Plasma-Assisted Milling of FeSiAl/Si3N4 Soft Magnetic Composites: Frequency-Tuned Interfacial Coupling For Synergistic Magnetic, Electrical, and Thermal Performance

Abstract

FeSiAl-based soft magnetic composites (SMCs) for medium- and high-frequency electromagnetic devices require high permeability, low core loss, and efficient thermal management. However, current insulation coating strategies are limited by poor coating continuity, weak interfacial adhesion, and insufficient thermal stability. In this work, FeSiAl/Si₃N₄ SMCs were fabricated via two-step plasma-assisted milling (PAM) combined with magnetic-field-assisted orientation forming, with the vibration frequency in the second PAM step used to regulate the microstructure, interfacial characteristics, and overall performance of the composite powders and compacts. The results show that Si₃N₄ was successfully introduced onto the powder surface while preserving the crystal structure of the FeSiAl matrix, thereby establishing chemically bonded interfaces involving Si–N, Si–O–N, and M–N (M = Fe, Al). First-principles calculations confirm that the FeSiAl(110)/Si₃N₄(120) heterointerface exhibits a binding energy of −5.08 J/m², with its stability originating from charge transfer and the formation of Fe–N, Al–N, and Si–N bonds. As vibration frequency increased, the continuity of the Si₃N₄ coating layer and the interlayer insulating network first improved and then deteriorated. The 1300 rpm sample exhibited an effective permeability of 99.8 at 1 MHz, with a decline of less than 2% over 1 kHz–1 MHz, together with a saturation magnetization of 165.2 emu/g, an electrical resistivity of 192.1 Ω m, a total core loss of 72.8 kW m⁻³ at 50 mT/100 kHz, and a thermal conductivity of 28.3 W m⁻¹ K⁻¹. These results indicate that regulating the PAM frequency provides an effective route for coordinating the magnetic, electrical, and thermal properties of FeSiAl-based SMCs.