Plasma nitriding of Ti6Al4V alloy for improved water erosion resistance

Abstract

Water droplet erosion (WDE) is a severe problem encountered in the operation of gas turbines. The damage is caused when small water droplets hit the surface of the compressor blades at very high velocities. In this work, RF plasma nitriding of Ti6Al4V alloy was performed with the aim of improving its resistance to the WDE. Process parameters, including temperature and gas atmosphere, were varied to evaluate the influence of nitriding conditions on the WDE resistance. Structural and morphological changes of the samples were investigated by scanning electron microscopy (SEM), phase composition by X-ray diffraction, and cross-sectional hardness distribution by nanoindentation. Water droplet erosion tests were performed using a rig that simulates WDE at working conditions of the compressor blades. Each sample was subjected to 6 min of WDE tests. After every minute of the experiment the erosion area of the sample was evaluated by the optical microscope and the specific mass loss was determined. Damage was analyzed using SEM and confocal laser scanning microscopy. The results show significantly higher resistance against WDE of Ti6Al4V nitrided at low temperature, especially in the case of hydrogen-free nitriding atmosphere. The incubation time for these samples was extended four times, as compared to the bare substrates or to the samples nitrided at higher temperatures.