Nanomagnetic Materials Processed by Plasma Cold spraying

IC-WNDT-MI’10 Topic :3. Industrie des métaux et alliages, Technologie des métaux et alliages Oral presentation

Nanomagnetic Materials Processed by Plasma Cold spraying

N. Fenineche^1*, M. Cherigui¹, W. Li²

1 LERMPS -UTBM, Site de Sévenans, 90010 Belfort Cedex, France.

2 Shaanxi Key Laboratory of Friction Welding Technologies, NPU, Xi'an, PR China.

* Corresponding author:

Email: [email protected] Abstract

Cold spraying (CS) is a radical departure from conventional thermal spray (TS) techniques in that the deposition process relies purely on kinetic energy rather than on a combination of thermal and kinetic components [1-3]. The most advantage of this process over TS is the ability to generate dense coatings retaining initial material chemistry and phase composition with a very little oxidation. Also, low temperature process (no bulk particle melting) eliminates solidification stresses and enables thicker coatings [4]. However, hard brittle materials like ceramics can not be sprayed without using ductile binders.

In this study, magnetic alloys such as FeSiBNbCu also called Finemet and FeSiBNbCu-Al with various percentages of Aluminum coatings were synthesized using cold spray technique in order to produce ferromagnetic materials. Ultra- fine grain coatings were obtained using FINEMET nanostructured powders mixed with Aluminum as ductile binder in order to improve adherence.

Keywords: Nanomaterials, Cold spray, FINEMET, Magnetic properties, Microstructure.

1 Introduction

Nanostructured materials have been receiving increasing attention due to their impressive combination of strength and toughness, as well as magnetic properties [5,6]. They also exhibit reduced thermal conductivity and favourable oxidation resistance [7,8]. Meanwhile, the processing of nanocrystalline materials is often difficult and, thus, fairly expensive. This prevents some of their potential applications in many engineering components. However, coatings containing nanocrystalline grains can be synthesized by several deposition techniques, without incurring extra expenses or altering the attributes of the components [9-12]. Particularly, nanocrystalline coatings deposited by thermal spraying have been the subject of intense research activities during the last decade [8,13,14]. Thermal spraying has also recently been proved to be a successful technique for producing thick, near net shaped, nanostructured or ultra-fine grained deposits [15-16]; which could be of useful for some magnetic applications.

However, the powder melting during thermal spray is a major problem due to the total change of the coating microstructure [16]. In this way, the cold spray technique was used to preserve the microstructure of the initial nanostructured powder. This technique is a recent development to produce dense, oxide-free metallic and cermet coatings with properties not achievable by

established atmospheric thermal spray (TS) techniques. Little or no thermal component (i.e. high temperature) is introduced in the CS process; deposit formation relies mainly on dynamic compaction as particles impact the substrate. However, cold spray posed a problem to project the Finemet powders characterized by its high hardness. Aluminum characterized by its low hardness, was added to obtain a coating formed of an Aluminum matrix and randomly distribution of Finemet grains. Aluminum is chosen like an additive element for the following reasons: (i) Al is a nonmagnetic element, (ii) it is a low density metal which can be accelerated to very high velocities by CS; (iii) it is characterized by a low hardness, and (iv) Al powders are available commercially in a variety of compositions.

2. Experimental procedures

A commercial cold spray gun (CGT GmbH, Germany) was used. The high-pressure compressed air (2.6 MPa) was used as the accelerating gas and argon was used as a powder carrier gas and a temperature of about 490^oC.

The standoff distance from the nozzle exit to substrate surface was 20 mm. A FeSiBNbCu nanostructured powder produced by “Imply alloys - ARCELOR”

mixed with Aluminum 2319 was used as a feedstock.

Table 1 shows the chemical composition of Finemet and Al 2319. Copper plates were used as substrates

and sandblasted using alumina grits prior to spraying.

The traverse speed of the spray gun was 160 m/s relative to the substrate.

Finemet

Iron Silicon Boron Niobium Copper

73.5 13.5 9 3 1

Aluminum

Copper Aluminum each

5.5 Remainder 1.3

Table.1. Chemical composition of FeSiBNbCu and Al 2319.

Observations of the powders and coatings were done using an optical microscope (OM) (Nikon, Japan) and a scanning electron microscope (SEM). The coated samples were sectioned using a low speed diamond saw and polished prior according to the SEM investigation.

The SEM microstructure observations of the coatings presented in this paper were carried out under electron backscattering imaging conditions using JEOL JSM- 5800 LV. X-ray diffraction (XRD) traces were obtained from the powders and the coated surfaces using a SIEMENS D5000 X-ray diffractometer (λ = 0.1789 nm).

The diffraction traces were recorded at a speed of 0.05 s^-1 and magnetic measurements with a hysteresismeter Bull M 2000 SIIS apparatus at ambient temperature.

3. Results and discussion 3.1. Characteristics of powders

Four types of powder were used, FeSiBNbCu(0wt.%Al), FeSiBNbCu-10wt.%Al, FeSiBNbCu-25wt.%Al and FeSiBNbCu-60wt.%Al. Fig. 1 shows the micrography of FeSiBNbCu-60wt.%Al mixture powders. This morphology seems to be appropriate for feeding the powders using the optimized cold spraying parameters.

Fig.1. Morphology of FeSiBNbCu-60Al% powders.

The size of the small powder particle is about a few hundreds of nanometers while the biggest ones are about

50 µm in size. However, the grains are not clearly defined.

Fig. 2 shows the hysteresis loop of FeSiBNbCu- 25wt.%Al powder. One can observe a very low coercivity despite Aluminum presence which can be regarded as nonmagnetic inclusions. These inclusions act on the magnetic behavior of the Finemet alloy by forming shutting domains which contribute to stop the Bloch’s wall move and, consequently, increase the coercivity. This low value of coercivity is due then to the very low grain size of Finemet. This coercivity can be related to the increase of Aluminum content.

Indeed, ordered Fe-40Al at% is paramagnetic at room temperature. However it becomes ferromagnetic when plastically deformed [17].

Fig.2. Hysteresis loop of FeSiBNbCu-25%Al powder.

Fig.3. Coercivity of powders.

The coercivity values of the four powders are plotted in Fig. 3 as a function of Aluminum concentration.

In general, the powder coercivity increases with the amount of nonmagnetic element addition of Aluminum.

(1) Finemet particles

(2) Aluminum particles

4ππππM

-4,50E+05 -3,00E+05 -1,50E+05 0,00E+00 1,50E+05 3,00E+05 4,50E+05

-2100 -1400 -700 0 700 1400 2100 H

-4,E+02 -2,E+02 0,E+00 2,E+02 4,E+02

-0,4 -0,2 0 0,2 0,4

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8

FeSiBNbCu FeSiBNbCu- 10%Al FeSiBNbCu- 25%Al FeSiBNbCu-60%Al

Pow der type

Coercivity Hc (Oe)

3.2. Coating characterization

As it was already mentioned at the beginning, the elaboration of FeSiBNbCu coatings by cold spray was not possible due to the high hardness of Finemet.

FeSiBNbCu-25wt.%Al, was easily carried out with a great adhesion with the substrate. It is also possible to observe the dispersion of FeSiBNbCu grains in the Aluminum matrix (Fig. 4).

For the FeSiBNbCu coating, the diffractogram reveals that the majority phase has the cubic structure Fe3Si (Fig.5). This is characteristic for the low silicon content in Fe-Si based alloys. This result agrees with those obtained by HVOF thermal spraying [20], by rapidly quenched Fe100-xSix alloys [18] and mechanically alloyed FeSi powders [19].

Fig.4. SEM micrography of the FeSiBNbCu-25wt.%Al coating of transverse section showing local morphology.

1 and 2 indicate respectively two zones of gray dark and gray clear contrasts.

Comparing the structural state of the cold spray coatings with those produced by HVOF thermal spraying [20], it can be noted the presence of the same picks at a same positions (2θ) and the formation of the same phase (Fe3Si). However, the grain sizes in the case of cold spray coatings are clearly lower compared to those produced by HVOF technique.

Indeed, in the case of HVOF, the structural state is completely or partially changed due to the high temperature of the flame which is largely higher than the melting temperature of the FeSiBNbCu alloy.

Fig.5. X-ray diffraction patterns of cold sprayed coatings.

However, the cold spray temperature is less higher and much lower than the melting temperature of Finemet which allows the preservation of the structural state of alloy after spraying and consequently the preservation of the grain size of the initial powders.

Fig. 6 shows the hysteresis loop of the FeSiBNbCu- 25%Al coating with a coercivity of about 1 Oe.

However, the coercivities of other coatings were clearly increased compared to those of the powders (Fig. 7).

Fig.6. Hysteresis loop of FeSiBNbCu-25%Al coating.

Fe Si

Cu B Nb

(1) (2)

Al

Cu

1

2

75 wt.% FeSiBNbCu + 25 wt.% Aluminum

Intensity

40 wt.% FeSiBNbCu + 60 wt.% Aluminum

20 30 40 50 60 70 80 90 100 110

2 θθθθ

75 w t.% FeSiBNbCu + 25 wt.% Aluminum

Intensity

40 wt.% FeSiBNbCu + 60 wt.% Aluminum

20 30 40 50 60 70 80 90 100 110 2 θθθθ

A

l A

l Fe3S

i

Fe3S i

Fe3S i

Al

1

2

-400 -300 -200 -100 0 100 200 300 400

-2700 -1800 -900 0 900 1800 2700

H 4ππππM

-0,2 0 0,2 0,4

-2 -1 0 1 2

In the case of FeSiBNbCu-60%Al coating, the coercivity is higher compared to that of initial powder. This increase is probably due to the melting of little quantity of Aluminum. Indeed the concentration of Aluminum is very important and the temperature melting of this element is lower compared to that of Finemet. A partial fusion can occur and consequently, the melted aluminium particles will cover partially or completely the Finemet particles.

Fig.7. Coercivity of coatings. 4 Conclusion

In this study, the microstructure and magnetic properties of FeSiBNbCu and FeSiBNbCu-Al cold sprayed coatings elaborated from nanostructured powders were investigated.

(1) It has been shown that FeSiBNbCu-Al powders present a soft ferromagnetic character despite the presence of Aluminum up to 60 wt.% Al which is considered as a nonmagnetic material.

(2) The addition of Aluminum contributes to facilitate the elaboration of metallic coatings by cold spray technique.

25% of Al was considered as ideal to produce a homogenous coating with suitable magnetic properties.

Acknowledgement

We would like to thank T. WAECKERLE from Imphy alloys, ARCELOR (France), concerning the preparation of FINEMET powder and J. CRAVEN from NIPSON (France), for his assistance concerning magnetic measurements.

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0 5 10 15 20 25

FeSi BNbCu FeSi BNbCu-10%Al FeSi BNbCu-25%Al FeSi BNbCu-60%Al

Coating type

Coercivity Hc (Oe)