Microstructure, mechanical, and corrosion properties of Zr_1-xCr_xB_y diboride alloy thin films grown by hybrid high power impulse/DC magnetron co-sputtering

Abstract

We study microstructure, mechanical, and corrosion properties of Zr_1-xCr_xB_y coatings deposited by hybrid high-power impulse/DC magnetron co-sputtering (CrB₂-HiPIMS/ZrB₂-DCMS). Cr/(Zr + Cr) ratio, x, increases from 0.13 to 0.9, while B/(Zr + Cr) ratio, y, decreases from 2.92 to 1.81. As reference, ZrB_2.18 and CrB_1.81 layers are grown at 4000 W DCMS. ZrB_2.18 and CrB_1.81 columns are continual from near substrate toward the surface with open column boundaries. We find that the critical growth parameter to achieve dense films is the ratio of Cr⁺-dominated ion flux and the (Zr + B) neutral flux from the ZrB₂ target. Thus, the alloys are categorized in two groups: films with x < 0.32 (low Cr⁺/(Zr + B) ratios) that have continuous columnar growth, rough surfaces, and open column boundaries, and films with x ≥ 0.32 (high Cr⁺/(Zr + B) ratios) that Cr⁺-dominated ion fluxes are sufficient to interrupt continuous columns, resulting in smooth surface and dense fine-grain microstructure. The pulsed metal-ion irradiation is more effective in film densification than continuous Ar⁺ bombardment. Dense Zr_0.46Cr_0.54B_2.40 and Zr_0.10Cr_0.90B_1.81 alloys are hard (>30 GPa) and almost stress-free with relative nanoindentation toughness of 1.3 MPa√m and 2.3 MPa√m, respectively, and remarkedly low corrosion rates (∼1.0 × 10^-6 mA/cm² for Zr_0.46Cr_0.54B_2.40 and ∼ 2.1 × 10^-6 mA/cm² for Zr_0.10Cr_0.90B_1.81).