Effects of the tantalum intermediate layer on the nanomechanical properties and biocompatibility of nanostructured tantalum/tantalum nitride bilayer coating deposited by magnetron sputtering on the nickel titanium alloy

Nanostructured tantalum nitride coatings are widely used in biomedical engineering due to the good hardness, wear resistance, and corrosion resistance. In this work, TaN and Ta/TaN nanostructured coatings were deposited on the NiTi alloy by magnetron sputtering for 150 min at a pressure of 3.8 × 10^–3 mbar using a power of 143 W. The phase, structure, morphology, and thickness were evaluated by X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), respectively. The nanomechanical properties such as Young's modulus (E) and hardness were determined by nano-indentation and nano-scratch tests. To assess the biological properties of the coatings, the antimicrobial activity was monitored by the disk diffusion antibiogram technique using E.coli as the model gram-negative bacteria and S. aureus as the gram-positive bacteria and the biocompatibility was also studied. A homogeneous, uniform, and crack-free of TaN monolayered and Ta/TaN bilayered coating with a thickness 1050 and 1310 nm, respectively, were produced by magnetron sputtering. As a result of the Ta middle layer in the Ta/TaN coating, the hardness increased by 22% from 6.3 ± 0.5 GPa to 7.6 ± 0.5 GPa under a load of 700 µN and 6% from 6.1 ± 0.2 GPa to 6.5 ± 0.3 GPa under a load of 1000 µN. The critical load also increased from 3.7 ± 0.2 to 4.1 ± 0.6 N in the presence of the Ta intermediate layer which improved the adhesion strength of the Ta/TaN bilayer compared to the TaN monolayer. In addition, the Ta intermediate layer increased the flexibility leading to a adhesive wear mechanism for the friction behavior of Ta/TaN. The bacteriostatic ability against gram-negative bacteria and bacteriocidal ability against gram-positive bacteria increase. Our results demonstrate that the nanostructured Ta/TaN bilayer coating provides a better bioactive surface for the growth and proliferation of MG-63 ossicular osteosarcoma cells growth as well.