Thermochemical surface hardening of Ti-6Al-4V: On the role of temperature and treatment media

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out:

1) carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 °C. Treatment at 1027 °C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiC_xO_1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiC_xO_1-x phase by the incorporation of nitrogen, yielding values up to ~2,900 HV and multi-layered structures.

2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO₂ gas mixture at (intermediate) temperatures in the range 677 - 777 °C. Treatment at 777 °C resulted in a surface hardness of ~1,900 HV and a ~60 μm-thick diffusion zone.

In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23:

3) plasma (carbo-nitro)oxidizing was performed in a CO₂ and N₂ gas mixture at 750 °C and (carbo-)oxidizing in CO₂ gas at 650 °C.

4) Plasma nitriding was conducted in a N₂/H₂ gas mixture at 750 °C and 850 °C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys.