Properties of nitrided layers formed during plasma nitriding of commerciallypure Ti and Ti–6Al–4V alloy

An investigation was carried out on commercially pure titanium and on a Ti–6Al–4V alloy plasma nitrided at 730 °C according to different conditions. Diffusion of nitrogen and formation of compound layer were very limited for the shortest processing times of 20 h in both materials while the combination of diffusion periods after active nitriding led to a clear improvement of nitriding efficiency only for the Ti–6Al–4V alloy. Extension of nitriding times to 76 h generated a significantly thicker compound layer composed of a combination of TiN and Ti2N phase while TiN became predominant at nitriding times of 156 h. Ti2AlN was also supposed to be present in the outermost layers of the Ti–6Al–4V alloy nitrided in both conditions. Al-enriched and Venriched regions were detected beneath the above layers. Modifications observed in extension and chemistry of the nitrided layers also resulted in different hardness and scratch resistance properties. The thin TiN layer found in the soft CP titanium nitrided to 20 h clearly cracked due to extensive deformation produced during scratch testing. On the contrary, shallower scratch tracks were generated with no evidence of surface cracking in the harder Ti–6Al–4V alloy. Increasing the nitriding times led to generation of small cracks at root of track, more developed after nitriding for 156 h. It was speculated that the different fractions of TiN and Ti2N found for the two processing times could have an effect on damage sensitivity, promoting cracking where the most brittle TiN was predominant.