Effect of H3PO4 Coating, Polyimide Binder, and MoS2/Graphite Lubricants on the Formability and Electromagnetic Properties of Fe-5.0 wt.%Si SMC Toroidal Cores

This study examined the effects of phosphoric acid (H3PO4), polyimide (PI), and lubricants (MoS2, graphite) on the phase stability, microstructure, and magnetic performance of Fe-5.0 wt.%Si soft magnetic composites (SMCs). Warm compaction (≤550 °C) and annealing at 700 °C were applied to samples prepared under a full factorial design. X-ray diffraction confirmed stable α-Fe(Si) phases without secondary phases. SEM and TEM–EDS revealed interfacial insulating layers mainly composed of Si-O, with localized phosphorus and carbon. Additive composition strongly influenced magnetic and physical properties. Increasing H3PO4 and PI reduced the density from 7.50 to 7.27 g/cm3 and lowered the permeability (from 189 at 1 kHz to 156), due to thicker interparticle layers that restricted metallic contact and domain wall motion. In contrast, Q-values rose significantly with frequency: for H3PO4 0.25 wt.% + PI 0.25 wt.% + graphite 0.3 wt.%, Q increased from 0.39 (1 kHz) to 2.91 (10 kHz), reflecting effective eddy current suppression. Lubricant type further influenced performance: graphite consistently outperformed MoS2, with 0.3 wt.% graphite providing the best balance of high density, permeability, and a frequency-stable Q-value. Overall, Fe-5.0 wt.%Si performance is governed not by bulk phase changes but by the trade-off between densification and insulation at particle interfaces. The optimal combination of low H3PO4 and PI with 0.3 wt.% graphite offers practical guidelines for designing high-frequency, high-efficiency motor materials.