Evolution of the mechanical properties of Ti-based intermetallic thin films doped with different metals to be used as biomedical devices

Abstract This study reports the assessment of the mechanical properties of intermetallic titanium thin films, doped with different amounts of aluminium, copper, silver, and gold, aiming their use as biopotential electrodes for non-invasive physiological monitoring. The four binary thin film systems, Ti-Me (Me = Al, Cu, Ag, Au), were prepared by DC magnetron sputtering, placing different number of Me pellets on a pure Ti target. The use of a Ti-composed target gave rise to a wide range of compositions, resulting in three distinctive zones of (micro)structural features, identified in all the prepared systems. In the first zone, a Ti-rich one, the films behaved like solid solutions, developing Ti-like microstructures. As the Me/Ti atomic ratio increased, the formation of intermetallic phases played the leading role and it became possible to observe two different microstructural trends, clearly related to the Me type. This zone was identified as an intermetallic region. In the third zone, a Me-rich one, the microstructures displayed by the different films (Me/Ti > 1.0) showed to be dependent on the Ti solubility into Me. The assessment of the mechanical properties revealed an improved hardness and stiffness with the Me addition, directly related to the formation of intermetallic compounds in different degrees of crystallinity. Moreover, the adhesive strength between the substrate and the coating was higher for the films deposited in the intermetallic zone, more evident in the films prepared with Au, Cu and Ag, in this order. The hardness enhancement was especially evident for the thin films presenting microstructures typical of thin film metallic glasses (TFMGs), Ti-Au and Ti-Cu, about twice the values exhibited by the Ti-Al and Ti-Ag ones. Furthermore, the toughness of these metallic glass-like thin film systems was remarkable, more evident within the Ti-rich zone, presenting H/E ratios close to 0.1 and good elastic recoveries. In contrast, the typical columnar morphologies, combined with the brittle intermetallic structures of the Ti-Ag and Ti-Al films, proved to be less resistant to the plastic deformation (H/E ≤ 0.04), despite the improved elasticity presented by the Ag-rich films.