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AES studies of antimony segregation on the surface of a FeSiC alloy

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Page 1: AES studies of antimony segregation on the surface of a FeSiC alloy

Vacuum/volume 43/numbers 5-7/pages 449 to 45111992 0042-207x/92$5.00+.00 Printed in Great Britain @ 1992 Pergamon Press Ltd

AES studies of antimony segregation on the surface of a Fe-Si-C alloy M Jenko and F Vodopivec, Institute of Metals andAlloys, Lepipot 17, 61000 ljubljana, Slovenia

and

B PraEek, Institute for Electronics and Vacuum Techniques, Teslova 30, 67 1 I7 ljubbljana, Slovenia

Antimony segregation has been studied on the surface of a nonoriented electrical steel sheet: a polycrystalline Fe-Si-Sb-C alloy heated in ultra-high vacuum in the temperature range from 20 to 850°C. Two differently heat- treated samples were used: (i) a cold rolled strip and (ii) a vacuum annealed strip. The kinetics of segregation has been measured on surfaces, which had been cleaned previously by ion sputtering. The surface concentrations were determined with dependence on bulk concentration and temperature by AES. During the heating process Sb, C, S and P segregate consistently with their diffusivities in u-Fe. Segregation of antimony on the surface of the investigated alloy in the temperature range from 500 to 650°C proceeded with perceivable velocity and increases with increasing temperature. The segregated antimony thin film was estimated to be 0.3 nm thick.

1. Introduction

The structure and composition of surfaces and interfaces affect many material properties during production processes and their use due to the segregation of alloying elements and impurities. Some of these elements have a selective effect on the processes which start at the surface. Adsorption, corrosion, adhesion, sur- face diffusion, recrystallization etc., are strongly influenced by surface composition’~‘.

It has been shown, that small antimony addition (0.34.1%) into the silicon electrical steel, Fe&X, alloy results in sub- stantial texture improvements in a nonoriented electrical sheet and causes a decrease in core losses* I’. The possible explanation for this effect is that antimony, being a surface active element, segregates on grain boundaries or free surfaces and affects the recrystallization behaviou?.

The purpose of present study is to examine the kinetics of antimony segregation on the surface of a nonoriented electrical sheet in the temperature range from 400 to 850°C in ultra-high vacuum, using the AES surface analytical method.

The kinetics of segregated antimony layer growth on the sur- face of a nonoriented electrical sheet has been studied up to a

thickness of 0.3 nm. Antimony has segregated on the surface with perceivable

velocity in the temperature range from 500 to 650°C.

2. Experimental

The vacuum induction melted steel of composition Fe-l .87% Si, 0.21% Al, 0.3% Mn, 0.03% C, 0.002% S, 0.016% P and 0.052% Sb with minimal content of uncontrolled elements and impurities, was manufactured at our research laboratory. Ingots were hot rolled to 2.5 mm thick strips and then cold rolled with inter- mediate recrystallization annealing to a final thickness of 0.125 mm.

Auger electron spectroscopy (AES) was applied to the study of antimony surface segregation on a polycrystalline nonoriented

electrical sheet between 400 and 850°C. Two kind of samples, differently heat treated, are used : (i) a cold rolled strip and (ii) a vacuum annealed strip, both of the same dimensions of 1.5 mm x 30 mm x 0.125 mm.

In ultra-high vacuum the sample was heated resistively, with a maximum dc current of 20 A.

The thermocouple Fe-CuNi 0.1 mm in diameter, spot welded to the rear side of the sample, was used to control the tem- perature ; a constant temperature was attained at about 3-5 min after establishing a fixed heating current.

By alternative argon ion sputtering (pAr = 5 x lo- ’ mbar, EAr = 1 and 3 keV, respectively, ion current = l-10 ,uA cm-*) and annealing almost all impurities were removed. The sputtering rate was estimated on the Sb thin film with a known thickness.

The AES analysis was made with an additionally equipped Physical Electronics Scanning Auger Microprobe, SAM 545 A, using a static electron beam of 3 keV, 1 PA, 45 pm in diameter at an incidence angle of 30”.

3. Results and discussion

In Figure 1 AES spectra of a cold rolled nonoriented electrical sheet during the heating process in region from 200 to 800°C are shown. Before heating, the surface was cleaned previously by ion sputtering and all impurities, except carbon and oxygen, were removed.

At the temperatures 200 and 400°C the Auger peaks of C (272 eV),O(510eV),Fe(703eV),Al(1391 eV)andSi(1619eV)were recorded. The Sb concentration was lower than the sensitivity of AES method. Only after heating T > 500°C is the antimony enrichment found by AES measurements. It is shown that the process of antimony segregation on the surface has proceeded with perceivable velocity at 500°C and has increased with increas- ing temperature. At the temperature of 800°C there is a dimu- nition in the Sb Auger line intensity, probably provoked by surface evaporation losses.

449

Page 2: AES studies of antimony segregation on the surface of a FeSiC alloy

MJenko et al: Antimony segregation

800°C

0 (5101 1 Fe(7031

---v-+-m Al(l396) 51 (1919:

w----w+

+L-f@+* Al (13961 SI(19191

--v@- Al (1396) SI (1919)

Electron energy (ev)

Figure 1. Auger electron spectra of a cold rolled nonoriented electrical steel sheet during the heating process in region from 200 to 800’C. The process of antimony segregation on the surface has proceeded with perceivable velocity at 500°C and has increased with increasing tem- perature. At 800°C there is a dimunition in the Sb Auger line intensity, probably provoked by surface evaporation losses.

The surface concentration of oxygen during the heating pro- cess in the region from 400 to 800°C was constant. The chemical shift (-4.5 eV) for the Sb oxide state was not found and it was not found for Fe oxides.

The kinetics of Sb segregated thin film growth on the surface of a cold rolled nonoriented electrical sheet was studied by AES direct measurements, following the time dependence ratio of Sb (454 eV) and Fe (703 eV) Auger line intensities, Is,,/I,,, at

temperatures of 500, 700 and 850°C. The Sb segregation is per- ceivable at 500°C ; at 700°C the maximal ratio Ish/ZFe of 0.50 was reached and at 850°C the segregated Sb began to evaporate from the surface (Figure 2).

The concentration of elements Sb, Fe, C, 0, and S during the heating process from 20 to 850°C is shown in Figure 3.

In the second part of the experiment Sb segregation on the

Time (min)

Figure 2. The kinetics of Sb segregated thin film growth on the surface of a cold rolled nonoriented electrical sheet at the temperatures of 500, 700 and 850-C.

450

*ff$k&J& 0 200 LOO 600 800 1

Temperature (“Cl

O(

Figure 3. The concentration of elements : Sb, Fe, C. 0 and S during the heating process from 20 to 850°C in uhv.

surface of a vacuum annealed nonoriented electrical sheet was investigated. After ion sputtering and annealing all impurities, except carbon, were removed. The carbon surface concentration increased during heating in the range from 300 to 500°C. Above this temperature almost no carbon was detectable; at the tem- perature of 700°C only Sb and Fe were present, while at 800’C the P surface concentration increased significantly.

In Figure 4 the surface Sb segregation rate in the vacuum annealed nonoriented electrical sheet at 650, 700 and 800°C is shown. Sb segregates at the surface of the investigated alloy with a perceivable velocity at 650°C. The maximal antimony segregation rate was reached during the heating process at a constant temperature of 700°C. We assume that the lower Sb segregation rate at 800°C is a consequence of the antimony sur- face evaporation process.

The thickness of the segregated Sb thin film, 0.3 nm-which corresponds approximately to one monolayer of S&was esti- mated after heating the sample for 30 min at 700,C and cooling to about 150-C. The thickness of segregated Sb was estimated in the AES depth profile in Figure 5.

4. Conclusions

The surface antimony segregation was studied in the con- centration and temperature range of stability of the a-solid solu- tion. Only adsorption phases can be formed under these conditions, i.e. no three-dimensional phases and compounds’.‘.

0 5 10 15 20 25 30

Time (min)

Figure 4. The segregation rate of antimony on the surface of a vacuum annealed nonoriented electrical sheet at the temperatures of 650,700 and 85o’-c.

Page 3: AES studies of antimony segregation on the surface of a FeSiC alloy

M Jenko et al: Antimony segregation

0 0 5 10 15 20 25

Sputtering time (secl

Figure 5. AES depth profile of a Sb segregated thin film : I keV Ar+. IS mA, rastered region 5 x 5 mm.

The adsorption structure is formed in the range of a monolayer on the surface with a degree of coverage corresponding to Sb/Fe < 1.

On the surface of the laboratory nonoriented electrical steel

sheet, the antimony segregation increased in region from 500 to

700°C. The thickness of 0.3 nm of the segregated Sb thin film was estimated with AES depth profile analysis. The calculated thickness of one Sb monolayer is 0.3 nm.

Besides Sb, the segregation of C in region 300 to 500°C and P at higher temperature was found.

On the basis of the results obtained it is assumed that Sb segregation decreases the surface energy of grains emerging to the surface of the sheet and also the kinetics of the grains.

References

’ H J Grabke, ISIJ Znt, 129, 529 (1989). ‘H Viefhaus and M Rusenberg, Surfhce Sci, 159, I (1985). 3 H J Grabke, W Paulitscke, G Tauber and H Viefhaus, Surfuce Sci, 63, 377 (1977). 4H Viefhaus and H J Grabke, Surfuce Sci, 109, I (1981). ’ B Egert and G Panzner, Ph.rs Rev, B29, 2091 (1984). ‘G Panzner and W Diekman, Surface Sri, 160,257 (1985). ‘S Hofmann, Vucuum, 40,253 (1990). ‘G Lyudkovsky and P K Rastogi, Metull Truns, 15A, 257 (1984). ‘H Shimanaka, T Irie, K Matsumura and H Nakamura, J Mugn Mugnef Mawr, 19,22 (1980). ‘“P Marko, A Solyom and V FriE, J Mqn Mugnet M&v-, 41, 74 (I 984). ” F Vodopivec. F MarinSek, D Gnidovec, B PraEek and M Jenko, J Magn Magnet Maw, 97,281 (1991). IL M P Seah and W A Dench. Swfuce Interface Analysis, 1, 2 (1979).

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