Powder Metallurgy Progress, Vol.12 (2012), No 3 181

MICROSTRUCTURE AND FRACTURE OF MAGNETIC COMPOSITES WITH VITROPERM ADDITION

R. Bureš, M. Fáberová, P. Kollár, J. Füzer, M. Strečková

Abstract Soft magnetic composite (SMC) is a well known group of materials for applications as cores (in transformers, electromotors and electromagnetic circuits, sensors, electromagnetic actuation devices, low frequency filters, induction field coils, magnetic seal systems and magnetic field shielding) with three dimensional isotropic ferromagnetic behaviour. The Vitroperm 800 (Fe73Cu1Nb3Si16BB

7) supplied by Vacuumschmelze was chosen as an excellent soft ferromagnetic material and Somaloy 700 as successful SMC concept brand from Höganäs. Composites based on mixtures of Vitroperm and Somaloy powders were prepared by cold die pressing at 800 MPa followed by heat treatment at 530ºC, 1 hour in Ar or air. The goal of this study was to evaluate the influence of powder morphology on magnetic properties. Vitroperm is a flaky powder containing relatively large particles d

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0.5=200 �m, but 15-20 �m in thickness. Somaloy is a mixture of irregular spheroidal particles bonded by a lubricant. Mechanical properties of Vitroperm/Somaloy composites were analysed. Composites based on mixed powder morphology lead to a specific fracture morphology. The values of Young‘s modulus and bend strength were correlated with microstructures and morphologies of the fracture surfaces. Mechanical properties were compared with other physical characteristics of the composites. Mechanical properties of Somaloy-Vitroperm composite are higher after heat treatment in air in comparison to heat treatment in Ar. Fractographic investigation was used to explain this beahviour. Keywords: soft magnetic composites, powder metallurgy, powder morphology

INTRODUCTION Soft magnetic composites (thereinafter SMC) prepared using powder metallurgy,

are particle composites in which ferromagnetic particles are coated with an electric insulator. Pure metallic particles of Fe, Co, Ni and their alloys, magnetic alloys FeSi or FeP, are used as ferromagnetic cores. The more advanced soft magnetic materials of core particles are amorphous and nanocrystalline materials as Vitroperm or Permalloys [1,2,3]. Insulation is based on inorganic (SiO2, Al2O3, MgO), organics (thermoplastic or thermosetting resin) or hybrid inorganic-organic composite layer [4]. The advantages of SMC are 3D isotropy of magnetic properties, high resistivity and low eddy current losses. Applications of SMC are in the fields of high speed, high frequencies, miniaturization and

Radovan Bureš, Mária Fáberová, Magdaléna Strečková, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic Peter Kollár, Ján Füzer, Institute of Physics, Faculty of Science, Pavol Jozef Šafárik University in Košice, Košice, Slovak Republic

Powder Metallurgy Progress, Vol.12 (2012), No 3 182 low power electrotechnics. Hysteresis losses, maximum flux density and mechanical properties are reasons for intensive research of SMC [5]. The motivation of this work is preparation and characterization of the model material to study magnetic properties of powder composites. The goal of the presented work is to contribute to explanation of microstructure dependence of mechanical properties of SMC based on mixed morphology of the core particles.

MATERIALS AND METHODS Vitroperm 800 powder (thereinafter VPM) supplied by Vacuumschmelze

Germany was used. Commercial premixed powder system Somaloy®700 was supplied by Höganäs AB Sweden.

Size distribution of powder particles is presented in Table 1. Vitroperm particles have flat and flaky morphology as shown in Fig.1. Somaloy powder morphology is based on aggregates of iron particles and wax based binder.

Tab.1. Particle size distribution.

Powder/Particle size

+425 μm [%]

212-425 μm [%]

150-212 μm [%]

75-150 μm [%]

-75 μm [%]

Vitroperm 800 0.2 35 29 31.7 4.1 Somaloy 700 0 39 29 29 3

(a) (b)

Fig.1. Morphology of (a) Vitroperm 800 powder; (b) Somaloy 700.

Somaloy powder was mixed with 10, 20, 30 and 50 wt.% of VPM in a Turbula mixer for 20 minutes. Mixtures were uniaxially pressed at 800 MPa to the required shapes. Cylindrical samples with 10 mm of diameter and 3 mm of height were prepared for measurement of resistivity and evaluation of microstructure. Bar samples of 4x5x20 mm size were used for measurement of mechanical properties. Green compacts were heat treated at 530ºC for 1 hour in air and in a protective atmosphere of Ar. Pure Somaloy samples were heat treated in Ar and in air, as well.

Microstructure of composites was analysed using light optical microscope (LOM) Olympus GX-71. Scanning electron microscope (SEM) Jeol 7000F was used for analysis of powder morphology and fractographic documentation. Measurement of Young’s modulus was performed using Buzz-o-Sonic system for non destructive testing by impulse excitation technique according to ASTM E1876. Bend strength was tested by 3 point bending on the universal testing machine Tiratest 2100. He pycnometer AccuPyc II 1340 was used for density measurements.

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RESULT AND DISCUSSION Composites based on mixture of Somaloy and Vitroperm powders were

investigated. Mechanical properties of the composites heat treated in Ar atmosphere and in air are documented in Figs.2-4. Transverse rupture strength (TRS) of composite heat treated in Ar decreased with increased VPM content as well as Young’s modulus (E). Mechanical properties of composites heat treated in air are several time higher than in the case of composite after heat treatment in Ar. Somaloy composite heat treated in air shows more than 2 times higher TRS and 3 times higher E than Somaloy heat treated in Ar. The most significant differences between heat treatment in Ar and air were achieved in the case of Somaloy with 10 wt.% of Vitroperm (S10VPM). Results of density measurements show that density of composite decreasing with increase of VPM content in both cases of heat treatment. Decrease of mechanical properties does not fit to dependence on density. Microstructure of composites was investigated to explain the changes of mechanical properties in dependence on VPM content. Figure 5 shows that microstructure observed using LOM is related to the density of composites.

Fig.2. Elastic modulus of composites. Fig.3. Transverse rupture strength.

Fig.4. Measured density of prepared composites (ρSomaloy = 7.14 g/cm3, ρVitroperm =

7.20 g/cm3).

SEM observations of fracture surfaces, presented in Fig.6, show the differences in morphology of the fracture after 3 point bend test. Detailed observation of fracture surfaces at higher magnification, Fig.7, reveals the reason for differences in mechanical properties between samples after heat treatment in Ar and in air. Interface connections created during heat treatment in air are responsible for improvement of mechanical properties of the composite. Heat treatment in air leads to formation of oxide phases with connection to phosphate phase of Somaloy. Oxide-phosphate phases create the bridge between Somaloy and Vitroperm particles. These processes are not relevant during heat treatment in neutral atmosphere of Ar.

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S10VPM, air S20VPM, air S30VPM, air

S10VPM, Ar S20VPM, Ar S30VPM, Ar

Fig.5. Microstructure of Somaloy-Vitroperm composites, LOM.

S10VPM, air S10VPM, Ar

S20VPM, air S20VPM, Ar

S30VPM, air S30VPM, Ar

Fig.6. Fracture surface of Somaloy-Vitroperm composites, SEM.

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S10VPM, air S10VPM, Ar

S20VPM, air S20VPM, Ar

S30VPM, air S30VPM, Ar

Somaloy, air Somaloy, Ar

Fig.7. Detail observation of fracture surface of Somaloy-Vitroperm composites, SEM.

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CONCLUSIONS The composites based on Somaloy and Vitroperm powders were prepared using

heat treatment in air and in neutral atmosphere of Ar. Basic mechanical properties of Somaloy-Vitroperm composite were measured and the following results were obtained: • The values of the mechanical properties of Somaloy-Vitroperm composite are higher

after heat treatment in air in comparison to composite heat treated in Ar. • Transverse rupture strength decreases with increase of Vitroperm content.

Fractographic evaluation identified the reasons of different mechanical properties: • Higher content of Vitroperm leads to lower density of composite. • Heat treatment in Ar produces higher porosity and voids among Somaloy and Vitroperm

particles. • Interparticle connection, as well as mechanical strength, are based on oxide phases. Low

content or missing oxides at interphases Somaloy-Vitroperm leads to lower mechanical properties of composites.

Mixed morphology of the source particles is not a barrier to densification during compaction of Somaloy-Vitroperm mixtures. Density improvement could be achieved in two ways: • Warm compaction of the mixture and short time of the heat treatment in Ar. • Cold compaction following heat treatment in air.

Somaloy-Vitroperm composite with sufficient level of density is a good model material for study of magnetic properties of soft magnetic composites.

Acknowledgement This work was supported by the Slovak Research and Development Agency under

the contract No. APVV-0222-10 MAGCOMP, VEGA 2/0155/12.

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