Understanding Liquid Impingement erosion behaviour of nickel–alumina based thermal spray coatings

Hydroturbines and other fluid machineries are generally subjected to cavitation and slurry erosion environment resulting in the degradation of impellers, vanes, nozzle, spear, labyrinth seal, and buckets. Although hydroturbine steel (13Cr4Ni) provides sufficient resistance against cavitation, however, it degrades severely under slurry erosion. Generally thermal spray coatings are used for protection against slurry erosion, however their performance under cavitation erosion is not appreciable. It is generally observed that the material removal process in cavitation and Liquid Impingement erosion is alike. In this paper, Liquid Impingement erosion performance of composite coatings of nickel and alumina, mixed in various proportions, has been investigated and compared with the uncoated 13Cr4Ni steel. Coatings were deposited using the High Velocity Flame Spray (HVFS) technique. Effects of micro hardness, fracture toughness, work-hardening index, residual stresses and porosity of coatings on the Liquid Impingement erosion performance are discussed. In depth study of the erosion mechanism in each coating and their work hardening capabilities were studied using SEM/EDS and micro-hardness tester. It was observed that fracture toughness and work hardening index of coatings correlate well with the erosion resistance. The effect of alumina content on erosion mechanism of the developed coatings was also investigated. The content of alumina was found to be having significant effect on the erosion response and the degradation mechanism of the coatings.