Corrosion resistance, nano-mechanical properties, and biocompatibility of Mg-plasma-implanted and plasma-etched Ta/TaN hierarchical multilayered coatings on the nitrided AZ91 Mg alloy

Abstract

A tantalum/tantalum nitride (Ta/TaN) multilayered coating is deposited on plasma-nitridedAZ91 Mg alloy. The top TaN layer undergoes O₂+ Ar plasma etching to improve the antibacterial properties and Mg plasma immersion ion implantation (MgPIII) is performed to enhance the biocompatibility and wound healing capability. A uniform, compact, homogeneous, and columnar nanostructured MgPIII and plasma-etched TaN layer with a cluster size of about 17 nm, surface roughness of 0.28 nm, and needle morphology is observed. Although, plasma etching increases the corrosion current density (i_corr) from 0.02 to 0.19 µA cm^-2due to larger surface roughness and different potentials between sharp points and smooth points, MgPIII decreasesi_corrfrom 0.19 to 0.02 µA cm^-2besides a more positive corrosion potential. The amounts of Mg⁺²released to the simulated body fluid (SBF) diminishes from 89.63 ± 0.54 to 60.30 ± 0.47 mg l^-1cm^-2indicating improved corrosion resistance. Under fever conditions (40 °C),i_corrdecreases by 63%, but the open circuit potential does not change due to the constant chemical composition of the surface as well as thicker double layer and less defects, as confirmed by the larger amount of Mg⁺²of 71.49 ± 0.22 mg l^-1cm^-2leached to the SBF. In the self-healing process which occurs via the reactions between the tantalum intermediate layer and electrolytes and penetrating ions through the defects as well as formation of oxide compounds, creation and propagation of defects are deterred as shown the 24 h destructive polarization test in SBF. The combination of plasma etching and MgPIII enhance not only the bacterial resistance and biocompatibility of the super-hard TaN layer by providing the rougher surface on TaN-P-Mg, but also the nano-mechanical properties and anticorrosion properties. As a result, the hardness increases by 7%, elastic modulus decreases by 19%, and the stiffness increases by 21%.

Keywords: AZ91 Mg alloy; Mg plasma immersion ion implantation; Ta/TaN multilayered coating; biocompatibility; corrosion resistance; etching; plasma.