Effects of Mg addition on the phase formation, morphology, and mechanical and tribological properties of Ti-Nb-Mg immiscible alloy coatings produced by magnetron co-sputtering

Abstract

The addition of Mg to Ti-based alloys is a promising method for improving bioactivity in this type of alloys because Mg is absorbed by the body and can biologically activate the implant material, facilitating its adaptation. However, the fabrication of Ti-Nb-Mg alloys by conventional processes is extremely difficult due to the very low solubility of Mg in Ti and the very large difference in their melting points. Therefore, non-equilibrium processing methods such as magnetron sputtering can be used to produce metastable supersaturated Ti-Mg alloys with low density and attractive mechanical properties. In the present study, Ti85-xNb15Mgx (x = 5, 8, 16, 25 at.%) coatings were successfully produced by magnetron co-sputtering deposition on AISI 316L SS substrates. The effects of Mg content on the phase formation, morphology, and mechanical and tribological properties of the Ti-Nb-Mg coatings were investigated. The coatings with lower Mg contents (5 and 8 at.%) presented a predominance of the α (hexagonal close packed - HCP) phase (near α-Ti), the coating with 16 at.% Mg had near equal relative quantity of the α and the β phases (α + β type), and the coating with 25 at.% Mg had almost only the β phase (β-Ti type). A greater stabilization of the β-Ti phase with the increase of Mg content yielded a decrease in both the elastic modulus (102 to 88 GPa) and the hardness values (2.7 to 2.1 GPa). Tribological tests did not detect any detachment of the coatings, due to the high plastic joint strain of the coating/substrate systems, that ensures integrity and good adhesion. Failure was observed only on the coating and depended on α and β relative amounts in the coating.