**3. Results**

## *3.1. As-Deposited Equiatomic Ru–Zr Coatings*

Table 1 lists the chemical compositions of the as-deposited equiatomic Ru–Zr coatings prepared at various substrate holder rotation speeds of 1–30 rpm. The samples were denoted as Ru*x*Zr1−*x*(R*y*), or R*y*, where R*y* indicated that the sample prepared using the substrate holder was rotated at *y* rpm. All the coatings exhibited similar atomic ratios Ru/(Ru + Zr) within 0.46–0.50 after being examined using EDS on the surface, and a thickness of 870–920 nm after being evaluated using FE-SEM in the cross section. Oxygen content in the as-deposited coatings was 0.1–0.5 at.% because of weak oxidation caused by the residual oxygen in the vacuum chamber.

**Table 1.** Chemical compositions, thickness values, laminated period, mechanical properties, and surface roughness values of Ru*x*Zr1−*x*(R*y*) coatings as-deposited and annealed at 600 ◦C in 1% O2–99% Ar for 30 min.


Figure 1 shows cross-sectional SEM images of the as-deposited Ru–Zr coatings, which exhibit a columnar structure. Laminated structures stacked along the growth direction were observed in the Ru0.50Zr0.50(R1) and Ru0.49Zr0.51(R3) coatings, for which the equilibrated laminated layer periods were 35 and 12 nm, respectively, as determined using the thickness recorded from the SEM observation divided by the number of laminated layers; in other words, the number of revolutions of the substrate holder. Each equilibrated laminated layer period formed as a result of cyclical gradient concentration deposition. The laminated structures of the Ru–Zr(R*y*) coatings prepared at higher substrate holder rotation speeds such as Ru0.47Zr0.53(R10) and Ru0.46Zr0.54(R30) exhibited narrower equilibrated laminated layer periods that could not be evaluated through SEM images.

Figure 2 shows the XRD patterns of the as-deposited Ru–Zr(R*y*) coatings. The Ru0.50Zr0.50(R1), Ru0.49Zr0.51(R3), Ru0.48Zr0.52(R5), and Ru0.47Zr0.53(R10) coatings exhibited reflections of hexagonal Ru [ICDD 06-0663], cubic RuZr [ICDD 18-1147], and hexagonal Zr [ICDD 05-0665] phases, implying that these coatings consisted of laminated sublayers. The equilibrated laminated layer periods for the R5 and R10 coatings were 7.2 and 3.6 nm, respectively. By contrast, XRD patterns of the as-deposited Ru0.46Zr0.54(R15), Ru0.47Zr0.53(R20), and Ru0.46Zr0.54(R30) coatings exhibited a RuZr phase dominant structure. The cubic RuZr phase exhibited XRD reflections of (110), (200), and (211), which are comparable with previous XRD results reported by Mahdouk et al. [21]. RuZr exhibited a B2 structure (CsCl type) [21–25]. Figure 3 depicts a cross-sectional TEM image of the as-deposited Ru0.46Zr0.54(R15) coating, which comprises a columnar structure without evident laminated sublayers; the diffraction pattern of the selected area shows a cubic RuZr phase. The equilibrated laminated layer periods for the as-deposited Ru0.46Zr0.54(R15), Ru0.47Zr0.53(R20), and Ru0.46Zr0.54(R30) coatings were 2.4, 1.8, and 1.2 nm, respectively, which were too thin to construct the laminated structure. Under such conditions, the equilibrated laminated layer periods were equal to a variation period of cyclical gradient concentration. Because the substrate temperature was sustained at 400 ◦C during cosputtering, the

deposited atoms formed an intermetallic RuZr compound, as observed by the XRD patterns. In our previous study [26], B2-RuAl phase was observed for Ru–Al multilayer coatings prepared at 400 ◦C.

**Figure 1.** Cross-sectional SEM images of the as-deposited (**a**) Ru0.50Zr0.50(R1); (**b**) Ru0.49Zr0.51(R3); (**c**) Ru0.47Zr0.53(R10); and (**d**) Ru0.46Zr0.54(R30) coatings.

**Figure 2.** XRD patterns of the as-deposited Ru–Zr(R*y*) coatings.

**Figure 3.** Cross-sectional TEM image and selected area diffraction pattern of the as-deposited Ru0.46Zr0.54(R15) coating.
