Tantalum-Doped Co-Cr Alloys: Multiscale Effects on Mechanics, Surface Potential, and Biological Responses

Cobalt-chromium (Co-Cr) alloys are valued in biomedicine for their mechanical performance and relative bioinertness. Here, Co-Cr alloys containing tantalum (Ta; 0, 3, 6, and 9 wt %) were synthesized by electric-arc melting to assess how Ta content influences bulk and surface properties across scales. X-ray Diffraction (XRD), Vickers Microhardness, Wettability, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), and Atomic Force Microscopy/Kelvin Probe Force Microscopy (AFM/KPFM) were employed. Bioactivity and biocompatibility were evaluated by immersion in simulated body fluid (SBF) and cell-viability assays. Ta addition promoted the coexistence of Co allotropes (εCo and αCo) and the formation of a secondary TaCo₂ phase, resulting in a dendritic microstructure. Microhardness increased significantly, reaching 466 ± 27 HV0.5 for 9 wt % Ta (≈77% above the Ta-free alloy). Surface-potential mapping revealed ≈1500% higher local potential differences and composition-dependent work function distribution, indicating Ta-driven electronic heterogeneity. The alloys were cytocompatible under extract exposure conditions, suggesting that Ta-induced surface changes may indirectly modulate the biological response. SBF tests confirmed improved in vitro bioactivity, with apatite nucleation on all alloys; the 6 wt % Ta composition produced the thickest Ca/P-rich layer (∼1.3 μm). Overall, Ta addition modulated the microstructure, localized deformation resistance, surface electronic response, and in vitro biological behavior of Co-Cr alloys, with 6 wt % Ta showing the most balanced response among the investigated compositions.