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.

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 (icorr) from 0.02 to 0.19 µA cm-2due to larger surface roughness and different potentials between sharp points and smooth points, MgPIII decreasesicorrfrom 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),icorrdecreases 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%.

Enhanced corrosion resistance, antibacterial properties, and biocompatibility by hierarchical hydroxyapatite/ciprofloxacin-calcium phosphate coating on nitrided NiTi alloy.

Multi-functional hierarchical coatings are deposited on the nitrided NiTi alloy. The nitrided layer is first deposited by nitrogen plasma immersion ion implantation and a middle layer containing porous hydroxyapatite and ciprofloxacin (Cip) is produced before the top calcium phosphate coating is deposited by the sol-gel method. The thicknesses of the coating and nitrided intermediate layer are about 1.54 µm and 160 nm, respectively and Cip penetrates to a depth of about 530 nm. Calcium phosphate reduces surface defects resulting in a surface roughness of 17 ± 2 nm compared to 34 ± 5 nm of the porous hydroxyapatite coating. The corrosion resistance is improved due to reduced defects and localized corrosion as manifested by the decrease in the Ni2+ release rate by 11.6% from 0.0198 to 0.0175 mg L-1 cm-2. The bacterial resistance against E. coli is also improved by about 88 times on account of Cip release and good biocompatibility is confirmed by proliferation of MC3T3 cells. This multi-functional hierarchical coating has large potential in orthopedic and dental applications.