*2.4. Compositional Characterization of the Coating by High-Resolution Elastic Recoil Analysis and Microscale X-ray Photoelectron Spectroscopy*

An elastic recoil detection analyzer (ERDA; HRBS1000; Kobelco, Hyogo, Japan) was used for depth profiling of the hydrogen content of DLC-coated disk specimens. The ion type, acceleration voltage, incident angle and scattering angle were N+, 500 kV, 67.5 and 45.6◦, respectively. The main chamber was maintained at a pressure less than 1 × <sup>10</sup>−<sup>5</sup> Pa during the measurements. A multi-channel plate was used as the detector in this study. A beam of 500 keV N+ ions was irradiated against the surface of the specimens, and hydrogen ions recoiled at 45.6◦ were measured by the 90◦ sector-type magnetic spectrometer. To reject the scattered N+ ions, a Mylar foil was set in front of a multi-channel plate detector. The energy of hydrogen ions recoiled from the surface region of the implants was ca. 61 keV. Amorphous carbon materials with 20 at.% hydrogen were used as the standard sample. The standard sample was also measured under the same measurement conditions. The hydrogen contents of the specimens relative to carbon were compared with that of the standard sample. This enabled the depth profile of the contents to be calculated because the change in energy of the hydrogen ions corresponds to their depth from the surface.

The surface and in-depth composition of the control and DLC-coated disk specimens were analyzed by micro-XPS (Quantera II; Ulvac-Phi, Kanagawa, Japan) using Al Kα radiation with a 25-W beam power. The pressure of the main chamber was maintained at less than 1 × <sup>10</sup>−<sup>6</sup> Pa. Measurements on a 100 μm2 area of the disk specimens were conducted from 0–1100 eV at a step size of 0.2 eV. The counting time was 20 ms for each step, and the number of sweeps was 5, i.e., the total counting time was 100 ms at each step. Argon-ion sputtering was used for depth profiling measurements. The ion sputtering area was 2 × 2 mm2, and the measurements were taken at the center of the area. The sputtering rate of a SiO2 layer under the same conditions was 13 nm min<sup>−</sup>1. The *sp*<sup>2</sup> (for graphite) and *sp*<sup>3</sup> (for diamond) contents were determined using the software bundled with the XPS apparatus.

#### *2.5. Mechanical Properties of the Coating from Nanoindentation and Three-Point Bending Testing*

The external surfaces of DLC-coated wire specimens were examined with a nanoindentation apparatus (ENT-1100a; Elionix, Tokyo, Japan). The specimens were fixed to the specimen stage with adhesive resin (Superbond Orthomite; Sun Medical, Shiga, Japan). Nanoindentation testing was carried out at 28 ◦C using a Berkovich indenter for depth analyses at 20 and 70 nm (*n* = 10). Linear extrapolation methods (according to the ISO Standard 14577 [23]) were applied to the unloading curve between 95% and 70% of the maximum test force to calculate the elastic modulus. The hardness and elastic modulus of the wire specimen surfaces were calculated using the software bundled with the nanoindentation apparatus.

#### *2.6. Evaluation of the Elastic Modulus of the DLC-Coated Wires by the Three-Point Bending Testing*

A three-point bending test was carried out for non-coated and DLC-coated wires (*n* = 10). A 12-mm span was chosen for the wire segments in accordance with the ANSI/ADA Specification No. 32. All samples were loaded following the same protocol on a universal testing machine equipped with a 20 N load cell (EZ Test; Shimadzu, Kyoto, Japan) at room temperature (25 ◦C). Each wire was first loaded to a deflection of 1.0 or 1.5 mm and then unloaded at a rate of 0.5 mm min−1. Following a three-point bending test, a specimen was inspected with a stereoscopic microscope (SMZ1500; Nikon, Tokyo, Japan) to observe the detachment of the DLC layers.
