Post-processing of Ni-Cr-Al2O3 thermal spray coatings through friction stir processing for enhanced erosion–corrosion performance

Abstract

Thermal spray coatings are widely used for addressing erosion problems in fluid machineries. However, the presence of splat boundaries, non-homogeneous microstructure and element segregation limits their performance. In this study, we developed Ni-Cr-40Al2O3 coatings on stainless steel (SS316L) substrate using highvelocity flame spray followed by post-processing using friction stir processing. The performance of as-sprayed and processed coatings was evaluated in slurry erosion, erosion– corrosion and pure corrosion in 3.5% NaCl solution. As-sprayed coating showed lower erosion and erosion– corrosion resistance compared to stainless steel. Friction stir processing resulted in significant microstructure refinement concurrent with enhanced hardness and fracture toughness of the developed coating. At oblique impingement angle, the processed coating showed nearly 30% to twofold higher erosion and erosion–corrosion resistance compared to stainless steel substrate. During pure erosion at normal impingement, both as-sprayed and processed coating showed higher volume loss. However, during erosion–corrosion at normal impingement, the processed coating was able to demonstrate similar volume loss as that of the substrate. In addition, the processed coating showed significant improvement in the corrosion performance as well. The study showed that friction stir processing could be a promising pathway to unravel the deleterious effects of inherent non-homogeneities in thermal spray coatings.