On Surface Characteristics and Microstructural Development of Soft Magnetic Composite Powder and Components

Sammanfattning: Soft Magnetic Composite (SMC) products manufactured by traditional Powder Metallurgical (PM) techniques, are strong candidate materials for electromagnetic applications. Their advantages are based on cost and energy efficient production methods, shape complexity realization and uniquely uniform and isotropic three dimensional (3D) magnetic properties. SMC powder grades consist of individually encapsulated iron powder particles with an ultra-thin, electrically insulating surface coating. Component manufacturing procedure involves compaction of the admixed SMC base powder with a lubricant substance to a final shape, as well as a subsequent heat-treatment that aims at the relaxation of stresses induced during the compaction. The concept of SMC utilizes the insulating properties of the surface coating by creating a 3D laminated stack in a powder form, which essentially aims in a higher reduction of the total core losses of the application. In this manner, products with comparable or superior magnetic performances can be produced as opposed to the more traditional laminated steels and ferrites, especially for high frequency applications. The electrical insulating coating, as well as the internal microstructure of the matrix powdered material, constitute two of the most important factors that control the final performance of an SMC product. Their state and its development through processing is crucial for tailoring the material to the desired properties. In the case of the coating, its morphology, thickness, cohesion to the powder surface and durability during compaction and annealing steps are all important aspects that contribute to the improvement of its functionality. Moreover, the degree of deformation, recrystallization and grain boundary interface related to the base material after treatment, are also heavily linked to its magnetic behavior. In the present study, methodologies along with theoretical modeling were developed initially based on analytical techniques, in order to assess and relate the aforementioned material properties to processing parameters such as compaction pressure, annealing temperature and composition of process atmosphere. For such purposes, both loose SMC base powder as well as compacted components were investigated before and after processing treatments in various conditions. The results of this thesis address the nature of the insulating coating, in terms of chemical composition and microstructure, while pointing out the underlying mechanism responsible for its stability under heat treatment. Furthermore, the development of the deformation state and internal microstructure of the SMC processed components was assessed in respect to their processing parameters and correlated to their measured core loses. These findings are important contributions, especially from an industrial point of view, in achieving optimal properties for such composite materials as well as designing an efficient processing route. The methods and knowledge developed here can also prove to be of interest for a wider range of PM concepts.