Thermochemical Nitriding and Oxynitriding of Ti Alloys.

Nitrided and oxynitrided coatings that formed on alloy (c.p.-Ti), near- alloy (Ti-2.1Al-2.5Zr), ( + ) alloy (Ti-6Al-4V), and alloy (Ti-6Al-2Zr-1Mo-1V) were microstructurally characterized. The nitriding at 950 °C and ═ 10 Pa for 5 h formed TiN, Ti₂N, and -Ti(N) layers from the surface. The nitriding tendency increased in the order of alloy, ( + ) alloy, near- alloy, and alloy. The Ti-N coatings transformed to Ti-N-O coatings when the nitrided alloys were exposed to ═ 10 Pa during cooling at the final stage of the nitriding. This oxynitriding process led to the formation of TiNO, Ti₂N, and -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.