Physical, morphological and bioactive properties of Co-Cr-W-Ta alloys: Influence of insertion of tantalum and surface thermochemical treatment on bioactivity

The development of bioactivity in bioinert metallic alloys is a field of interest aiming to improve some aspects of these materials for implant applications. New Co63Cr28W9-xTax alloys with different Ta concentrations (x = 0, 2, 4, 6, and 9% w/w) were synthesized in the work reported here. The alloys were characterized by x-ray diffraction, volumetric density, Vickers microhardness, atomic force microscopy, scanning electron microscopy (SEM), and energy-dispersion x-ray spectroscopy (EDS). Bioactivity properties were evaluated by in vitro tests with simulated body fluid (SBF). In vivo assays were performed to assess biocompatibility. The influence of surface thermochemical treatment and Ta insertion on the bioactive properties of the alloys was investigated. The results showed that the alloy structure comprises epsilon Co and alpha Co phases, with cobalt as a matrix with Cr, W, and Ta as a solid solution. TaCo2 phase is observed in the alloys with 4, 6, and 9% w/w of Ta, and its amount increase as Ta concentration increases. Volumetric density is reduced (from 8.78 +/- 0.06 to 8.56 +/- 0.09 g/cm(3)) as Ta concentration increases (from 0% to 9% w/w) mainly due to the lower density of the tantalum compared to the tungsten metal. On the other hand, the TaCo2 phase contributes to the increase of Vickers's hardness by similar to 17.6% for the alloy with 9% Ta (394.7 +/- 8.1 HV) compared with Co63Cr28W9 (336 +/- 5 HV). The topographic analysis showed increased roughness and adhesion due to the nucleation of Ta1.1O1.05 and Ca2Ta2O7 crystals after surface thermochemical treatment. The roughness and adhesion increase from 16.9 +/- 0.6 nm and 8.3 +/- 1.8 nN (untreated surface) to 255.7 +/- 17.7 nm and 24.1 +/- 12.6 nN (treated surface), respectively, for the Co63Cr28Ta9 alloy. These results suggest that thermochemical treatment provides surface conditions favorable to hydroxyapatite (HA) nucleation. The SEM and EDS data showed the nucleation of spongy structures, consistent with HA, composed mainly of Ca and P, indicating that oxides tantalum promoted a bioactive response on the sample's surface. The biological assay corroborated the alloy's safety and applicability, highlighting its potential in biomedical application since no harmful effects were observed.