Surface nirtiding process of zirconium cathode for arc-heated plasma wind tunnel

Zirconium cathode was nitrided using an induction furnace to ensure a long and stable plasma plume of oxygencontaining gas in an arc-heated plasma wind tunnel. The surfaces of the cathode turned golden after nitriding at 1200 and 1450 degrees C. ZrN layer on the surface was nitrided faster at higher temperatures. ZrN layer thickness was uniform at 1200 degrees C, whereas the thickness varied significantly by location at 1450 degrees C. Operation tests were conducted with the prepared cathodes, using air as the main working gas. Stable operation for more than 30 min were achieved, which was significantly longer than the previous results. The cathode nitrided at 1200 degrees C had only one arc spot during 30 min of operation, showed extremely stable plume and almost no corrosion of both cathode and anode. On the other hand, for the 1450 degrees C cathode, the plume was occasionally unstable during the 30 min of operation. This was due to discharges occurring at several locations. In particular, the anodes showed severe corrosion, causing intermittent green sparks in the plume. The optimal cathode condition for arc-heated wind tunnel should have an appropriate thickness and a uniformness of the nitride layer, which has achieved 26.8 mu m of thickness by nitriding at 1200 degrees C.