Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Magnetic and microstructural properties of Fe 3 O 4 -coated Fe powder soft magnetic composites Katie Jo Sunday a , Francis G. Hanejko b , Mitra L. Taheri a, ⁎ a Department of Materials Science and Engineering, Drexel University, Philadelphia, PA19104, United States b Hoeganaes Corporation, Cinnaminson, NJ08077, United States ARTICLE INFO Keywords: Soft magnetic composites Core loss Inorganic coating Magnetic properties ABSTRACT Soft magnetic composites (SMCs) comprised of ferrite-coated ferrous powder permit isotropic magnetic flux capabilities, lower core losses, and complex designs through the use of traditional powder metallurgy techniques. Current coating materials and methods are vastly limited by the nonmagnetic properties of organic and some inorganic coatings and their inability to withstand high heat treatments for proper stress relief of core powder after compaction. Ferrite-based coatings are ferrimagnetic, highly resistive, and possess high melting temperatures, thus providing adequate electrical barriers between metallic particles. In this work, iron powder was coated with Fe 3 O 4 particles via mechanical milling, then compacted and cured in an inert gas environment. We find density and coercivity to improve with increasing temperatures; however, core loss greatly increases, which is attributed to the formation of a more conductive iron-oxide phase and less resistive Fe volume. Our work begins to exemplify the unique qualities and potential for ferrite-based coatings using traditional powder metallurgy techniques and higher curing temperatures for electromagnetic devices. 1. Introduction The continuous development of SMCs is crucial to electromagnetic device potential application at high applied frequencies, nominally 400 Hz. SMCs incorporate electrically insulated particles with the capability of high magnetic permeability and low core losses depending on material selection, processing, and curing temperature. Utilizing powder material all for several worthy advantages of SMCs, which include three-dimensional magnetic flux carrying capabilities, lower core losses, and structural freedom for complex designs [1–3]. In addition, manufacturing cost and waste material can be reduced since minimal processing steps are required and subtractive methods are not necessary. The limiting factor in current SMC designs is the thermal instability of the coating material at high curing temperatures, which leads to large eddy current paths and potential failure from overheating [4]. The coating material needs to be electrically resistive, thermally stable, and compressible at low compaction pressures and iron stress- relieving temperatures (570–775 °C). Several groups have studied organic coatings such as epoxy resins or silicone polymers, which cannot withstand heating above 450 °C before they begin to degrade [5,6]. Organic coatings are thus not viable for applications requiring low coercivity because the internal strain and dislocations brought on by compaction cannot be relieved via curing below 500 °C [7–10]. Therefore, inorganic coatings (mostly oxides) are more suitable due to higher thermal stability and improved soft magnetic properties if minimal coating material is applied or ferrimagnetic material is used [11–14]. Ceramic materials have high melting points and are electri- cally insulating, corrosive and wear resistant, making them good coating selections [12]. These materials, however, are dia- or anti- ferromagnetic which decrease magnetic permeability considerably by decreasing the overall magnetic volume [15]. Therefore materials that exhibit ferro- or ferrimagnetism are optimal for soft magnetic applica- tions as core and coating materials. These two types of magnetic materials have limited frequency ranges because of their electrical resistivity, which ultimately affects the core loss and overall performance. Ferromagnetic materials (iron-, nickel-, cobalt-alloys) are applicable at low frequencies ( < 2 kHz) because of their high electrical conductivity, while ferrimagnetic materials (ceramic oxides or soft ferrites) are applicable at much higher frequencies [16]. The frequency range at which each material is applicable vastly depends on eddy current loss and thus electrical resistivity. NiZn-ferrites can be employed at UHF (ultra-high fre- quency) between 300 MHz and 3 GHz because of a high electrical resistivity (ρ) of ∼10 Ω·cm 6 at room temperature [17]. Likewise, MnZn- ferrites (ρ ∼ 10 Ω·cm 2 ) are limited to below VHF (very high frequency) between 30 and 300 MHz because of more ferrous ions present http://dx.doi.org/10.1016/j.jmmm.2016.09.024 Received 3 June 2016; Received in revised form 25 August 2016; Accepted 2 September 2016 ⁎ Corresponding author. E-mail address: mtaheri@coe.drexel.edu (M.L. Taheri). Journal of Magnetism and Magnetic Materials 423 (2017) 164–170 0304-8853/ © 2016 Elsevier B.V. All rights reserved. Available online 15 September 2016 crossmark