Effect of Phase Transformation Mechanism on the Microstructure of Cold-sprayed Ni/Al-Al2O3 Composite Coatings during Post-spray Annealing Treatment

NiAl-Al2O3 intermetallic-based composite coatings were prepared by cold spraying of Ni/Al-Al2O3 composite powders followed by post-spray annealing treatment. The phase transformation mechanism from Ni/Al mechanical alloy to intermetallic compounds and its influence on the microstructure were examined to aim at controlling the coating microstructure. Results show that with the porous Ni/Al and Ni/Al-Al2O3 green compacts, a self-propagating high-temperature synthesis (SHS) reaction is ignited at a temperature of 500-600 A degrees C. However, a SHS reaction could not be ignited for the cold-sprayed dense Ni/Al and Ni/Al-Al2O3 alloy coatings with or without a substrate. The phase transformation from Ni/Al mechanical alloy to NiAl intermetallic compound during post-spray annealing evidently can be attributed to a diffusion mechanism. Compared to the quite porous structure of the composites resulting from SHS, the composite coatings via diffusion phase transformation during post-spray annealing treatment present a relatively dense microstructure with neither large pores nor micro-sized cracks.