Formation and characterization of Ti-Zr surface alloys obtained by magnetron sputtering deposition of thin layers of Zr on Ti substrates and subsequent electron beam treatment

Ti-Zr binary alloys are currently being studied primarily for applications in the biomedical field as a valid substitute for alloys containing Al and V. Although the Ti6Al4V alloys are widely used in the biomedical field, their biocompatibility is considered suspicious due to the toxicity of Al and V ions, while it is generally accepted that the zirconium compounds do not give rise to local or systemic toxic effects. In recent years, studies were also carried out on the anodic oxidation of Ti-Zr alloys with the purpose of obtaining nanostructured surfaces in order to improve the biocompatibility or to investigate their behavior as a dielectric for capacitors. In other areas, the use of Ti-Zr alloys for hydrogen storage systems has been considered. A very interesting technique for the formation of surface alloys is based on the magnetron sputtering deposition of thin metal layers onto a metal substrate, followed by treatment with low-energy high-current electron beams (LE-HCEB, or for brevity EB) The EB treatment is a vacuum technique that allows to modify the surface of metal components, for example by improving the mechanical and corrosion resistance properties through microstructural modifications of the surface layers. The EB treatment of a metal surface causes the melting and rapid solidification of a layer, on a large area (electron beam diameter ca. 10 cm), of a thickness up to approximately ten micrometers, thus providing the reduction of roughness and the decrease of the content of impurities in the surface layer. The EB treatment is useful for obtaining smooth surfaces, especially in the case of components with complex shapes, with increased resistance to wear and corrosion. The EB treatment, applied after the deposition of metal films onto a metal substrate, allows to obtain surface alloys by mixing the deposited layers and the substrate in the liquid phase, reducing the problems of poor adhesion of the coating and their possible delamination. The EB or PVD + EB techniques have found use in increasing the wear resistance of high speed steels, and of WC-Co and WC-TiC-Co inserts, in the treatment of Ti6Al4V or 61Co31Cr6Mo alloys for implants and, in the realization of various types of surface alloys and in the control of the corrosion behavior of steels and magnesium alloys. The present work is an investigation on the possibility of using the combined technique PVD + EB for making Ti-Zr surface alloys on titanium substrates. The surface alloys were obtained by deposition, by PVD magnetron sputtering, of Zr layers of different thickness onto titanium grade I substrates, followed by EB treatment. The Ti-Zr surface alloys have been investigated by X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOES). The changes in morphology and composition were observed by scanning electron microscopy (SEM) and the mechanical properties were evaluated by means of microindentations experiments. Ti-Zr surface alloys of thickness in the range between 2 and 4 micrometers were obtained as a function of the number of EB pulses. The number of pulses has also an effect on the morphology and homogeneity of the surface alloys as observed by SEM and EDS. The alloys are characterized by a composition gradient: the concentration of zirconium from the surface decreases (from 43 to 20 at.%) going into the bulk of the sample. Correspondingly, to the formation of Ti-Zr alloys, there is a very significant increase in the microhardness. The analysis of the crystalline structure by X-ray diffraction identified a crystal structure of cubic type with some distortion and in none of the analyzed samples it was possible to assign the diffraction pattern to an hexagonal close packed crystal structure. This work demonstrates that it is possible to obtain Ti-Zr alloys with high content of zirconium having a distorted cubic crystal structure even at room temperature.