Surface modification of Ti–6Al–4V alloy by means of combined plasma nitriding and oxidising treatments

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Abstract
The high reactivity that titanium and its alloys show with respect to nitrogen, oxygen, and carbon allows them to be used to improve the poor wear resistance of these materials. Nitriding and oxidising are two of the most widely used techniques for surface alloying with nitrogen and oxygen, as they produce modified surface layers characterised by improved hardness and wear resistance. A study of contemporary glow discharge nitriding and oxidising treatments was carried out with the purpose of evaluating the effect of oxygen introduced into the atmosphere of the nitriding process. Oxynitriding treatments were performed on Ti–6Al–4 V alloy specimens in atmospheres with variable contents of nitrogen and oxygen (0–21 vol.-%02) at four different temperatures (973, 1073, 1123, and 1173 K). Treated specimens showed a modified surface layer, constituted by an outer compound layer and an innerdiffusion layer: the characteristics of these layers depend on treatment temperature and gas composition. The compound layer consisted essentially of TiO2 and TiNxOy, though Ti2N was also present when treatment was performed with low oxygen concentrations (2) The diffusion layer was constituted mainly by interstitial rich (α-Ti crystals. It should be noted that the presence of oxygen in the treatment atmosphere leads to thicker diffusion layers characterised by higher hardness values (up to 1300 HK) and smoother hardness gradients in comparison with results obtained by means of the plasma nitriding process.