HVAF vs oxygenated HVAF spraying: Fundamental understanding to optimize Cr₃C₂-NiCr coatings for elevated temperature erosion resistant applications

High velocity sprayed Cr₃C₂-NiCr coatings are a viable solution to improve the life of power plant components and are often influenced by the choice of processing conditions. The new generation convertible high-velocity air-fuel (HVAF) process offers higher versatility to spray wide ranging particle sizes including the standard high-velocity oxy-fuel (HVOF) grade. However, the use of such oxygenated spraying termed as ‘HVAF(O)’ mode necessitates better understanding for reliable and repeatable coating. Single particle splat morphology and fracture mechanisms of HVAF and HVAF(O) sprayed Cr₃C₂-NiCr coatings were correlated with the microstructural features, mechanical properties and erosion performance with two different particle sizes. Among the coatings studied, HVAF(O) sprayed coarse Cr₃C₂-NiCr and HVAF sprayed fine Cr₃C₂-NiCr provided the least thermal degradation, and better mechanical properties which collectively ensured enhanced erosion performance. The overall coating performance can be attributed to the appropriate choice of process conditions and suitable particle size. In comparison with conventional HVOF process that generally induces a certain degree of thermal degradation within the hard carbides, oxygenated HVAF(O) sprayed Cr₃C₂-NiCr coating still retained most of the starting carbide phase with minimal thermal degradation, which resulted in superior erosion performance.