Evaluation of mechanical properties and in vitro biocompatibility of TiZrTaNbHf refractory high-entropy alloy film as an alternative coating for TiO2 layer on NiTi alloy

TiZrTaNbHf refractory high-entropy alloy (RHEA) film with a thickness of 750 and 1500 nm was deposited on NiTi alloy by RF magnetron sputtering technique and compared with the thermally grown TiO2 film in terms of mechanical properties and in vitro biocompatibility. RHEA film with the amorphous and homogenous micro-structure, outstanding mechanical properties, and enhanced adhesion strength displayed potential to be used as a protective film preventing Ni ion release from the NiTi implants, particularly in long-term applications. Furthermore, RHEA film exhibited an accelerated and promoted hydroxyapatite (HAp) forming ability sug-gesting excellent bioactivity as well as good bone-bonding ability than thermally grown TiO2 film. The presence of various oxides and sub-oxides played an indispensable role in the rapid nucleation and development of HAp upon RHEA coated specimens' surfaces. The obtained results revealed that RHEA film appears to be the viable alternative coating for TiO2 films on the NiTi biomaterials.