Shaping the structure and properties of titanium and Ti6Al7Nb titanium alloy in low-temperature plasma nitriding processes

Abstract

Plasma nitriding of titanium Grade 2 and Ti6Al7Nb titanium alloy at 730 °C was used to produce nitrided surface layers with an outer zone consisting of nanocrystalline titanium nitride - TiN, and a Ti2N zone right above the αTi(N) diffusion area. The paper presents a comparison of structure (TEM, SEM), phase composition (XRD, SAED) surface topography (AFM, optical profilometer), corrosion (potentiodynamic method), tribological properties (‘ball-on-disc’), and adhesion (scratch-test) of TiN + Ti2N + αTi(N) type nitrided surface layers produced on Grade 2 titanium and Ti6Al7Nb titanium alloy using conventional glow-discharge nitriding (so called glow discharge nitriding at the cathode potential) and with the use of an ‘active screen’ (glow discharge nitriding at plasma potential). A reduction of cathodic sputtering in the plasma potential process made it possible to produce surface layers maintaining a high smoothness of the titanium surfaces. Due to the low surface roughness and the high compressive residual stress of the sample after nitriding at plasma potential, ‘ball-on-disc’ test results showed the lowest coefficient of friction and the lowest penetration depth for this layer and very good adhesion of the layer to the substrate – i.e. Ti6Al7Nb titanium alloy. Nitriding of titanium alloy at cathode potential increased the corrosion potential value and slightly decreased corrosion current density. The corrosion current density of the sample nitrided at plasma potential was similar to initial state Ti6Al7Nb titanium alloy. The nitriding process type (cathode potential or plasma potential) used to produce TiN + Ti2N + αTi(N) surface layers on Ti6Al7Nb titanium alloy has an influence on the microstructure of the layer, its residual stress, in particular its TiN outer titanium nitride zone, its thickness, and on its properties such as wear and corrosion resistance.