EXAFS

 

 

Exafs determination of the composition and structure of transitional metal aluminides multilayer coatings

Related publications:
1) I. Arčon, M. Mozetič, A. Zalar, A. Kodre, J. Jagielski, Nucl. Instrum. Methods, B 199 (2003) 222-226
2) M. Mozetič, A. Zalar, J. Jagielski, I. Arčon, P. Panjan Surface and Interface Analysis, 34 (2002) 365-368
3) I. Arčon, M. Mozetič, A. Kodre, J. Jagielski, A. Traverse, J. Synch. Radiation 8 (2001) 493-495

Introduction

Transitional metal aluminides are of great interest owing to their industrial application. The Fe-Al solid solution, in particular, shows good wear resistance and excellent resistance to oxidation, sulphidation and corrosion [1]. Fe-Al intermetallic phases can be used as heat- and corrosion-resistant coatings for bulk materials [2,3].
Thin coatings of pure FeAl phase can be prepared in different ways including co-deposition of both materials, implantation of one ion species into the layer of the other metal, and consecutive deposition of Fe and Al thin films [4,5]. The multilayers are heated to appropriate temperature in order to obtain a uniform coating. During the deposition at room temperature some migration of Fe into Al occurs. The FeAl phase is reported to start growing at the temperature of about 200°C, but rapid nucleation and growth of FeAl is observed at the temperature of about 300°C. Between 400 and 600°C the FeAl phase is transformed into the Fe3Al lattice. At 700°C the -Fe phase becomes dominant [5].
In this paper we present a study of the phase formation in the Fe/Al multilayers by ion beam mixing at different temperatures. The composition of the coatings is determined by AES depth profiling and the atomic structure of the coating by Extended X-rays Absorption Fine Structure (EXAFS).

Fig. 1. AES depth profiles of Fe/Al samples ion beam mixed at room temperature and at 400°C.

 

Experiment

Four layers of Fe and Al were alternately sputter-deposited on well-polished silicon substrates in order to obtain Fe/Al multilayer. The respective layer thickness was 20 and 30 nm, to ensure a 1:1 atomic composition of as-deposited coating. The samples were ion beam mixed with the dose 3x10^15 ions/cm2 of Ar ions with the kinetic energy of 330 keV, at different sample temperatures between room temperature and 400°C. The composition of samples was determined by AES depth profile analysis with the scanning Auger microprobe (Physical Electronics Ind SAM 545 A).
Fe K-edge EXAFS spectra of the Fe/Al films and a reference Fe foil were recorded at the X1 station in HASYLAB at DESY (Hamburg, Germany). A Si(111) double-crystal monochromator was used with 1 eV resolution at the Fe k-edge (7112 eV) in combination with fluorescence detection technique for thin film samples. The EXAFS spectra are obtained as the ratio of the fluorescence detector signal and the signal of the incident photon beam from the ionization cell filled with nitrogen at abient pressure. Typical measuring time for each fluorescence spectrum was 3 hours. The EXAFS spectrum of 7 micron thick Fe foil was measured in a standard transmission mode.

Fig. 2. The k3 weighted Fe K-edge EXAFS spectra measured on the Fe/Al multilayer samples after ion mixing at room temperature (RT) and at 200°C, 300°C and 400°C. For comparison the spectrum of Fe metal foil is added.

 

Results and Discussion

AES depth profiles of the as deposited sample and the sample mixed at 400°C are shown in Fig. 1. Although the deposition was carried out at room temperature, the rather wide interfaces between the layers (Fig. 1a) indicated that some migration of atoms between the layers occurred prior to ion beam mixing. Ion beam mixing at room temperature does not produce substantial additional migration. However, extensive migration of atoms between the layers is observed during ion beam mixing at elevated temperatures. At 300°C some of the layered structure still persists, while at 400°C the coating becomes almost uniform (Fig 1b). It is interesting that the composition of the Fe:Al in the sample mixed at 400°C is not 1:1 as one would expect from atom ratio in as-deposited samples, but rather 0.55:0.45. Some aluminum is evidently lost - probably sputtered off during the ion beam mixing.
EXAFS analysis is performed with the University of Washington programs using FEFF6 code for ab initio calculation of scattering paths [6,7]. The basic facts about the structure can be deduced already from the Fourier transforms of the Fe K-edge spectra (Fig. 2), even before the detailed quantitative analysis is performed. We can see that in the thin film mixed at room temperature Fe exhibits pure bcc crystal structure, same as in the reference metal foil. However, with increasing substrate temperature, a gradual change from pure Fe bcc to a new Fe-Al alloy structure is observed. The mixing on the atomic scale is therefore more efficient at higher substrate temperatures as already expected from the AES depth profiles.

Fig. 3.The k3 weighted Fourier transforms of the Fe K-edge EXAFS spectra (k = 4.5 A-1 to 11.5 A-1) measured on the Fe/Al multilayer samples after ion mixing at room temperature (RT) and at 200°C, 300° C and 400°C. For comparison the spectrum of Fe metal foil is added. Solid line - experiment, dashed line – EXAFS model.

 

Quantitative EXAFS analysis is used to determine the ratio of the two phases in each coating. FEFF model of the bcc crystal structure of Fe metal is constructed from the crystallographic data (the lattice constant a = 2.87 A [8]). The Fe atom is surrounded by 8 atoms at 2.49 A, 6 at 2.87 A, 12 at 4.06 A, 24 at 4.76 A, and 8 at 4.97 A in the first five consecutive neighbor shells. The FEFF model comprises all single scattering paths from this shells and all multiple scattering paths up to 5.7 A. The model is calibrated by the Fe foil spectrum, yielding an excellent fit for the region from 1.5 to 5.0 A (Fig. 2) with just five variable parameters: the lattice expansion , the amplitude reduction factor ( = 0.81(5)), the zero-energy shift DEo, and the Debye temperature in modeling the Debye-Waller factors of all paths [6], except the first two, for which a separate factor () is introduced. The shell coordination numbers and unperturbed radii are fixed at their bcc values. Best fit values of the parameters are included in Table 1.
The room temperature Fe/Al EXAFS spectrum is completely described by the same Fe bcc model as in the Fe metal foil. Self-absorption effects of fluorescence measurements, which reduce the EXAFS amplitude by a factor of 1.7, agree well with the estimate for the given experimental setup by program code ATOMS [9].
EXAFS fit of the spectra of the FeAl coatings treated at higher temperatures show that an additional aluminum-rich phase is present beside the metallic Fe (Table 1, Fig.2). In this phase Fe atoms are coordinated to Al atoms in the first coordination shell at 2.5 A and Fe atoms in the second coordination shell at 3.1 A. Eventual further coordination shells could not be distinguished from the signal of the Fe metal phase. In Fe/Al coatings, treated at 200°C and 300° C the Fe metal phase is still prevailing (80%), while at 400 C about half of Fe atoms in the coating are incorporated in the Fe-Al phase. In this case EXAFS fit was successful only up to 4 A from the central Fe atom. The last peak in the spectrum at 4.7 A is a combination of single and multiple scattering contributions from both phases and cannot be reliably modeled. Additionally, in the Fe bcc phase of the samples ion beam mixed at higher temperatures, the Debye - Waller factors are increased, indicating stronger static disorder in the metallic Fe bcc phase.
From the structural parameters obtained by EXAFS fit it is not possible to determine if the Fe-Al phase is fully crystallized or if only sub-nano scale crystallites are formed. Likewise, there is not enough information to draw definite conclusions on the crystal structure of the observed Fe-Al phase. The obtained local structure around Fe atoms is similar but not equal to the one in the cubic FeAl crystal [11]. The presence of cubic Fe3Al phase [11] can be completely excluded.

Table 1. Parameters of the nearest neighbors up to 3 A from the central Fe atom in the Fe foil and in the Fe/Al multilayer samples after ion mixing at room temperature (RT) and at 200°C, 300°C and 400°C.: w – fraction of Fe bcc crystal phase in the sample; (Dr/r) - Fe bcc lattice expansion, R – neighbor distances in Fe-Al phase, and si2 - Debye-Waller factor for individual shells. Uncertainty of the last digit is given in parentheses.

 

Conclusions

A study of the phase formation in the Fe/Al multilayers by ion beam mixing at different temperatures was performed. AES depth profiles of the samples treated at room temperature and at 100, 200, 300 and 400C showed gradual mixing of the layers with increasing temperature. At 400°C the mixing was complete. Fe K-edge EXAFS data showed that at room temperature there was no mixing on the atomic level. At higher temperatures a new Fe-Al phase appeared. Up to 300°C the fraction of this phase was only about 20%, while at 400°C it increased to 50%. Although the AES depth profile of the sample mixed at 400°C showed a uniform coating, the mixing on the atomic level was not complete.

Acknowledgment
Support by the Slovenian Ministry of Education, Science and Sport and by Internationales Buero BMBF (Germany) is acknowledged. N. Haack of HASYLAB (Germany) provided expert advice on the X1 beamline operation.

 

 

E-mail:iztok.arcon@p-ng.si Last change: 02-Jun-2006