Characterization of Ti-6Al-4V Alloy Modified by Plasma Nitriding Process

Titanium alloys are gaining widespread acceptance in aerospace industry because of its high specific strength, corrosion resistance and good fatigue properties. Dovetails of the compressor blades, gears, splines etc. are some aerospace components that fail through premature fatigue crack initiation and propagation under the action of wear, fatigue and fretting fatigue. The fretting failure originates from the surface or near surface and leads to the damage of the components. Therefore the surface of components can be modified to improve the tribological performance by plasma nitriding which improve the titanium alloy by forming layer of hard TiN and Ti₂N phases on surface. Plasma nitriding of titanium alloys has several advantages over gas and liquid nitriding methods where the phase formation and the depth of nitriding can be controlled. In this study Ti-6Al-4V alloy is taken as surrogate material for gas turbine application and its surface is modified by plasma nitriding at three different temperatures 500, 700 and 800 °C with N₂:H₂ ratio of 4:1 at 5 mbar pressure for 5 hrs. It is observed from XRD that at 500 and 700 °C temperature, nucleation of ε-Ti₂N and δ-TiN started and complete titanium nitride layer formed at 800 °C. Nucleation and growth mechanism was studied by surface and cross section SEM analysis. Nitride layer of around 0.5 μm with ε-Ti₂N and around 2 μm thick of both ε-Ti₂N and δ-TiN phases were formed at 700 and 800 °C respectively. It is observed that the surface roughness increases with increasing the temperature for plasma nitriding. Vickers microhardness (HV_0.1) is observed to be increased from 393.7 HV to 1016.4 HV by plasma nitriding at 800 °C.