Surface Characterization of Soft Magnetic Composite Powder and Compacts

• Main Author: Oikonomou, Christos
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2014
• Subjects: Materials science
• ISBN: 1392506409; 9781392506400

Soft Magnetic Composite (SMC) components produced based on traditional Powder Metallurgical (PM) techniques, are strong candidate materials for electromagnetic applications. Their advantages are based on profitable and energy efficient production methods, shape complexity realization and uniquely uniform and isotropic 3D magnetic properties. SMC powder grades consist of individually encapsulated iron powder particles with an ultra-fine, electrically insulating surface coating. Component manufacturing procedure involves compaction of the admixed SMC base powder with a lubricant to a final shape, as well as a subsequent heat-treatment that aims on the relaxation of stresses induced during the compaction. The concept of SMC is based on taking advantage of the insulating properties of the surface coating and creating a laminated stack in a powder form. In this manner, products with comparable or superior magnetic performances can be produced as opposed to the more traditional laminated steels and ferrites, due to the higher reduction of eddy currents especially at high frequency applications. The insulating coating constitutes the paramount feature of the SMC technology. Its morphology, thickness, cohesion to the powder particles surface and durability during processing, are critical aspects to the properties of a finished component. Hence, a methodology based on analytical techniques was developed and implemented on commercially available SMC base powder and finished components, in order to address these matters for such insulating coatings on a micro-level. Standard methods previously used for the powder surface characterization have been significantly modified due to the non-conductive nature of the surface coating. A theoretical model for thickness determination of the surface layer for powders, based on X-ray photoelectron spectroscopy (XPS) depth profiling, was further developed and experimentally evaluated. The theoretical results showed only minor deviations of the order of 3% with the experimental values. The novelty of the latter lies in its ability to take into account the artifacts imposed to an analysis due to the geometries of the sample and of the current experimental arrangement. The surface analysis of the SMC base powder revealed the presence of a uniform, inorganic, iron phosphate based coating with good overall coverage and coherence to the core. Its chemical depth profile was evaluated and its thickness was determined by implementing the theoretical model. Additionally, the thermal stability of the insulating coating of finished SMC components was investigated under different temperature regimes. The implementation of the previously developed methodology highlighted the difference of the heat-treatment effect on the interior and exterior regions of the components in terms of surface chemistry. In this context, increased oxidation was observed in the area close to the surface of the parts, as opposed to the center, while the analysis also showed that incomplete de-lubrication is taking place at temperatures below 500°C.