*3.3. Mechanical Properties of the Coating*

The mechanical properties of DLC-coated wire specimens obtained from nanoindentation testing at two analysis depths (ca. 20 and 70 nm) are summarized in Table 3. The mean values of the mechanical properties (hardness and elastic modulus) obtained for the external surface regions (at ca. a 20-nm depth) were lower than those for the inner surface regions (at ca. 70 nm depth) for both types of DLC-coated specimens. The DLC-1 tended to show higher mechanical properties at the external surface region and lower mechanical properties at the inner surface region compared with the DLC-2, although only the elastic modulus at the inner surface region was significantly different.

**Table 3.** Mechanical properties of DLC-coated wires obtained from nanoindentation testing (GPa).


Notes: Values are presented as the mean ± SD; <sup>1</sup> Student *<sup>t</sup>*-test.

Table 4 summarizes the elastic modulus for the non-coated and DLC-coated wire specimens obtained by the three-point bending test. Both DLC-coated wires had a significantly higher elastic modulus (181–188 GPa) than the non-coated wire (170 GPa). For the 1.5-mm bending condition, the DLC-1 (188 GPa) had a significantly higher elastic modulus than the DLC-2 (181 GPa). Micrograph images taken following this three-point bending test revealed that the coating layer had been removed from the inner core for both DLC-coated wire specimens; none of the DLC-2 wire coatings were damaged after three-point bending at 1.0 mm (Figure 5).

**Figure 5.** Stereomicroscope images of DLC-coated wires after the three-point bending test. (**a**,**c**) DLC-1 and (**b**,**d**) DLC-2. The first row shows specimens after the 1.0-mm bending and the second row specimens after the 1.5-mm bending. Original magnification: 50×.


**Table 4.** Elastic modulus for non-coated and DLC-coated wires obtained by the three-point bending test (GPa).

Notes: Values are presented as the mean ± SD; Identical letters indicate that mean values were not significantly different (*p* < 0.05) by one-way ANOVA followed by the Games–Howell test.
