*3.1. Crystal Structures and Morphological Features of the Coating Layers*

Figure 1 displays representative XRD spectra of non-coated and DLC-coated wire specimens. No peak was obtained for either DLC-coated specimen due to their amorphous structures. The XRD spectra for the non-coated wire specimen contained peaks associated with the austenite phase (γ-Fe) (ICDD PDF 01-071-4649) and a non-indexed peak at 21.6◦.

Representative SEM images of the non-coated and DLC-coated wire specimens are shown in Figure 2. The thin DLC layers on the wire specimen surfaces were ca. 300 nm thick for both the DLC-1 and DLC-2 cases. Good interfacial adhesion was observed between all DLC-deposited layers and bulk materials.

**Figure 1.** X-ray diffraction patterns obtained from the surfaces of non-coated and diamond-like carbon (DLC)-coated wire specimens.

**Figure 2.** Scanning electron microscopy images of cross-sectioned disk (first raw) and wire specimens (second raw): (**a**,**c**) DLC-coated specimens (DLC-1) and (**b**,**d**) DLC-coated specimens (DLC-2). DL, DLC layer; D, disk; W, wire; E, epoxy resin. Original magnification: 20,000×.

#### *3.2. Compositional Characterization of the Coating*

Figure 3 shows hydrogen depth profile (concentration relative to carbon) by the elastic recoil detection analysis (ERDA) to a depth of 600 Å (60 nm) for the DLC-coated disk specimens. A higher hydrogen concentration was detected for DLC-2. The average hydrogen contents from the top surface to a depth of 600 Å were 23% for DLC-1 and 27% for DLC-2; the external surface regions contained 29% for DLC-1 and 33% for DLC-2.

Figure 4 shows the C 1s spectra obtained by XPS for the DLC-coated disk specimens. Gaussian-Lorentzian curve fitting was used to deconvolute the spectra into three peaks corresponding to *sp*<sup>2</sup> for graphite-like (284.5 eV) and *sp*<sup>3</sup> for diamond-like (285.3 eV) and CO-contaminated (283.56–288.43 eV). The amounts of *sp*<sup>2</sup> and *sp*<sup>3</sup> and the *sp*2/*sp*<sup>3</sup> ratio (area) for each sputtered layer are summarized in Table 2 (a single layer was ca. 13 nm thick). The C 1s spectra almost disappeared from 40 layers because of the exposure of stainless steel surface to Ar-ion sputtering. The DLC-1 had a higher *sp*2/*sp*<sup>3</sup> ratio (0.343) at the external surface region, although the value decreased (to 0.235) for four sputtered layers, which was similar to that for DLC-2 (0.283). On the other hand, DLC-2 had a

graphite-rich external surface (*sp*2/*sp*<sup>3</sup> ratio: 0.181), although the value increased (to 0.343) after nine sputtered layers, which indicated a diamond-rich surface.

**Figure 3.** The hydrogen depth profile (concentration relative to carbon) to a depth of 600 Å (60 nm) determined from elastic recoil analyses of DLC-coated disk specimens.

**Figure 4.** Gaussian-Lorentzian curve fitting of X-ray photoelectron spectroscopy C 1s spectra obtained for DLC-coated disk specimens.


