The Strength and Fracture Behaviour of Soft Magnetic Composites in Bending on Different Length ScalesWednesday (28.09.2016) 11:30 - 11:45 Part of:
The use of Soft Magnetic Composites (SMC) in devices for energy conversion offers high magnetic permeability, high resistivity, and thus small eddy current losses, as well as reduced weight and size of devices. These unique properties of the SMCs lead to new design options for 3D components which demand besides the magnetic properties of SMC a certain mechanical robustness of the material.
Typically, SMCs consist of large iron particles coated with a fairly thin inorganic layer. This insulating layer leads to an increase in resistivity reducing eddy current losses. The iron particles dominate in a favourable manner the magnetic properties. The combination of soft iron particles with a brittle insulating layer causes, however, a rather poor mechanical behaviour of the SMC. For instance, macroscopic samples fail in tension at stresses lower than 100 MPa in a brittle fashion whereas in compression the material can be deformed plastically at flow stress of more than 300 MPa . In macroscopic bending, the transverse rupture strength is 125 MPa .
In this work, SMCs consisting of pure iron particles coated by a thin inorganic, phosphorous layer were studied. The SMCs were annealed after compaction in order to form a strength-enhancing oxide layer at particle boundaries. Transmission Electron Microscopy images show that the resulting boundary structure can be classified in three categories according to their degree of oxidation. In order to study the influence of the boundary structure on the mechanical properties micro-cantilever bending tests were performed. Using focused ion beam preparation, micro-cantilevers, containing only one boundary, were produced and then tested using a nanoindenter. The load-displacement curves and subsequent scanning electron microscopy observations reveal the different failure behaviour for each type of boundary. The fracture morphologies observed range from “no fracture” for well-oxidized boundaries to “brittle fracture” along the boundary for non-oxidized boundaries. For the latter, the fracture toughness can be calculated and compared to macroscopic bending tests.
 H. G. Nguyen, G. D., A. Hartmaier (2013). Grenze der Einsetzbarkeit eines weichmagnetischen Pulververbundwerkstoffes aus Sicht der Mechanik. 19. Symposium Verbundwerkstoffe und Werkstoffverbunde. Karlsruhe.
 Högänas, product information, available at www.hoganas.com/globalassets/media/sharepoint-documents/BrochuresanddatasheetsAllDocuments/Somaloy_Technology_for_Electric_Motors.pdf, access date: March 2016