*3.1. Color Measurements*

Figure 2 shows the changes in representative diffuse reflectance curves for the specimens. The lightness (*L\**) and two opponent color (*a*\*, *b*\*) values are summarized in Table 1. The reflectance values (%) increased with increasing wavelength for all specimens. Non-coated specimens showed significantly higher reflectance values in the 350–800 nm range and *L\**, due to the polished bright surface acting as a mirror. Among the BG-coated specimens, the one prepared using AC 15 V showed the highest values for both reflectance in the 350–800 nm range and *L\**, followed by specimens coated at DC 15 V, AC 10 V, and DC 10 V. The specimens coated at higher voltage (15 V) showed significantly higher reflectance and *L\** values than those coated at lower voltage (10 V). Similar *a*\* and *b*\* values were obtained for most BG-coated specimens.

**Figure 2.** Diffuse reflectance curves of the various specimens.

**Table 1.** Commission International de L'clairage (CJE) lightness (*L*\*) and color (*a*\*, *b*\*) values for the specimens.


Notes: Values are mean and standard deviation (S.D.), *n* = 7. One-way ANOVA followed by Tukey–Kramer multiple range test. a–e Idential letters indicate that mean values were not significantly different (*p* < 0.05).

#### *3.2. Crystal Structures, Morphological Features, and Compositions of the Coating Layers*

Figure 3 displays representative XRD spectra of non- and BG-coated specimens. Weak broad feature at around 32◦ was obtained for the BG-coated specimen (AC 15V) because of the amorphous structure. On the other hand, two peaks at 2θ = 43.5 and 51.0◦ associated with the austenite (γ-Fe) phase (ICDD PDF 01-071-4649) were observed for the BG-coated specimen (DC 15V), while the broad feature at around 32◦ was observed.

**Figure 3.** Representative X-ray diffraction (XRD) spectra of the non- and BG-coated specimens.

Figure 4 shows SEM photomicrographs and qualitative compositional maps obtained by EDS of BG-coated cross-sectional specimens. The thickness of the BG coating layers formed on the disk surfaces was ca. 1.0–4.0 μm and the specimens coated at higher voltage (15 V) tended to have thicker BG coating layers than those coated at lower voltage (10 V). Additionally, the thickness of the BG coating layers were similar in both specimens coated with AC and DC. Good interfacial adhesion was observed between all BG coating layers and the bulk materials. Si and O, the major components of BG, were enriched in all of the coating layers.

**Figure 4.** Scanning electron microscopy (SEM) photomicrographs and qualitative compositional maps obtained by energy-dispersive X-ray spectroscopy (EDS) of cross-sections of the BG-coated specimens.

#### *3.3. Evaluation of the Mechanical Properties of the Coating Layers by Nanoindentation*

Table 2 summarizes the mechanical properties of surface regions for non- and BG-coated specimens. The hardness and elastic modulus of the BG layers were significantly lower than those of the non-coated specimen. The specimens coated at higher voltage (15 V) showed significantly higher the hardness and elastic modulus of the BG layers than those coated at lower voltage (10 V). The elastic modulus of the BG layers formed by AC were significantly higher than those for the BG layers formed by DC.



Notes: Values are mean and standard deviation (S.D.), *n* = 7; One-way ANO VA followed by Tukey–Kramer multiple range test. Identical letters indicate that mean values were not significantly different (*p* < 0.05).

#### *3.4. Frictional Properties Measured by the Progressive Load Scratch Test*

Figure 5 shows the change in representative frictional force obtained by the progressive load scratch test. The BG-coated specimens (DC 15 V) showed significantly higher frictional force than the other specimens when measured over 5 mm. Over approximately the first 4 mm of the measurement distance, the other BG-coated specimens displayed slightly higher frictional force than the non-coated specimen.

**Figure 5.** Frictional forces obtained by the progressive load scratch test.
