Microstructure and high temperature tribological behaviour of self-lubricating Ti-TiBx composite doped with NiBi

It is a well-established fact, that the tribological performance of titanium and its alloys at high temperatures is poor, especially for self-mated titanium‑titanium pairs. Previous work has shown a significant improvement in mechanical and high temperature tribological behaviour (700–900 °C) for pure Ti sintered with TiB2 ceramic, occasioned by the in-situ formed boric acid lubricant. The current work explores the possibility to extend the lubrication temperature range (below 700 °C) of Ti-TiB2 composites through the incorporation of a nickel‑bismuth mixture (NiBi) as a solid lubricant using laser melting. To this end, phase equilibrium diagrams have been calculated for the Ti-B-NiBi quaternary system using the CALPHAD method to evaluate the possibility of the formation of different phases due to dilution between the substrate and deposit, as well as their possible interactions. The self-lubricating NiBi samples feature a formation of Bi-Ni-Ti-TiBx functional surface. The functional surface demonstrates a compact and unique microstructure of a hard B-rich phase encapsulated inside a lubricous Bi-rich phase. A two-fold increase in hardness of the surface in comparison to the underlying substrate was noted for NiBi laser-melted samples. Further, the NiBi samples were studied under a reciprocating sliding configuration against a Ti-alloy flat-pin counterbody at temperatures including RT, 400 and 600 °C. The resulting wear scars were analysed utilizing SEM, XRD and 3D profilometry to understand the prevalent wear mechanisms. A significant decrease in coefficient of friction and wear rate was observed for the NiBi laser-melted samples sliding at 400 °C and 600 °C due to the formation of a self-lubricating tribo-oxide layer rich in Bi-compounds. The results are reported in comparison to the unmodified reference Ti-TiB2 composite.