Effect of Boron Additives on the Structure and Properties of Soft Magnetic Composites Produced from Nickel-Clad Iron Powders

Abstract

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 chemicothermal 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 a nickel-phosphorus coating. Moreover, the cladding of iron powder with nickel–phosphorus substantially decreases (2–2.5 times) the magnetic loss in ac field at a frequency of 50 Hz. It is shown that the microstructure of the Fe–Ni–P–В composites with different boron contents is heterophase and consists of iron-based ferritic grains, pores, and the liquid phase based on the eutectic γ-Fe+Fe₂B, α-Fe+Fe₃Р, and Ni + Ni₃В. The boron content of the material must 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 characteristics of the material produced.