In situ formation of TiN coating on TA2 pure titanium via laser surface nitriding

Laser surface nitriding of TA2 pure titanium was worked out via surface radiation in high pressure N₂ atmosphere. Nitriding layer with a thickness of about 200μm was obtained when the laser power was 800W used in the experiment. The nitriding layer is consisted of two different areas: the TiN formed layer and the remelted layer. Microstructure observations by optic microscope and scanning electron microscope show that the remelted layer presents equiaxed grain structure with fine α plate in inner grains. Obvious difference structure was found in the TiN formed layer, where TiN dendrites were found below a continuous TiN layer. X-ray diffraction analysis result also shows the appearance of Ti₃O in the outmost surface of the nitriding layer, which indicates the oxygen content is still too high in the N₂ atmosphere to inhibit the oxidation of the surface. Microhardness testing indicates that the nitriding layer has a hardness of about 4 times higher than the TA2 pure titanium substrate. Friction and wear testing also shows a better performance of the nitriding layer. The mechanics of the formation of nitriding layer and the improvement of microhardness and the friction and wear property was discussed.