*2.1. Materials*

Mechanically-polished stainless steel disk specimens (diameter: 14 mm; thickness: 2 mm; Nogata Denki Kogyo, Tokyo, Japan) and as-received stainless steel orthodontic wires with cross-sectional dimensions of 0.017 × 0.025 in<sup>2</sup> (stainless steel archwire; 3M Unitek, Monrovia, CA, USA) were purchased and subjected to DLC coating. These stainless steels were confirmed to be Type 304 austenitic stainless steel (ISO No. 4301-304-00-I) by X-ray fluorescence analysis. As-received, preadjusted stainless steel orthodontic brackets (Mini Uni-Twin; 3M Unitek) for the upper canine teeth were used for friction tests. Non-coated specimens served as a control.
