Laser gas alloying of Ti-6Al-4V in air for tribological applications

Abstract Hard and wear-resistant TiOxNy/α-Ti composite coatings were fabricated on Ti-6Al-4V alloy via laser gas alloying in air for tribological applications. The effects of laser power, scan speed, and hatch spacing on the surface morphology, geometry and microstructure of the composite coatings were investigated systematically. The formation of laser-sustained plasma above the melt surface could lead to the dissociation of N2 and O2 molecules into N and O atoms. The atoms diffused and mixed into the melt, and subsequently reacted with molten Ti to form TiOxNy dendritic reinforcements, while the remaining melt solidified to form the α-Ti solid solution matrix. A compositional gradient was observed in the transitional zone between the TiOxNy dendrite and α-Ti matrix. The transitional zone was also found to have a crystallographic relationship with the TiOxNy dendrite. The microhardness and wear resistance of the composite coating were 2.8 and 10.8 times those of Ti-6Al-4V base metal, respectively. The excellent wear resistance of the composite coatings was attributed to the formation of hard TiOxNy reinforcements and α-Ti solid solution, as well as the strong TiOxNy/α-Ti interface.