Nano Particle Embedded High Temperature FGM Coatings by Hybrid Spray Process

Thermally sprayed NiCr alloy coatings are commonly used for high temperature coal fired boiler applications. For a stable chromia scale formation a minimum of 20% Cr is required in the NiCr coatings. However, Ni – 50%Cr is the recommended composition for actual boiler coating applications in the industries. Conflicting to this requirement high Cr content of NiCr (50/50) deteriorates the creep properties. In the present work we developed nano ceramic particle embedded NiCr functionally gradient (FGM) composite coatings for high temperature applications using a one step Hybrid Spray coating technology; that combines the twin-wire arc spray and High Velocity Oxy Fuel (HOVF) spray techniques, conceptualized at the University of Michigan. Using liquid precursors and NiCr wire feed stock, in-flight synthesis and simultaneous embedment of nano ceramic particles (SiO₂, Cr₂O₃ and Al₂O₃) in the NiCr matrix was carried out to develop composite coatings. For comparison purposes Arc sprayed NiCr coatings were also developed. All the coatings were then characterized using X – ray diffraction, Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) to understand the nano particulate phase composition, dispersion and the size distribution within the metal matrix along with their thermal stability at high temperatures. FGM coatings showed higher densities compared to the arc spray coatings. Functional property characterization of FGM coatings demonstrated superior corrosion, erosion and oxidation resistances in dry and wet conditions compared to the arc spray coatings, which are otherwise commonly used for boiler applications. Uniform size distribution and dispersion along with dense packing of nano ceramic particles not only helped obtaining denser coatings but also improved the corrosion, wear and oxidation resistance, especially SiO₂ in NiCr matrix enhanced the creep resistance through grain boundary pinning.