Slurry Erosion of Thermal Spray Coatings: Effect of Sand Concentration

Abstract

Slurry erosion is a major problem in hydroturbines and other fluid machineries wherein erodent particles entrained in carrier fluid impinge upon the target surface to cause its degradation. Several possible alternative solutions are available to control slurry erosion; however, surface coatings developed using thermal spraying are mostly used owing to their versatile nature. In this work, Ni based thermal spray coatings were deposited on a commonly used hydroturbine steel (CA6NM), with Al2O3 mixed in different proportions. Coatings were developed using high velocity flame spray process (HVFS). Slurry erosion performance of prepared coated and bare steel was investigated using a specially designed jet type test rig with sand as erodent particles. To investigate the effect of concentration on erosion behavior, sand was mixed in water at two different levels, 0.1 wt % and 0.5 wt. %. It was observed that all the coatings helped in improving the erosion resistance of the steel with one containing 40 wt % Al2O3 showing the maximum improvement. This was related to the microstructure, which controlled the micro hardness and fracture toughness of the coatings. Shielding effect of the rebounding particles was found to have profound effect as erosion rates did not show similar augmentation with concentration. Fracturing of alumina phase along with removal of coating in form of splats was observed to be prominent erosion mechanism.