Surface Chemical Analysis of Soft Magnetic Composite Podwers

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 encapsulated iron powder particles with an ultra-fine, electrically insulating surface coating. Component manufacturing procedure involves compaction of the admixed SMC 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 dielectric properties of the surface coating and creating a laminated stack in a powder form. In this manner, products with 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. It is obvious that the insulating coating is the paramount feature of the SMC technology. Its morphology, thickness, cohesion to the powder particle and durability during compaction and heat-treatment, are critical for the properties of a finished component. Hence, a methodology based on surface sensitive analytical techniques was developed in order to address these matters on a micro-level for such insulating coatings. Standard methods previously used for the powder surface characterization have been significantly modified due to the non-conductive nature of the surface coating, which led to charging during analysis. Charge compensation was confronted by adding slow electrons and software correction. Additionally two commonly used sample preparation techniques were implemented and compared. Surface chemical analysis and coating’s morphology characterisation were performed on commercially available water atomized SMC powder grades that were coated with a phosphate based inorganic glass. High resolution imaging combined with spectroscopic techniques and depth profiling (HR-SEM, EDX and XPS) provided complementary results for the coating characterisation. The chemical states of the main constituent elements of the coating, namely oxygen (O1s), iron (Fe2p) and phosphorous (P2p), were determined. Furthermore, two approaches to evaluate the coating thickness were followed based on the relationship between the normalized intensity of the iron metal peak and the etch depth where phosphorus and oxygen intensities reach half the difference of their maximum and minimum values. The results indicated the thickness of the coating to have an average value of approximately 30 nm. Moreover, HR SEM imaging as well as EDX analysis conditions for optimum analysis was established using low acceleration voltage (~5 kV) and inLens detector for secondary electron imaging. Results revealed a uniform coating morphology without the presence of any artifacts with good overall coverage and coherence to the iron core