**2. Materials and Methods**

Ru–Zr coatings with a Cr interlayer were fabricated by using magnetron cosputtering onto silicon wafers. Pure metal targets of Ru (99.95%), Zr (99.9%), and Cr (99.95%) with diameters of 50.8 mm each were adopted as source materials for sputtering. The sputter guns were inclined to focus plasma on the circular track of the substrate holder, as described in detail in a previous study [13]. The target-to-substrate distance was maintained at 90 mm for all sputtering runs. The chamber was evacuated down to 2.7 × <sup>10</sup>−<sup>4</sup> Pa, followed by the inlet of argon gas as a plasma source. The substrate holder was heated to 400 ◦C and the Ar flow rate was controlled at 20 sccm; the resulting working pressure was 0.7 Pa. The substrate holder was rotated at 1 rpm for depositing the Cr interlayer. Then, Ru–Zr coatings with fixed DC sputtering powers (*W*Ru = 100 W and *W*Zr = 200 W) and various substrate holder rotation speeds were deposited on the Cr interlayer for 25 min. To investigate the internal oxidation phenomenon after performing heat treatments, the Ru–Zr coatings were further annealed at 600 ◦C in a 1% O2–99% Ar atmosphere by introducing O2–Ar mixed gas into a quartz tube furnace.

Chemical composition analysis was conducted by using energy dispersive spectrometry (EDS, Horiba, Kyoto, Japan) equipped with a scanning electron microscope (SEM, S3400N, Hitachi, Tokyo, Japan) on the surface. Surface morphology and thickness evaluation of the coatings were performed by using a field emission scanning electron microscope (FE-SEM, S4800, Hitachi, Tokyo, Japan) at a 15-kV accelerating voltage. A conventional X-ray diffractometer (XRD, X'Pert PRO MPD, PANalytical, Almelo, The Netherlands) with Cu Kα radiation was adopted to identify the phases of the coatings, using the grazing incidence technique with an incidence angle of 1◦. The Cu Kα radiation was generated from a Cu anode operated at 45 KV and 40 mA. The nanostructure was examined by using a transmission electron microscope (TEM, JEM-2010F, JEOL, Tokyo, Japan) at a 200-kV accelerating voltage. The TEM samples were prepared by applying a focused ion beam system (FEI Nova 200, Hillsboro, OR, USA) operated at an accelerating voltage of 30 kV with a gallium ion source. A Pt layer was deposited to protect the free surface in the sample preparation. The chemical states of the constituent elements were examined by using an X-ray photoelectron spectroscope (XPS, PHI 1600, PHI, Kanagawa, Japan) with an Mg Kα X-ray beam (energy = 1253.6 eV and power = 250 W) operated at 15 kV. The XPS spectra of O 1s, Ru 3d, and Zr 3d core levels were recorded. Ar+ ion beam of 3 keV was used to sputter the coatings for depth profiling. The surface hardness and Young's modulus of Ru–Zr coatings were measured with a nanoindentation tester (TI-900 Triboindenter, Hysitron, Minneapolis, MN, USA). The nanoindenter was equipped with a Berkovich diamond-probe tip. The applied load was controlled to produce an indentation depth of 80 nm, which is 1/10 of the film thickness [18]. The loading, holding, and unloading times were 5 s each. The nanoindentation hardness and elastic modulus of each indent were calculated using the Oliver and Pharr method [19]. The standard deviations for nanoindentation data were calculated from 8 measurements made at different locations on one sample. The surface roughness values of the coatings, *R*a [20], were evaluated by using an atomic force microscope (AFM, Dimension 3100 SPM, NanoScope IIIa, Veeco, Santa Barbara, CA, USA). The scanning area of each image was set at 5 × <sup>5</sup> <sup>μ</sup>m2 with a scanning rate of 1.0 Hz.
