Static Fracture Behavior and Mechanical Strength Fluctuation of Powder Magnetic Core

Abstract

Powder magnetic core is a densely-packed material produced by compacting soft magnetic metal powders coated with an insulating surface layer tens of nanometers thick. This material exhibits low eddy current loss due to its high electrical resistivity, and can provide opportunities for new three-dimensional designs of electrical machines because of the magnetically isotropic characteristics and complex shape forming capabilities. However, the mechanical strengths of the magnetic powder cores are usually lower by a factor of one-third or one-tenth than those of the typical Fe-Cu-C sintered materials. In this study, the mechanical strengths of three typical powder magnetic cores were examined by three-point bending test, by tensile test and by fatigue test to discuss the basic mechanical strength characteristics of the magnetic powder cores. The fractured surfaces of the specimens were observed with an optical microscope and scanning electron microscope, and the mechanical strength fluctuation was discussed by Weibull analysis. As a result, the fracture of powder magnetic cores was generated in the insulating layer on the metal powder boundary and/or at the boundary of insulating layer and the metal powder. The Weibull moduli were approximately in 30 to 45 range, which is higher than that of the conventional ceramic materials. The relationship between static strength and fatigue strength of the powder magnetic cores was similar to that of a low-density sintered metal.