Comparison of auxiliary cathode and conventional plasma nitriding of gamma-TiAl alloy

• Formation of nitroxide passive films during nitriding of γ-TiAl is limited by ACN (auxiliary cathode).
• About 70% of process time is saved by ACN in comparison of CPN (conventional plasma nitriding).
• Higher density of N ions in ACN causes the formation of thicker nitride layers.
• The ACN surface hardness and its compound layer uniformity are more than CPN.

Effect of auxiliary cathode (ACN) and conventional plasma nitriding (CPN) processes on surface modification of gamma-TiAl has been investigated. Both processes were done at temperature of 800 °C under gas ratio of N2/H2 = 1. The process duration was selected to be 3, 6 and 9 h for ACN, while it was 10, 20 and 30 h for CPN. Characterizations of the microstructure, phases, elemental distribution and hardness were performed using FESEM, XRD, GDOES, RBS and microhardness tests. Ti2AlN and TiN phases were detected as the main nitride compounds in both processes. Passive oxide films disrupted diffusion of nitrogen atoms in CPN process; hence a narrow nitrided zone was formed due to this surface oxidation. Higher ion intensity and oxidation protection emphasized the ACN process. The thicker and more uniform compound layers with more surface hardness were achieved using the auxiliary cathode. About 70% of nitriding process time was saved by ACN, in comparison with the CPN process.