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 + ß) alloy (Ti-6Al-4V), and ß alloy (Ti-6Al-2Zr-1Mo-1V) were microstructurally characterized. The nitriding at 950 °C and PN2 ═ 105 Pa for 5 h formed TiN, Ti2N, and α-Ti(N) layers from the surface. The nitriding tendency increased in the order of ß alloy, (α + ß) alloy, near-α alloy, and α alloy. The Ti-N coatings transformed to Ti-N-O coatings when the nitrided alloys were exposed to PO2 ═ 10-2 Pa during cooling at the final stage of the nitriding. This oxynitriding process led to the formation of TiNxO1-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.

Gas nitriding and subsequent oxidation of Ti-6Al-4V alloys.

Ti-6Al-4V alloys consisting of α-Ti grains and intergranular ß-Ti islands were nitrided at 850°C for 1 to 12 h under a nitrogen pressure of 1 Pa. With increasing nitriding time, the Ti-N compound layer became thicker, and the α-Ti diffusion zone containing dissolved nitrogen became wider. In the Ti-N compound layer, the initially formed Ti2N became TiN as the nitriding progressed. The nitride layers were oxidized to rutile-TiO2 after oxidation at 700°C for 10 h in air.