**1. Introduction**

Multilayer nitride coatings with nanoscale layer thickness have exhibited extremely high mechanical hardness due to dislocation blocking by layer interfaces and Hall–Petch strengthening [1]. By contrast, the hardness enhancement in the Y2O3/ZrO2 superlattice has been limited because oxides are brittle materials that are deformed by fracture mechanisms [2]. Two metallic multilayer coatings deposited by cosputtering for immiscible systems, W–Cu [3,4] and Cu–Ta [5,6], have developed a phase-separated nanostructure. However, Ru/Al multilayers have been deposited to fabricate a B2-RuAl intermetallic compound through annealing at approximately 600 ◦C in a vacuum or Ar [7,8]. Oxide-dispersion-strengthened platinum materials [9] and Ag-oxide-based electric contact material [10] are conventional applications of internal oxidation [11]. Our previous studies [12–15] investigated the internal oxidation of Ru-based alloy multilayer coatings annealed at 600 ◦C in oxygen-containing atmospheres for the application of protective coatings on glass molding dies. The specific cosputtering processes, which were performed using a substrate holder rotating at a slow speed of one to seven revolutions per minute, have been examined in detail for fabricating Ru–Ta coatings [14]; the fabricated coatings had exposed substrates alternately to the sputter sources without shutter shielding, forming a multilayer structure with a cyclical gradient concentration period at a nanometer scale. An oxidized laminated structure formed because of the inward diffusion of oxygen during the annealing process; this structure comprised alternating oxygen-rich and oxygen-deficient sublayers stacked adjacent to the surface. The inward diffusion of oxygen at 600 ◦C was dominated by lattice diffusion in the active element-enriched regions [13,16,17]. Because the elements were stacked on the substrate with an alternating gradient concentration, the O atoms could easily diffuse through the paths in the transverse direction, thereby forming oxide sublayers. After the oxygen content in the oxide sublayers reached a saturation level, the grainboundary diffusion along the original columnar structure drove oxygen

to the next period of the laminated structure. During an annealing process conducted at 600 ◦C in a 1% O2–99% Ar atmosphere, internal oxidation occurred for Ti–Ru, Zr–Ru, Nb–Ru, Mo–Ru, Hf–Ru, Ta–Ru, and W–Ru coatings, which were prepared using a substrate holder rotating at one revolution per minute [15]; the mechanical properties of the annealed coatings depended on the characteristics of the oxide sublayers. The nanoindentation hardness of the annealed Zr0.30Ru0.70 coating exhibited a relatively high value of 18.4 GPa. The widths of the oxide sublayers were restricted by the Ru-dominant sublayers [16,17]; therefore, the internally oxidized coatings can be categorized as nonisostructural oxide/metal multilayers [1]. The substrate holder rotation speed in sputtering affects the stacking period of the laminated structure [14]; therefore, assessing the effect of the stacking period on the mechanical properties of the internally oxidized Ru–Zr coatings is imperative.
