Fig. 1. Schematic illustration of coating thickness quantification methods under dry friction. (a) Coating thickness quantification method based on the Raman signal of the silicon substrate. (b) Light wavelength shift on the sites of Raman scattering. (c) Coating thickness quantification method based on the Raman signal of carbon films.

Fig. 2. Transmittance spectra (a) and reflectivity spectra (b) of as-grown a-C:H films of different thicknesses deposited on the glass substrate. Raman spectra of as-grown a-C:H films of different thicknesses deposited on silicon wafers ((c), silicon 1st band; (d), carbon band). Comparison between the measured thickness of as-grown a-C:H films by optical profilometer and calculated thickness based on Raman intensity of silicon band (e) and G peak of carbon band (f).

Fig. 3. (a) Intensity of silicon bands, intensity and position of G peak of carbon bands, and area ratios of D peak and G peak (AD/AG). AD/AG increases obviously in the center region of wear track indicating the increase of sp2 fraction. (b) Calculated wear profile curves based on the Raman intensity of silicon signal and carbon signal.

Fig. 4. (a-c) Comparison of the wear profile curves obtained by different methods (90 min tribo-test). Non-contact optical profilometer: wear profile curves before (blue line) and after depositing iridium layer (green line), and wear profile curves after FIB (purple line). Wear quantification method based on the Raman intensity of silicon signal (red line). Combination of FIB, SEM, and TEM measurements: actual thickness values (+) in the marked areas of (a) and (b). Grey areas are corresponding to targeted FIB areas.

Fig. 5. Comparison between calculated wear profile derived from Raman intensity of silicon and carbon bands and measured wear profile characterized by non-contact optical profilometer (before and after depositing iridium layer on top of a-C:H).

Fig. 6. TEM images showing the cross-sectional morphology of FIB lamellar specimens from the center area (a), side area (d) and unworn area (g) as marked in Fig. 5, evolution of C-K EELS (STEM) core-edge spectra recorded across the cross-sectional area from the wear center (b), side area (e) and unworn area (h) as marked in (a), (d) and (g) respectively, and evolution of the calculated EELS C-bonds fractions across the cross-sectional area from the wear center (c), side area (f) and unworn area (i) from the EELS C-K edges presented in (b), (e) and (h). Error bars denote s.d. of calculated bond fractions. TEM images of the cross-sectional morphology of the interface areas between a-C:H and Si wafer from the center area (j), side area (k), and unworn area (l). The thickness of the layer with nanovoid increases as it moves towards to wear center.

Fig. 7. (a-h) Comparison of the wear profile curves obtained by different methods (dry friction with test time range 10-110 min). Non-contact optical profilometer: wear profile curves before (blue line) and after depositing iridium layer (green line). Wear quantification method based on the Raman intensity of silicon signal (red line). The AD/AG ratios across the wear scars (grey line). (i) Evolution of width and depth of wear scars with test time.

Fig. 8. Schematic illustration of the coating thickness quantification methods under oil-lubricated condition.

Fig. 9. (a-d) Comparison of the wear profile curves obtained by different methods (oil-lubricated condition, PAO; test time 30-110 min). Non-contact optical profilometer: wear profile curves before (blue line) and after depositing iridium layer (green line). Wear quantification method based on the Raman intensity of silicon signal (black and red lines). (e) Evolution of width and depth of wear scars with test time. (f) Comparison of the wear profile curves obtained by different methods. Here, we drop PAO on top of the tested sample under dry friction with test time of 110 min as shown in Fig. 7(h). Non-contact optical profilometer: wear profile curves before (blue line) and after depositing iridium layer (green line). Wear quantification method based on the Raman intensity of silicon signal with oil on top (black and red lines).