Effect of boron additives on the structure and properties of soft magnetic composites produced from nickel-clad iron powders

The effect of boron additives on the structure, magnetic properties, and corrosion resistance of Ni– P-clad iron-based soft magnetic materials is studied. The iron powder (with a particle size of 250 to 350 μm) is clad with nickel–phosphorus through thermochemical reduction of nickel chloride by sodium hypophosphite, and boron (1 and 3 wt.%) is added to activate the formation of a liquid phase during high-temperature sintering and reduce the porosity of the prepared composites. It is shown that the cladding of iron powder with nickel–phosphorus increases its corrosion resistance by two points on the scale of ISO 11130:2010 and decreases the depth corrosion index from 0.6457 mm/year to 0.0269 mm/year, which is likely due to the high corrosion resistance of the nickel-phosphorus coating. Moreover, the cladding of iron powder with nickel–phosphorus substantially decreases (2–2.5 times) the magnetic loss in ac fields at a frequency of 50 Hz. The microstructure of the Fe–Ni–P–B composites with different boron contents is heterophase and consists of iron-based ferritic grains, pores, and a liquid phase based on the γ-Fe + Fe₂B, α-Fe + Fe₃P, and Ni + Ni₃B eutectics. The boron content of the material should not exceed 1% because a decrease in the volume of the ferromagnetic component reduces the magnetic induction and permeability. The addition of boron to the clad iron powder increases the hardness and strength of the material produced.