Development and characterisation of high-entropy alloys (HEAs)

This doctoral thesis presents a comprehensive investigation into the potential applications of High Entropy Alloys (HEAs) as protective coatings for biomedical implants and as electrocatalysts for water-splitting applications. The research spans five main chapters, each exploring different facets of HEAs, including their microstructure, mechanical properties, electrochemical behavior, biocompatibility, and catalytic performance. Chapter three highlights the deposition of Ti1.5ZrTa0.5Nb0.5Hf0.5 RHEA films on 316L, CoCrMo, and Ti6Al4V substrates, revealing amorphous, compact structures with superior mechanical properties and adhesion, particularly on the Ti6Al4V substrate, and demonstrating enhanced corrosion resistance in PBS solution. Chapter four focuses on different RHEA films on 316L substrates, identifying potential as protective coatings due to improved hardness, tribological performance, and biocorrosion resistance, with biocompatibility confirmed through in vitro tests. Chapter five discusses both undoped and Ag-doped Ti1.5ZrTa0.5Nb0.5W0.5 RHEA films, noting the mechanical and corrosion-resistant benefits of Ag nanoparticle inclusion. Chapter six details the development of antibacterial RHEA films doped with Ag nanoparticles, showing significant antibacterial efficacy against P. Aeruginosa and S. Aureus, and promising biocompatibility with C2C12 myoblast cells. Collectively, these studies underscore the potential of RHEA films as functional coatings for biomedical applications. The seventh chapter explores the preparation of CoCuFeNi-based HEAs through mechanical alloying (MA) and their evaluation as electrocatalysts for water splitting. The results show that CoCuFeNiMnMo1.5 exhibits the best OER performance, while CoCuFeNiMnMo0.5 demonstrates the best HER activity with lower overpotentials and excellent stability. The assembled CoCuFeNiMnMo1.5 (anode)∥CoCuFeNiMnMo0.5 (cathode) couple achieves a current density of 10 mA cm–2 at 1.76 V, with a Faradaic efficiency for generated H2 of more than 80%. In conclusion, this thesis provides valuable insights into the potential of HEAs as protective coatings for metallic biomaterials and as efficient electrocatalysts for water splitting, contributing to the advancement of HEA research and the development of novel materials with enhanced properties for biomedical and energy-related applications.