Effect of nitriding conditions of Ti6Al7Nb on microstructure of TiN surface layer

Abstract

Presented experiments were aimed at comparing microstructure of the TiN layer produced on Ti6Al7Nb alloy being gas nitrided (GN) or glow discharge nitrided with either active screen (GD-ASN) or at cathode potential (GD-CPN). They were treated at 620 °C, 680 °C, 740 °C and 830 °C for 6 h. The transmission electron microscopy investigations showed that thickness of the TiN layer depends less on average temperature of nitrided piece, but more on temperature of its surface, being the highest for the GD-CPN process. The growth of TiN layer during the GN treatment proceeds mainly towards the core and to a lesser extent at the surface. The former process is controlled by chemisorption and inward diffusion of nitrogen atoms, while the latter by outward diffusion of titanium. The factor controlling the growth of TiN during the GD-ASN treatment is exclusively the flux of the titanium atoms sputtered from the active screen. The thickness of the TiN layer produced during the GD-CPN process in the temperature range between 680 °C and 830 °C is controlled in the same way as during the GN, except the fact that bombardment of the processed material with nitrogen ions strongly raises the surface temperature.