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 TiN x O1- x , 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.