Research ArticleMETALS

Lowering coefficient of friction in Cu alloys with stable gradient nanostructures

See allHide authors and affiliations

Science Advances  09 Dec 2016:
Vol. 2, no. 12, e1601942
DOI: 10.1126/sciadv.1601942
  • Fig. 1 GNG structure and COF in the Cu-Ag sample.

    (A) Typical longitudinal-sectional scanning electron microscopy image of the as-prepared GNG Cu-Ag sample. (B) Bright-field TEM image about 3 μm below the surface in (A); inset shows a corresponding electron diffraction pattern. (C) Variation of longitudinal (dl) and transversal grain sizes (dt) and microhardness along depth from the surface. Error bars represent the SD of grain size and hardness measurements. (D) Variation of COFs with sliding cycles for the CG, NG, and GNG Cu-Ag samples sliding against WC-Co balls under a load of 50 N, a slide stroke of 1 mm, and a velocity of 10 mm/s. Inset shows COFs during the initial 100 cycles. (E) Variations of the steady-state COFs with the applied load for the three samples.

  • Fig. 2 Surface morphology after a single and repeated sliding.

    (A) Confocal laser microscopy images and 3D profiles for surface morphologies of the CG, NG, and GNG Cu-Ag samples after a single sliding. White arrows indicate the sliding directions. (B) Measured surface height profiles along the sliding direction in the CG, NG, and GNG Cu-Ag samples after different sliding cycles (as indicated), with corresponding confocal laser microscopy images for surface morphologies after sliding for 18,000 cycles (above). A load of 50 N, a slide stroke of 1 mm, and a velocity of 10 mm/s were applied for each sample. White double-ended arrows indicate the sliding directions.

  • Fig. 3 Variation of surface roughness and plastic strain gradients in the GNG and homogeneous NG samples.

    (A) Variations of average surface roughness (Ra and Rz) along the sliding direction with number of cycles in the CG, NG, and GNG samples. (B) Schematic variations of applied stress (dashed line, above) and plastic strain (εp; dashed lines, below) along the depth from the sliding surface in the GNG and NG samples, respectively. Measured variations of yield strength along the depth for the GNG (taken as approximately one-third microhardness) and NG samples are included (σy; solid lines). The depth of plastic deformation for the two samples was determined from subsurface structural observations.

  • Fig. 4 Friction-induced subsurface microstructure evolution.

    (A) Typical cross-sectional TEM image of the subsurface layer in the GNG sample after sliding for 27,000 cycles. (B) Variation of the mean grain sizes along the depth determined from TEM images in the GNG samples before and after sliding for 9000, 18,000, and 27,000 cycles, respectively. (C) Corresponding electron diffraction pattern in the topmost surface layers [as indicated in (A)]. (D) Typical cross-sectional TEM image of the subsurface layer in the NG samples after sliding for 18,000 cycles. (E) Corresponding electron diffraction pattern in the topmost surface layers [as indicated in (D)]. Sliding surfaces are outlined by dash-dotted lines, and the tribolayer/recrystallization interfaces by dashed lines. A load of 50 N, a slide stroke of 1 mm, and a velocity of 10 mm/s were applied for each sample.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/12/e1601942/DC1

    fig. S1. Measurement repeatability of COF.

    fig. S2. Measurement results of wear rates.

    fig. S3. Surface profiles and morphology of the CG sample under low-load single sliding.

    fig. S4. Effect of Ag addition on COF reduction—Measurement results in pure Cu samples.

    fig. S5. Counter surface analysis.

    fig. S6. COF measurement on the NG, GNG, and CG samples subsequently using exactly the same contact surface of a WC-Co ball.

    fig. S7. Stability of the subsurface microstructure in the GNG samples against sliding.

    fig. S8. Chemical analysis of the topmost NG surface layer.

    fig. S9. Subsurface microstructures in the CG Cu under sliding in the steady state.

    table S1. Surface roughness change after dry sliding for 18,000 cycles.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Measurement repeatability of COF.
    • fig. S2. Measurement results of wear rates.
    • fig. S3. Surface profiles and morphology of the CG sample under low-load single sliding.
    • fig. S4. Effect of Ag addition on COF reduction—Measurement results in pure Cu samples.
    • fig. S5. Counter surface analysis.
    • fig. S6. COF measurement on the NG, GNG, and CG samples subsequently using exactly the same contact surface of a WC-Co ball.
    • fig. S7. Stability of the subsurface microstructure in the GNG samples against sliding.
    • fig. S8. Chemical analysis of the topmost NG surface layer.
    • fig. S9. Subsurface microstructures in the CG Cu under sliding in the steady state.
    • table S1. Surface roughness change after dry sliding for 18,000 cycles.

    Download PDF

    Files in this Data Supplement:

Navigate This Article