Abstract

Water Droplet Erosion (WDE) is a mechanical degradation which is of great concern to power generation and aerospace industries, where water droplets interact with high-speed moving components. It is observed on multiple components of airplanes when flying in the rain, on steam turbine blades, and gas turbine compressor blades which is the focus of this study. Compressor blades in gas turbines suffer from WDE due to the recently implemented technology Inlet Fog Cooling. It is utilized to boost gas turbine efficiency in high ambient temperatures via the spraying of micro-sized water droplets into the intake air. Evaporation of water droplets cools the intake charge and increases the air density. The main drawback of inlet cooling is that part of the water droplets are carried into the compressor by intake air flow, which is called overspray. Repetitive impact between rotating blades and water droplets leads to WDE damage, which is a major problem.
This study attempted to improve WDE resistance of compressor blades made of Ti6Al4V. Two different approaches were considered: Studying WDE of TiAl alloy as a potential erosion resistant material for turbine blades and Surface engineering of Ti6Al4V via gas nitriding and HVOF spraying of WC-Co. To investigate WDE in this work, erosion experiments were carried out using the state-of-art WDE rig available at Thermodynamics of Materials Group (TMG), Concordia University. It enables control of erosion conditions such as impact velocity, impact angle, droplet size, and droplet impingement number.
In the first stage of this study, WDE of nearly fully lamellar TiAl alloy was directly compared to Ti6Al4V at relatively wide ranges of impact speed (i.e. 275m/s, 300m/s, 325m/s and 350m/s) and droplet size (i.e. 464µm and 603µm). TiAl demonstrated superior erosion performance (i.e. longer incubation period and slower material loss) compared with Ti6Al4V at all tested conditions. Herein, the relation of impact velocity and maximum erosion rates for TiAl and Ti6Al4V alloys was established and the damage threshold speeds of TiAl were determined. Afterwards, the erosion behaviour of TiAl was related to its mechanical properties and irregular microstructure. Lastly, in-depth investigation was carried out to reveal erosion damage mechanism of TiAl at different stages including incubation, material loss initiation, maximum erosion rate, and advanced stages.
In the second stage of this study, the influence of gas nitriding on water droplet erosion behaviour of Ti6Al4V was investigated. Ti6Al4V coupons were nitrided at two different temperatures (i.e. 900 and 1050°C) for 5 and 10 hours and the nitrided coupons were characterized. Their erosion behaviour in comparison with as-received and annealed Ti6Al4V was examined at two impact speeds (i.e. 300m/s and 350m/s). Mechanical properties, erosion performance and damage mechanism of treated Ti6Al4V were significantly influenced by the nitriding temperature. Exploring the potential of HVOF spray WC-Co coating to combat WDE was the last part of this study. Erosion performance of two WC-Co coatings was examined at 250m/s, 300m/s and 350m/s impact speeds in comparison with Ti6Al4V. The as-sprayed coating did not show notable WDE protection; however, removing its top layers and smoothing its surface notably improved the erosion performance.