Phase Transformation of Cold-Sprayed Ni/Al-Al2O3 Composite Coatings During Post-Spraying Annealing Treatment

NiAl-Al₂O₃ intermetallics based composite coatings were prepared by cold spraying of Ni/Al-Al₂O₃ composite powder followed by post-spraying annealing treatment. The phase transformation mechanism was explored to aim at controlling the microstructure of the composite coating. Results showed that with the porous Ni/Al and Ni/Al-Al₂O₃ composites obtained by pressing the alloy-based powder self-propagating high-temperature synthesis (SHS) reaction was ignited at a temperature comparable to the accelerating gas temperature during cold spraying. However, SHS reaction was not able to be ignited for the cold-sprayed dense Ni/Al alloy coatings with or without substrate. SHS reaction was even not ignited for the Ni/Al-60vol.%Al₂O₃ composite coating, although the thermal conductivity of the composite coating was significantly decreased by the addition of Al₂O₃ ceramic particles. The phase transformation from Ni/Al alloy to NiAl intermetallics during post-spray annealing can be evidently attributed to diffusion mechanism.