Enhanced bonding strength of Ti-6Al-4 V/PEEK hybrids via atmospheric pressure plasma treatment: influence of surface functionalization and oxidative modification

Hybrid metal–polymer structures combining titanium alloys and Polyether Ether Ketone (PEEK) have attracted increasing attention for biomedical implants because they can reduce stress shielding while maintaining sufficient mechanical strength. Direct bonding of Ti-6Al-4 V (Ti-64) titanium alloy to PEEK was successfully demonstrated via hot pressing following pre-treatment using a non-thermal atmospheric pressure RF plasma jet. This study investigated the surface modifications on both materials and their influence on joint strength, aiming to establish a viable method for hybrid biomedical implants. Plasma treatment applied solely to the Ti-64 side led to TiO 2 ​ and Al 2 ​O 3 ​ oxide layer formation but resulted in negligible bond strength (≈0.8 MPa). Conversely, plasma treatment on the PEEK side resulted in a significant enhancement, and the tensile shear strength reached a maximum of 11.2 MPa. This strength was high enough for failure to occur within the PEEK bulk itself. Surface analysis confirmed that the enhancement was primarily chemical. While the Ti-64 surface roughness ( R a) remained nearly constant, XPS analysis revealed that the plasma jet introduced new oxygen-containing functional groups (O-C=O) onto the PEEK surface. The resulting bond strength showed a strong correlation with the density of these polar groups. The bonding mechanism is considered to be consistent with hydrogen bonding and other secondary interactions between the oxygen-containing functional groups introduced on PEEK and the oxide sites on the Ti-64 surface. This method offers excellent strength and maintains sufficient biocompatibility, validating its application potential in hybrid implant technologies. Unlike commercially pure titanium (TP340), Ti-64 contains alloying elements such as vanadium, whose oxidation behavior and surface segregation characteristics significantly affect the chemistry of the passive oxide layer. In this study, particular attention is given to the role of vanadium oxides in modifying interfacial interactions, which has not been systematically analyzed in previous TP340/PEEK systems.