Characterization of electrically insulating coatings for soft magnetic composite materials by means of surface sensitive analytical techniques

Abstract

Powder metallurgy (PM) is a cost effective and efficient process for producing net-shaped parts of high tolerances and homogeneous properties in large production volumes. These are few of the reasons why this technique is preferred in manufacturing soft magnetic composite (SMC) materials for electromagnetic applications. The concept of SMC consists of encapsulating pure iron particles with an electrically insulating coating which yields compacts of high bulk resistivity and isotropic magnetic behaviour. Such materials provide great freedom in design and exhibit improved magnetic performance compared to more traditional laminated steels and ferrites for a wide range of frequency applications. In the present thesis, surface characterization of such type of composite powder in as-received state was performed using surface sensitive analytical techniques. Information regarding the nature of the surface layer is important in designing the process and tailoring the properties of a product towards a desired application. The material under examination was commercially available phosphated iron grade which was analyzed using X-ray photoelectron spectroscopy (XPS) and high resolution scanning electron microscopy (HR SEM) coupled with energy dispersive X-ray analysis (EDX). Based on these techniques, thickness estimation, morphology, coverage, chemical state and compositional depth profile analysis of the insulating coating was conducted. The aim of this investigation was to evaluate the effect of the surface characteristics and geometry of powder on such type of analyses. For this purpose, gas and water atomized high purity iron grades, having spherical and irregular shape respectively, were used. Both kinds of powder were covered with uniform oxides layers of different thicknesses and they were both divided into different particle size fractions. Furthermore, standards and flat samples of similar composition to the metal powders were prepared and evaluated accordingly using additional thermoanalytical and X-ray diffraction (XRD) techniques in order to acquire reference values. The results showed that the as-received state appears to have a greater contribution of oxide thickness than when it is fractioned. This is supposed to be a result of the fact that the unsieved powder has a significant difference in the particle size and this can cause more shading effects during the XPS analysis.Moreover, the SMC powder exhibited a complex surface layer structure consisting of a mixture of iron oxide and iron phosphate compounds of the order of few nanometres.