**Takeshi Muguruma 1, Masahiro Iijima 1,\*, Masahiro Kawaguchi <sup>2</sup> and Itaru Mizoguchi <sup>1</sup>**


Received: 2 April 2018; Accepted: 22 May 2018; Published: 24 May 2018

**Abstract:** This study investigated a diamond-like carbon (DLC) coating formed on stainless steels (disk and wire specimens) using a plasma-based ion implantation/deposition method with two different parameters (DLC-1, DLC-2). These specimens were characterized using high-resolution elastic recoil analysis, microscale X-ray photoelectron spectroscopy and nanoindentation testing to determine the hydrogen content, *sp*2/*sp*<sup>3</sup> ratio and mechanical properties of the coating. Three-point bending and frictional properties were estimated. DLC-1 had a diamond-rich structure at the external surface and a graphite-rich structure at the inner surface, while DLC-2 had a graphite-rich structure at the external surface and a diamond-rich structure at the inner surface. Mean mechanical property values obtained for the external surface were lower than those for the inner surface in both types of DLC-coated specimens. The hydrogen content of DLC-2 was slightly higher versus DLC-1. Both DLC-coated wires produced a significantly higher elastic modulus according to the three-point bending test versus the non-coated wire. DLC-2 produced significantly lower frictional force than the non-coated specimen in the drawing-friction test. The coating of DLC-1 was partially ruptured by the three-point bending and drawing-friction tests. In conclusion, the bending and frictional performance of DLC-coated wire were influenced by the hydrogen content and *sp*2/*sp*<sup>3</sup> ratio of the coating.

**Keywords:** diamond-like carbon; frictional property; hydrogen content; surface modification; *sp*2/*sp*<sup>3</sup> ratio
