Microstructural and tribological properties of multilayered coatings developed through heat treatment on the surface of CP-Ti

It is well known that applying Ni Ti intermetallic coatings on the surface of different alloys can improve their tribological behavior. In this paper, various intermetallic phases of Ni 3 Ti, NiTi, and NiTi 2 and a Ti-rich pearlite-like eutectoid structure were in situ synthesized on the surface of commercially pure (CP) titanium grade 2. Nickel electroplating was performed on titanium samples followed by heat treatment at a temperature of 800 °C for various times up to 8 h. The formation of various phases and structures along the depth were studied using SEM, EDS and XRD. Heat treatment at a temperature of 800 °C for 4 h resulted in the formation of a multi-layered composite of Ni 3 Ti, NiTi, and NiTi 2 intermetallic films with a thickness of 5, 9, and 2 μm, respectively. A nickel diffusion layer (NDL) with a pearlitic structure containing nanolayers of αTi-NiTi 2 was also formed beneath the NiTi 2 layer even after 30 min of heating. Among the different layers formed on the surface of CP-Ti, the titanium-rich NiTi (51–52 at.% Ti) showed the highest hardness of more than 400 HV. Sliding wear tests were performed under a normal load of 10 N and the surfaces of wear tracks were studied using SEM. The results showed higher wear resistance of all intermetallic layers and NDL than the titanium substrate, with the highest wear resistance for Ni 3 Ti top layer. • Raising the heat treatment temperature reduced Ni 3 Ti thickness, increased NiTi, and kept Ni 2 Ti unchanged. • Eutectoid Ti+NiTi 2 and a proeutectoid region with ∼300 HV hardness were detected beneath NiTi 2 . • The dense NiTi film had a hardness of 420 HV, while porous Ni 3 Ti measured 250 HV. • Ti-rich NiTi phase (mostly B19’) was worn out through plastic deformation. • All developed layers had lower wear than the substrate, with Ni 3 Ti showing the lowest.