Thermochemical Nitriding and Oxynitriding of Ti Alloys

Abstract

Nitrided and oxynitrided coatings that formed on alpha alloy (c.p.-Ti), near-alpha alloy (Ti-2.1Al-2.5Zr), (alpha + beta) alloy (Ti-6Al-4V), and beta alloy (Ti-6Al-2Zr-1Mo-1V) were microstructurally characterized. The nitriding at 950 degrees C and P-N2 = 10(5) Pa for 5 h formed TiN, Ti2N, and alpha-Ti(N) layers from the surface. The nitriding tendency increased in the order of beta alloy, (alpha + beta) alloy, near-alpha alloy, and alpha 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 TiNxO1-x, Ti2N, and alpha-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.