF-STEM image indicates the existence of a nanoscale tribolayer with a thickness of 5 nm formed on the contact surface. (B) High-magnification BF-STEM image showing the tribolayer is mainly amorphous and well bonded to the steel surface as marked in (A). (C) HAADF-STEM image and the corresponding EDS maps showing the elemental distributions of Si-K, O-K, C-K, and Fe-K within the tribolayer. (D) The EELS Si-L, O-K, C-K, and Fe-L core edges acquired from the central position of the tribolayer as marked in (B).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/13/eaay1272/DC1

    Fig. S1. Surface morphologies and cross sections of the wear tracks produced on amorphous carbon film surfaces after the friction tests as shown in Fig. 1 (G and H).

    Fig. S2. Evolution of EELS low-loss spectra recorded across the shallow shear band as marked in Fig. 4F and the derived Ep values through peak fitting.

    Fig. S3. STEM and EELS characterization of the wear track formed on a-C:H film surface after sliding against bare SUJ2 steel ball in humid air (ACF-1, Fig. 1H).

    Fig. S4. FIB slicing and TEM characterization confirming the variable thicknesses of the tribolayer in different contact positions of the wear scar formed on bare steel ball surface after sliding against a-C:H:Si film in humid air (ACF-6, Fig. 1H).

    Fig. S5. STEM and EELS characterization of the wear track formed on a-C:H:Si film surface after sliding against bare SUJ2 steel ball in humid air (ACF-6, Fig. 1H).

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Surface morphologies and cross sections of the wear tracks produced on amorphous carbon film surfaces after the friction tests as shown in Fig. 1 (G and H).
    • Fig. S2. Evolution of EELS low-loss spectra recorded across the shallow shear band as marked in Fig. 4F and the derived Ep values through peak fitting.
    • Fig. S3. STEM and EELS characterization of the wear track formed on a-C:H film surface after sliding against bare SUJ2 steel ball in humid air (ACF-1, Fig. 1H).
    • Fig. S4. FIB slicing and TEM characterization confirming the variable thicknesses of the tribolayer in different contact positions of the wear scar formed on bare steel ball surface after sliding against a-C:H:Si film in humid air (ACF-6, Fig. 1H).
    • Fig. S5. STEM and EELS characterization of the wear track formed on a-C:H:Si film surface after sliding against bare SUJ2 steel ball in humid air (ACF-6, Fig. 1H).

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