Effect of C/N incorporation on structural and crystallographic properties of TaNbZr coatings deposited by double-glow plasma surface alloying on Ti-alloy substrate

The relatively low corrosion and wear resistance of Ti-alloy hinders full application of its meritorious low density-to-strength ratio. Here, we studied TaNbZr coatings by double-glow plasma surface alloying (DGPSA), with and without C- or N-incorporation, on TA15 substrate to enhance its tribological properties. The C-incorporation was introduced by adding a graphite target, which decreased the glow discharge on the Ta/Nb/Zr targets and thus reduced the deposition rate by -55 %. In comparison, the N-incorporation was introduced by feeding N2, partly replacing the Ar working gas flow, which reduced the sputtering yield of the metal targets that in turn reduced the deposition rate by -60 %. Hole and hill-in-hole structures have been observed on TaNbZrC with the size and density much higher than those on TaNbZr and TaNbZrN, which was attributed to undesired graphite particles ejected from the target and deposited directly onto the growing surface of the substrate. The localized ejection induced by the graphite particles upon the substrate-associated glow plasma gave rise to the hole and hill-in-hole structures. The C- and N-incorporation modified the surface alloying reactions, leading to different microstructure and smoothness on the surface, compositional changes of the 'buffer' layer at the coating/substrate interface, and varied crystallographic texturing of the coatings. Nanoindentation measurements at the cross-section across the coating/substrate interface provide evidence for the enhanced hardness, not only at the coatings but also at the near-interface regions of the substrate. These observations, together with bonding strength and tribology tests at elevated temperatures, shed new light on developing protective coatings on Ti-alloy by DGPSA.