Evaluation of Electromagnetic Properties of Fe–5.0 wt.%Si According to MoS2 and Graphite Lubricant Contents

Soft magnetic composites (SMCs) require a balance between internal density uniformity, microstructure, and electromagnetic performance, all of which are affected by lubricant composition. This study investigated Fe–5.0 wt.%Si SMC toroidal cores with nine MoS2/graphite compositions using FEA, electromagnetic measurements, SEM, and TEM–EDS. C3, containing 1.0 wt.% MoS2 and 0.3 wt.% graphite, exhibited the smallest relative-density Max–Min (0.439) and the highest relative-density Min (0.558). Its comparatively low powder–die friction reduced frictional resistance to load transmission and limited local low-density development near the inner die-contact boundary, resulting in a more uniform internal relative-density distribution. At 300 mT, C3 showed the lowest Pcv values of 113.50 and 173.70 kW/m3, Pcm values of 15.54 and 23.78 W/kg, and Hc values of 134.83 and 137.40 A/m at 1000 and 1500 Hz, respectively. SEM showed relatively uniform interparticle contact across the specimen height, while TEM–EDS revealed thin and similar interfacial regions in two C3 specimens. Together with the low graphite content, these densification and interfacial characteristics were associated with the lowest core loss. Overall, C3 showed the best combined performance among the nine compositions.