Evolution of phosphate coatings during high-temperature annealing and its influence on the Fe and FeSiAl soft magnetic composites

Phosphate layers were prepared as the insulation coating for Fe and FeSiAl soft magnetic composites (SMCs) and mechanisms of the phosphating process have been investigated. Effects of high-temperature annealing on the phosphate coating and the magnetic properties of the SMCs have also been revealed. For Fe SMCs, the insulating coating mainly consists of iron phosphate after surface passivation. It degrades into iron phosphide after annealing at relatively high temperatures (600 °C), leading to decreased electrical resistivity and increased eddy current for deteriorated magnetic properties of Fe SMCs. For the FeSiAl SMCs, the phosphate coating mainly contains Al(PO3)3, which converts into oxides such as Al2O3 and P2O5 after annealing. These oxides possess high electrical resistivity, and continue to serve as the insulation coating for the composites. Consequently, the resistivity of the FeSiAl SMCs increases with the annealing temperature, which is beneficial to decrease the eddy current loss for enhanced magnetic performance.