The durability of several mechanical components used in a broad range of industries is limited owing to wear-driven degradation. An accelerated wear rate is usually observed at elevated temperatures, resulting in premature failure of the industrial components and eventual shutdown. Therefore, such demanding environments necessitate the use of high-temperature, wear-resistant coatings for high durability and reliability. Thermal spraying is a widely accepted industrial process to deposit thick, dense, and hard overlay coatings. High-velocity thermal spray systems are usually preferred for depositing wear and corrosion-resistant coatings with desirable microstructural features such as better inter-splat bonding, less oxide content and reduced porosity. Among the high-velocity spray variants, High-velocity air fuel (HVAF) is finding increased industrial attention, which operates through the hypersonic combustion of compressed air and hydro-carbonaceous fuel. The innovative torch design of HVAF allows reduced gas temperature that shall be far lower than the competing High-velocity oxy-fuel spray (HVOF) and slightly higher than cold spray while maintaining the kinetic energy component. The above process characteristics enable better adhesion, less in-flight oxidation/degradation and further reduction in porosity.

HVAF sprayed Cr3C2-NiCr coating under certain processing conditions demonstrated superior coating characteristics and performances than competing deposition techniques. However, a detailed understanding of the role of HVAF process parameters on Cr3C2-25NiCr coating characteristics and wear performances is not explored. Accordingly, the present dissertation attempts to understand a detailed process-structure-property-performance correlation for HVAF-sprayed Cr3C2-25NiCr and identify conditions to deposit repeatable Cr3C2-25NiCr coatings with superior wear performance. New insights on the role of torch type, feedstock particle size, nozzle geometry, combustion mode and binder chemistry were obtained through systematically studying splat morphology, fracture mechanisms, microstructure, phases present, mechanical properties, wear rate and wear mechanisms involved in HVAF spraying of Cr3C2-25NiCr coatings. Coatings deposited using an optimal set of HVAF process parameters exhibited a disc-shaped splat, resulting in a coating microstructure with minimal defects, improved mechanical properties and ductile mode dominant fracture behaviour.

Additionally, the effect of post-heat treatment for as-deposited Cr3C2-25NiCr coatings is of interest, which typically exhibits fine carbide precipitation within the interacted carbide-binder regions. The role of such precipitates necessitates a better understanding of the wear performance since Cr3C2-25NiCr coatings are usually employed in practical applications at elevated temperatures. Finally, HVAF-sprayed Cr3C2-25NiCr coatings were compared with plasma and detonation spray deposition techniques. A comprehensive analysis was performed to understand the process-structure-property relationship of Cr3C2-25NiCr coatings based on the room and high-temperature erosion performance. Under elevated erosive conditions, an optimized HVAF sprayed Cr3C2-25NiCr coating provided 40-45% less erosion than boiler steel, 30-35% less erosion than detonation sprayed coating and 65% less erosion than plasma sprayed coating. Overall, the HVAF sprayed Cr3C2-25NiCr coatings with optimized microstructural features and mechanical properties yielded better room/high-temperature wear performance that can be potentially exploited in demanding power plants and other applications wherein high-temperature accelerated wear rate is a concern.