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
xZr
1−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.
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 Ru
0.50Zr
0.50(R1) and Ru
0.49Zr
0.51(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 Ru
0.47Zr
0.53(R10) and Ru
0.46Zr
0.54(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 Ru
0.50Zr
0.50(R1), Ru
0.49Zr
0.51(R3), Ru
0.48Zr
0.52(R5), and Ru
0.47Zr
0.53(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 Ru
0.46Zr
0.54(R15), Ru
0.47Zr
0.53(R20), and Ru
0.46Zr
0.54(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,
22,
23,
24,
25].
Figure 3 depicts a cross-sectional TEM image of the as-deposited Ru
0.46Zr
0.54(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 Ru
0.46Zr
0.54(R15), Ru
0.47Zr
0.53(R20), and Ru
0.46Zr
0.54(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.
3.2. Internally Oxidized Ru–Zr Coatings
Figure 4 shows the cross-sectional SEM image of the annealed Ru
0.50Zr
0.50(R1) coating, which exhibited a laminated structure with an equilibrated laminated layer period of 55 nm. However, the features of the other coatings could not be identified through SEM.
Figure 5 presents the XRD patterns of the Ru–Zr coatings after annealing in 1% O
2–99% Ar at 600 °C for 30 min; all patterns exhibited monoclinic ZrO
2 [ICDD 32-1484], tetragonal ZrO
2 [ICDD 42-1164], and Ru phases. The Ru:Zr atomic ratios were maintained at levels similar to those of the as-deposited coatings (
Table 1), implying that no volatile oxides were formed during annealing. The O content in the annealed coatings increased to within 58–62 at.%, indicating that extra O was trapped because the stoichiometric ratio of ZrO
2 was two, enabling partial Ru atoms to be oxidized.
Figure 6a–c illustrates the XPS spectra of O 1s, Zr 3d, and Ru 3d core levels, respectively, at various thickness levels of the annealed Ru
0.50Zr
0.50(R1) coating. The detected depth crossed six periods of the laminated layers. The O and Zr species were identified as O
2− and Zr
4+, whereas Ru was identified as Ru
0 except for the spectra near the surface region (depth < 13 nm), where the Ru
x+ and Ru
4+ signals were split. The binding energy value of Ru
0 3d
5/2 (279.96 ± 0.08 eV) was consistent with that of other coatings (279.69–280.16 eV) reported in the literature [
13,
16,
17,
27], whereas the binding energies of Ru
x+ and Ru
4+ 3d
5/2 were 280.45 ± 0.11 and 282.57 ± 0.15 eV, respectively. Previous studies reported 281.4–282.2 eV [
26,
28,
29,
30] for the binding energy of Ru
4+ 3d
5/2. Ru of 17%–20% exhibited the Ru
4+ state at a depth of 0–13 nm. Ru atoms remained in its metallic state beneath the near surface region.
Figure 6d shows the intensity variations of O
2− 1s, Ru
0 3d
5/2, and Zr
4+ 3d
5/2 signals along the depth, which indicates that the variation trend of the O
2− 1s profile coincides with that of the Zr
4+ 3d
5/2 profile and is in contrast to that of the Ru
0 3d
5/2 profile, implying that ZrO
2 is the dominant oxide. Therefore, the annealed Ru
0.50Zr
0.50(R1) coating comprised alternating oxygen-rich and oxygen-deficient layers stacked along the O-diffusion direction. The binding energy value of Zr
4+ 3d
5/2 deviated within 182.05–183.35 eV (
Figure 6e). Moreover, this range decreased to 182.71–183.35 eV after the data in the first laminated period had been excluded. Previous studies have reported 182.75 [
31], 182.8 [
32], and 182.9 eV [
33] for the binding energy value of Zr
4+ 3d
5/2. The binding energy value of O
2− 1s demonstrated a variation pattern similar to that of the binding energy value of Zr
4+ 3d
5/2 (
Figure 6e). The charging effect of analyzing insulators [
34] caused substantial deviation in binding energy. The binding energy difference Δ = (O
2− 1s − Zr
4+ 3d
5/2) was 347.92 ± 0.05 eV at the analyzed depth of 19.5–318.5 nm. This difference was highly consistent with the reported difference of 348.01 and 348.2 eV, calculated using 530.76 and 182.75 eV [
31] or 531.1 and 182.9 eV [
33] for O
2− 1s and Zr
4+ 3d
5/2, respectively. The periodic changes of nonoxidized metallic Ru suggested the influence of oxygen in the Zr-deficient sublayers.
Figure 7a,b shows the cross-sectional TEM images of the annealed Ru
0.48Zr
0.52(R5) coating; the laminated structure was evident.
Figure 7c shows a high-resolution TEM image of the near-surface region of the annealed coating. The lattice fringes of particular areas indicated that the annealed Ru
0.48Zr
0.52(R5) coating comprised ZrO
2- and Ru-dominant sublayers, which linked together across the original columnar boundaries such that the annealed Ru
0.48Zr
0.52(R5) coatings were laminated and the columnar boundaries were unresolved.
Figure 8a depicts the cross-sectional TEM image of the annealed Ru
0.47Zr
0.53(R10) coating. The laminated sublayers were curved, because of which the stacks of sublayers among neighboring columnar structures were disconnected.
Figure 8b shows the high-resolution TEM image of the middle region of the annealed Ru
0.47Zr
0.53(R10) coating. The Ru-dominant sublayers were two-nanometers thick only, and disconnected regions of the sublayers among neighboring columnar structures were observed. The fast variation of cyclical gradient concentration for the R10 coatings prepared with a quick substrate holder rotation speed resulted in the formation of grooved sublayers. For the coatings with thicker Ru sublayers, R1, R3, and R5, the sublayers became flat. The misaligned connections were more evident in the near-surface region (
Figure 8c).
Figure 9a shows a cross-sectional TEM image of the annealed Ru
0.46Zr
0.54(R30) coating, in which the original columnar boundaries are evident, but no laminated structures were observed. A high-resolution TEM image (
Figure 9b) revealed nanocrystalline grains of ZrO
2 and Ru, each approximately five nanometers in diameter, implying that a nanocomposite structure had been constructed. Furthermore, Ru grains, the dark regions in the image, tended to concentrate along the columnar boundaries.
Figure 10a–c illustrates the XPS spectra of the annealed Ru
0.46Zr
0.54(R30) coating. The XPS spectra of Ru 3d core levels indicated the presence of minor Ru
4+ (3d
5/2: 282.11 eV) in addition to Ru
0 (3d
5/2: 280.19 ± 0.07 eV) at the near-surface region (
Figure 10b), which was attributed to the incorporation of Ru into the ZrO
2 grains because RuO
2 and ZrO
2 possessed a similar tetragonal structure.
Figure 10d shows that the intensities of O
2− 1s, Ru
0 3d
5/2, and Zr
4+ 3d
5/2 signals were constant along the depth due to the limit of XPS analyses. Similar binding energy trends were observed for O
2− and Zr
4+ (
Figure 10e). The binding energy difference Δ = (O
2− 1s − Zr
4+ 3d
5/2) was 348.00 ± 0.02 eV at the analyzed depth (5.7–96.9 nm). Therefore, Zr reacted with O during annealing, and the annealed coating exhibited a nanocomposite comprising ZrO
2 and Ru grains.
3.3. Mechanical Properties of Internally Oxidized Ru–Zr Coatings
Figure 11 depicts the nanoindentation hardness variations of the as-deposited and internally oxidized Ru–Zr coatings prepared at various substrate holder rotation speeds through sputtering. The hardness of the as-deposited coatings increased from 9.1 to 13.1 GPa with the substrate holder rotation speed and decreasing equilibrated laminated layer period. This hardness increase was attributed to the decrease in crystalline size and structural variation. The nanoindentation hardness of all Ru–Zr coatings increased after annealing in 1% O
2–99% Ar at 600 °C for 30 min. The hardness variation curve of the internally oxidized Ru–Zr coatings exhibited three divisions representing a laminated structure, a disconnected laminated structure, and a nanocomposite region. The hardness increased from 9.1, 10.3, and 10.5 to 15.5, 16.1, and 17.2 GPa for the annealed Ru
0.50Zr
0.50(R1), Ru
0.49Zr
0.51(R3), and Ru
0.48Zr
0.52(R5) coatings, respectively, which exhibited equilibrated laminated layer periods of 55, 18, and 11 nm, respectively. This result indicates that the hardness of the internally oxidized Ru–Zr coatings, which exhibited crystalline phases identical to those identified through XRD analysis and appropriately maintained their multilayer structures, was affected by the layer period. These internally oxidized Ru–Zr multilayer coatings were categorized as nonisostructural oxide/metal multilayers [
1]. Dislocation could not be moved across oxide/metal interfaces because oxides are brittle materials that deform through fracture mechanisms, limiting the hardness enhancement [
2]; therefore, the hardness of oxide/metal multilayers approached that of the oxide ZrO
2. Gan et al. reported a nanoindentation hardness of 18 GPa for monoclinic ZrO
2 thin films [
35]. By contrast, the hardness of the annealed Ru
0.47Zr
0.53(R10) coatings with an equilibrated laminated layer period of 5.6 nm exhibited a relatively low level of 12.3 GPa. Although the internally oxidized Ru
0.47Zr
0.53(R10) coatings were laminated in each columnar structure, the same sublayers among neighboring columnar structures were misaligned and disconnected, which reduced the hardness. The internally oxidized Ru
0.46Zr
0.54(R15), Ru
0.47Zr
0.53(R20), and Ru
0.46Zr
0.54(R30) coatings exhibited high hardness within 16.1–17.9 GPa and were categorized as nanocrystalline composites consisting of hard ZrO
2 grains and soft Ru grains.
Figure 12 shows the variation in Young’s moduli of the as-deposited and internally oxidized Ru–Zr coatings. The Young’s moduli increased from 130 to 140 GPa for R1, R3, and R5 coatings, to 160 GPa for R10 coatings and 170–180 GPa for R15, R20, and R30 coatings. Because the internally oxidized Ru–Zr coatings exhibited similar phases, ZrO
2 and Ru, the differences in Young’s moduli among the annealed coatings were limited (i.e., 160–180 GPa). The surface roughness values of the Ru–Zr coatings are shown in
Table 1. When evaluating the mechanical properties of coatings, previous studies [
36,
37,
38] have reported that coatings with higher surface roughness exhibit larger standard deviation values or lower mean values. The effect of surface roughness on the mechanical properties of as-deposited Ru–Zr coatings was unclear. By contrast, the mechanical properties of the annealed coatings revealed larger deviations and higher surface roughness values than did those of the as-deposited coatings.