Transmission electron microscopy and atomic force microscopy characterisation of titanium-base alloys nitrided under glow discharge

The microstructure of titanium alloys (Ti–1Al–1Mn, Ti–6Al–4V) nitrided under glow discharge was characterised using light microscopy, X-ray diffraction and analytical electron microscopy. The investigations revealed a complex multilayered microstructure produced by diffusion at 850 and 900 °C at the surface. The layers formed on the Ti–1Al–1Mn alloy were identified as δ-TiN (face-centred cubic NaCl type), δ′-Ti2N (tetragonal body-centred), ε-Ti2N (tetragonal primitive) phases and α (N) solid solution containing up to 19 at.% nitrogen. The layers formed on the Ti–6Al–4V alloy also contained, besides the phases mentioned above, Ti2AlN and other minor Ti–Al–N ternary phases. The layer topography of both nitrided alloys, examined by atomic force microscopy, exhibited pronounced surface development. A relationship between the microstructure of the alloys and their micromechanical and tribological properties has been established. It shows that the improved properties are related to the presence of nanocrystalline δ-TiN in the outermost sublayer on the top of the layers composed of δ′ + ε-Ti2N and Ti2AlN crystals.