Microstructural analysis and crystallographic characterisation of plasma oxynitrided Ti-6Al-4V

Combined glow discharge oxidising and nitriding treatments allow the production on Ti-6Al-4V of modified surface layers with enhanced hardness properties compared with a traditional ion nitriding process. The treatment atmosphere has a marked influence on the properties of these hardened layers: as the oxygen concentration increases, thicker diffusion layers, characterised by greater hardness values and smoother gradients, are obtained. In the present paper the effects of treatment atmosphere and duration on the glow discharge oxynitriding of Ti-6Al-4V specimens are evaluated by performing tests at 1173 K with oxygen concentration varied in the range 0-21 vol.-% and for treatment durations in the range 0.5-8 h. The modified layers produced consist of an outer compound layer and an inner diffusion layer. The compound layer consists of nitrides, oxides, and oxynitrides of titanium and, in some cases, of alloy element oxides too. With increasing oxygen concentration and/or time, the compound layer develops a stratified structure and its adhesion to the substrate becomes poor; this fact can be attributed to the high content of titanium oxide, TiO2, in the layer. In the diffusion layer alpha-Ti crystals, rich in interstitial atoms, are present. The diffusion layers in the oxynitrided specimens are thicker and show higher hardness (up to 1300 HK) and smoother gradients than those produced by a traditional ion nitriding process under the same treatment conditions. Case depths of comparable thickness (around 45 µm) can be obtained by performing an 8 h nitriding treatment or a 2 h oxynitriding treatment with 6 vol.%O2. Moreover; alpha-Ti lattice parameters a, c were evaluated as a function of the experimental conditions, the values of these parameters can be correlated with changes in hardness values.