Novel surface engineering design enabled surface multi-functionalisation of metastable Ti-15-3 β-titanium alloy

The growing demand for high-performance and long-service components for challenging and resource-intensive applications has driven the development of high-strength metastable β-titanium alloys with multi-functional surfaces. In this study, a novel surface engineering strategy, termed integrated bulk heat treatment with surface functionalisation (IBTSF), is introduced. This approach combines bulk ageing treatment with catalytic ceramic conversion treatment (C3T) incorporating Ag or Au, thereby simultaneously imparting surface multi-functionalities (high hardness, desirable tribological properties and high antibacterial efficacy) while also enhancing the bulk mechanical properties. To this end, the metastable β-alloy Ti-15V-3Al-3Cr-3Sn (Ti-15-3) was selected as a representative alloy, and C3T was catalysed using either Au or Ag. Under a load of 20 N, the Au-catalysed C3T achieved near-zero wear and a low, stable coefficient of friction of ~0.3, owing to the formation of a lubricating tribo-film. In contrast, the Ag-catalysed C3T maintained stable tribological performance up to a load of 10 N while delivering high antibacterial efficacies of 99.878% and 99.999% against <em>E. coli</em> and <em>S. aureus</em> within 3 h and 6 h of contact, respectively, through passive Ag-ion release. Both treatments also enhanced the bulk tensile strength by approximately 50%, increasing from 872 ± 39 MPa to 1280 ± 40 MPa. This combination of exceptional wear resistance, potent antibacterial activity, and improved mechanical strength offers a promising and efficient pathway to the surface multi-functionalising metastable alloys for long-service, high-reliability applications.