**1. Introduction**

Metallic orthodontic appliances, such as brackets and archwires, typically show superior properties [1] and provide many clinical advantages, such as low frictional resistance and good bending performance as orthodontic archwires. They have been widely used in clinical orthodontics, although they have esthetic limitations compared to other orthodontic appliances made from ceramics and plastics. Another disadvantage of metallic orthodontic appliances is corrosion in the oral environment [2,3], because the release of metallic ions, such as nickel (Ni) and chromium (Cr), may cause an allergic reaction during orthodontic treatment [4–6].

The frictional force between the bracket and archwire (resistance to sliding) during tooth movement is a primary issue in orthodontics [7,8]. If the frictional force can be decreased, then the efficiency of the tooth movement can be improved. To improve the frictional characteristics and corrosion resistance, various surface modification techniques, such as diamond-like carbon (DLC) coating [9–12], plasma immersion ion implantation [7,13,14] and bioactive glass coating [15], have been investigated.

In recent years, DLC coating has become the subject of considerable research interest due to its bioinertness, extreme hardness, low friction coefficient and high wear resistance [16]. This technique has attracted significant attention for biomedical applications, such as artificial joints, cardiac stents and orthodontic archwires [17]. Concerning orthodontic applications, experimental DLC-coated orthodontic wires have been studied by several research groups [9–12,18–20]. One study reported that DLC layers protect against the diffusion of Ni and its release at the surface of Ni–Ti archwires and that these coatings are noncytotoxic in corrosive environments [18]. Other studies have investigated the effect of DLC coatings on the friction of orthodontic wires and found that DLC-coated wires produced less frictional resistance than non-coated wires [9–12,18–20]. The properties of a DLC coating depend on the hydrogen content, *sp*2/*sp*<sup>3</sup> ratio and presence of doping elements [21,22]. The properties of DLC-coated orthodontic materials are not well understood, and limited information is available regarding the hydrogen content and *sp*2/*sp*<sup>3</sup> ratio of DLC-deposited surfaces.

First, we deposited a DLC film onto orthodontic stainless steels using two different parameters and characterized the DLC films to determine their hydrogen content, *sp*2/*sp*<sup>3</sup> ratio and mechanical properties. The bending and frictional properties of the DLC-coated orthodontic stainless steels were also investigated.

#### **2. Materials and Methods**
