Thermochemical Nitriding and Oxynitriding of Ti Alloys

Nitrided and oxynitrided coatings that formed on α alloy (c.p.-Ti), near-α alloy (Ti-2.1Al-2.5Zr), (α + β) alloy (Ti-6Al-4V), and β alloy (Ti-6Al-2Zr-1Mo-1V) were microstructurally characterized. The nitriding at 950 °C and P_N₂ ═ 10⁵ Pa for 5 h formed TiN, Ti₂N, and α-Ti(N) layers from the surface. The nitriding tendency increased in the order of β alloy, (α + β) alloy, near-α alloy, and α alloy. The Ti-N coatings transformed to Ti-N-O coatings when the nitrided alloys were exposed to P_O₂ ═ 10^-2 Pa during cooling at the final stage of the nitriding. This oxynitriding process led to the formation of TiNO_1-, Ti₂N, and α-Ti(N,O) layers from the surface where a small amount of rutile-TiO₂ coexisted. Oxynitriding was more effective than nitriding in increasing the surface microhardness, with the former accumulating more compressive residual stress than the latter.