In this paper, diffusional surface hardening processes utilized to overcome the poor tribological performance of titanium and its alloys is briefly introduced. More specifically, surface treatments known as thermal oxidation, nitriding and boriding offering the advantage of producing graded surfaces comprising hard compound layer and diffusion zone by diffusion of interstitial atoms (oxygen, nitrogen and boron) are overviewed.

Gas nitriding of a commercial purity Grade 4 titanium at 1120°C for 5 h formed hard TiN layer (2450 HV) on the surface with a thickness of about 35 m. When compared to the as-received state, nitrided titanium exhibited excellent wear resistance along with a relatively low coefficient of friction (CoF) against Al2O3 ball under dry sliding conditions.

Commercial purity titanium samples were gas nitrided for 3, 5, 7 and 12 h at 1250°C. TiN layers with nitrogen diffusion zones were formed on the surfaces, which caused an increase in the surface hardness with respect to the processing time. In addition, nitriding provided a reduction in friction coefficient and an enhancement in wear resistance against an alumina ball under dry sliding conditions.

Samples of a Ti6Al4V alloy were treated by thermochemical gas nitriding at 1120°C for 7 h. Gas nitriding provided formation of titanium nitride layer with a hardness over 1800 HV. The titanium nitride layer exhibited excellent wear resistance along with a lower coefficient of friction against alumina ball under dry sliding conditions when compared to the as-received state.