Air treatment of pure titanium by furnace and glow-discharge processes

The high oxygen affinity of titanium can be used to improve the surface hardness and wear resistance of titanium and titanium alloy components by means of thermal treatments. In the present paper a comparison of the effects of glow-discharge and furnace processes, carried out using air as treatment atmosphere, on the microstructure and mechanical properties of commercially pure titanium samples is reported. Both treatment types, performed at 973 and 1173 K for 0.5, 2 and 4 h, produce modified surface layers: the outer layer (compound layer) consists essentially of TiO2 and small amounts of TiNxOy, while the inner layer (diffusion layer) consists of α-Ti crystals rich in interstitial atoms. The plasma treatment is shown to be more efficient in hardening titanium surface layers than the furnace process: by using the same treatment temperature and time, plasma treated samples have a thicker hardened layer, with higher hardness values than the furnace treated ones. A crystallographic characterisation of the α-Ti solid solution has also been performed and the a, c lattice parameters have been evaluated. Plasma treated samples show larger lattice parameters and c/a values than the furnace treated samples, suggesting a higher concentration of interstitial atoms due to the plasma treatment.