Effect of Heating on the Microstructure of NiAl + CrB₂ Coatings Deposited by Mechanical Embedding in a Ball Mill

Abstract

The thickness of NiAl + CrB2 coatings, produced by the mechanical embedding of powders, is limited due to the increasing brittleness of processed materials with milling time. Only the NiAl grain growth and resultant softening of the coating matrix could overcome this problem. Therefore, the effect of heating up to 750°C on the microstructure of NiAl + CrB2 coatings deposited in a ball mill rotating at 350 rpm was investigated through in situ TEM observations. The performed observations proved that defect annihilation starts at ~400°C in large intermetallic grains, which are first attached to the substrate. The growth in NiAl nanocrystallites forming most of the coating is activated only above ~600°C. The average crystallite size was measured to be 5, 14, and 19 nm at RT, 650°C, and 750°C, respectively. The first stage of nano-crystallite growth is relatively fast and connected with the reconstruction of crystallite boundaries using up the amorphous material accumulated in between them. The second stage is slower and involves the expansion of larger crystallites at the expense of smaller ones. The performed experiment proved that heating up to 750°C allows the microstructure recovery and grain coarsening of coatings to be activated.