Development of Iron-Based Soft Magnetic Composites with Novel Coating Materials
Soft Magnetic Composites (SMCs) are an exciting class of material, with the potential to replace steel laminations as the magnetic core material used in electric motors. The main advantage of SMCs is their isotropic magnetic properties, allowing for radically different design concepts as compared to the 2-dimensional magnetic properties of steel laminations. If this key characteristic can be effectively exploited, SMC materials may provide a step improvement in the performance of electric machines, progressing the global shift towards the electricifaction of transportation methods.
SMC materials consist of a base magnetic powder with an electrically resistive coating. The coating acts to contain eddy currents within each individual powder particle, greatly reducing core losses at increased frequencies. The coated powder is compacted into a solid using traditional powder metallurgy techniques. However, this compaction process induces microstresses into the base magnetic powder which are deleterious to magnetic performance. In order to remove these stresses, the component is subjected to a post compaction heat treatment process. Current coating systems degrade below the annealing temperature of iron, and therefore full stress relief is not achieved. An ideal coating system would be thermally stable above the annealing temperature of iron, as well as being able to form a thin, uniform layer on the surface of the base magnetic powder.
In this work, existing commercially available SMCs were studied to understand the key heat treatment mechanisms. Ancorlam - the flagship magnetic powder produced by GKN Hoeganes - was found to have reduced mechanical strength at high density, where magnetic properties are optimal. At such high densities, there is insufficient porosity for effective thermal oxidation bonding, the mechanism responsible for the increase in strength during heat treatment. A full understanding of the thermal oxidation bonding process was attained and used to increase the cured strength of Ancorlam.
This understanding of current SMCs and their heat treatment mechanisms was used to develop novel coating systems for use with iron-based SMCs. Two separate classes of coating material were developed each with indivudal merits over commercially available Ancorlam; liquid glass-based and silane based coatings. Liquid glass-based coatings produced superior mechanical properties whilst silane-based coatings produced superior magnetic properties. These new coating systems provide key steps in the development of SMCs towards being able to offer greater power density and ultimately enable new applications for electric motors.