- Email: [email protected]
GIXS studies of thin oxide films formed on FeCr alloys
......
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applied
surface science ELSEVIER
Applied Surface Science 103 (1996) 55-61
XPS/GIXS
studies of thin oxide films formed on Fe-Cr alloys
T. Kosaka a3*, S. Suzuki a, H. Inoue a, M. Saito a, Y. Waseda a, E. Matsubara b ’ Institute of Admnced Materials Processing, Tohoku University, Sendui 980.77, Japan b Graduate School, Kyoto Unic,ersi&, Kyoto 606, Japan Received 9 November
1995; accepted 27 November
1995
Abstract X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) have been used for characterizing thin oxide films formed on Fe- 10 _ 90mass%Cr alloys by heating at 873 K in air. The XPS depth profiles indicate that the oxide film of Fe-lOmass%Cr alloy consists of mainly an iron oxide, and a chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50mass%. In Fe-30mass%Cr alloy, the oxide film consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in the inner layer. The thickness of the oxide films appears to be insensitive to the bulk chromium concentration in the range between 30 and 90 mass% under the present oxidation condition. The GIXS results identify the main crystallographic structure of the oxide film with corundum (Fe,O, or Cr,O,) type structure, although it includes the preferential orientation and depends on the bulk chromium concentration.
1. Introduction Addition of chromium to iron or steels is very effective in improving heat-resistance and and therefore oxidation of corrosion-resistance, iron-chromium alloys have been widely investigated by surface analytical methods such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) [l-5]. In these works, changes in thickness and compositional distribution in oxide films formed on the surface were investigated for discussing the role of chromium or other alloying elements in oxidation-resistance. On the other hand, the layered structure of thick over a few micrometers
* Corresponding 217 521 I.
author. Tel.: +81 22 217 5168; fax: +81
0169-4332/96/$15.00 Copyright SSDI 0169-4332(95)00596-X
0 1996 Published
22
oxides formed on Fe-Cr alloys due to high temperature oxidation has also been investigated by analyzing their cross section using electron microprobe analysis [6,7]. In spite of further progress in surface analytical methods such as low energy electron diffraction (LEED) and reflective high energy electron diffraction (RHEED), information on the crystallographic structure of oxide films for Fe-Cr alloys still far from complete, because these methods are applicable to the structural analysis of thin films formed or deposited on single crystal samples. Saito et al. [8] have recently developed in-house grazing incidence X-ray scattering (GIXS) method, which utilizes total reflection of X-rays on the surface of materials occurred at small incidence angles less than the critical angle. This method enables us to give information on the depth profile of the surface crystallographic structure, since X-ray penetration depth changes from
by Elsevier Science B.V. All rights reserved
56
T. Kosaka et al./Applied
&face
several hundred nanometers to a few nanometers by varying the incidence angle. Using this method, structural features of oxide films formed on stainless steels [8] were provided from both X-ray reflection and diffraction curves. This prompts us to carry out systematic studies on the structure of thin oxide film grown on Fe-Cr alloys. The purpose of this work is to describe the compositional and structural data of thin oxide films formed on five Fe-Cr alloys obtained by XPS with argon ion sputtering and GIXS in order to clarify the growth mechanism of oxide films in this alloy system.
2. Experimental 2.1. Sample preparation Iron-chromium binary alloys with the chromium concentration of 10.0, 30.1, 50.7, 70.0 and 89.0 mass% were prepared by vacuum melting. They are referred to as Fe-lO%Cr, Fe-30%Cr, Fe-SO%Cr, Fe-70%Cr, Fe-90%Cr, respectively. Samples for XPS were shaped to plates of 3 mm thickness and 10 mm square, and samples for GIXS were plates of 3 mm thickness and 20 mm square. They were polished with emery papers in steps down to No. 4000 and with buffing with 1 pm alumina to obtain flat and smooth surface. The samples were heated at 873 K in air for 300 s or 3000 s for oxidation except Fe-lO%Cr, and then cooled to room temperature. 2.2. Measurement XPS analysis was carried out by using an apparatus of PHI Model 5600, in which monochromated Al Ka radiation was used. The angle between the axis of input lens of the analyzer and the surface normal was 45”. In order to estimate the thickness of the oxide layers and the elemental distribution in the oxide layers, XPS depth profiles of Fe 2p, Cr 2p, 0 1s and C Is were measured by following argon ion sputtering. The argon ion gun was operated with 3 kV and ion current was about 9 nA. The ion beam of about 1 mm diameter was rastered over an area of about 4 mm square, and the area of 0.8 mm diameter was analyzed. The thicknesses of the oxide layers
Science 103 (1996155-61 DifTractedBeam
4:
:?FY+z+__
(a)
Fig.
(b)
I (a) GIXS geometry and (b) Seemann-Bohlin
geometry.
were estimated by taking the oxide/metal interface as the depth reaching a half of the oxygen concentration in the oxide layer. The sputtering depth was estimated from the sputtering rate for films of iron oxide (Fe,O,) and chromium oxide (Cr,O,) of about 100 nm thickness deposited on Si wafer, of which the sputtering rates were determined to be 0.023 rim/s and 0.038 rim/s under the present condition, respectively. GIXS apparatus consists of an 18 kW rotating anode X-ray generator, a flat Ge(l11) single crystal monochromator and crossed double-axis diffractometers. In this measurement, MO-target was used for reducing the fluorescent X-rays from samples. Surface diffraction profiles of an oxide film on the samples were measured with the GIXS geometry (Fig. l(a)), where the incidence angle (Y and the take-off angle cr’ are selected so as to keep a value near the critical angle and the direction of scattering vector almost parallel to the sample surface. Therefore, the diffraction intensities from a crystallographic plane almost perpendicular to the sample surface can be detected. In order to determine the crystallographic orientation of the oxide film, measurements with the Seemann-Bohlin geometry (Fig. l(b)) were also carried out.
3. Results and discussion 3.1. XPS depth profiles Fig. 2(a) shows the XPS depth profile of FelO%Cr heated at 873 K for 60 s. This alloy is shortly oxidized, since its oxide film grows easily compared with other higher chromium alloys. Fig. 2(b)-(e) give the XPS depth profiles of Fe-30%Cr, 50%Cr, 70%Cr and 90%Cr heated at 873 K for 300 s,
51
T. Kosaka et al. /Applied Surface Science 103 (1996155-61
-.0
Cr in the oxide films for these two cases are mainly allocated to Fe3+ and Cr3+ [9]. A small amount of Fe*+ component is detected and this may be attributed to reduction of the oxides induced by argon ion sputtering. The thickness of the oxide films formed on the sample surface is plotted as a function of bulk chromium concentration in Fig. 5. Again the result for Fe-IO%Cr is only given for the thickness of the oxide film formed by heating for 1 min. These results indicate that the thickness of the oxide layers is almost unchanged in Fe-Cr alloys with chromium more than 30 mass%, although it is known to be reduced by the addition of chromium up to 20 mass%. It is interesting to note that the similar concentration independence of the thickness of the oxide films on Fe-Cr alloys has been observed in the oxidation at room temperature by using angle-resolved XPS [lo]. Then, it may safely be suggested that the oxidation of Fe-Cr binary alloys proceeds non-linearly against the chromium concentration both at room temperature and higher temperatures.
- -. 500
1000
1500
Sputtering Time(s) Fig. 2. The depth concentration profiles of thin oxide films formed on Fe-G alloys by heating at 873 K for 300 s. (a) Fe-lO%Cr heated at 873 K for 1 min only in this case, (b) Fe-30%Cr, (c) Fe-508Cr. (d) Fe-70%Cr, (e) Fe-90%Cr.
5” -_
_________._ ..--___.. ____ ..____.----<-
,y” CC ,,,,\ ,.____._______-/__.______/ I --.-_. 0
I&
respectively. Similar depth profiles for these four Fe-Cr alloys heated at 873 K for 3000 s were also obtained and the results are summarized in Fig. 3(a)-(d). In Fe-30%Cr heated for 300 s, the oxide film consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in the inner layer. On the other hand, a chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50% and it is almost independent of heating time. The similar effect of the chromium concentration on the layered structure has also been observed for relatively thick oxide formed on iron alloys with chromium less than 20% [6,7]. Fig. 4(a) and (b) show Fe 2p and Cr 2p XPS spectra from the oxide film formed on Fe-30%Cr heated at 873 K for 3000 s, respectively. The measurements were carried out prior sputtering and after sputtering for 480 s. The chemical states of Fe and
,_-+_--
1
(b)
”
”
I
Cr ---”
‘_ 1
-I
L
t .=
1
-
,111
e z
___-_--__A
--------
2 5o ___..___ ...____ ___.__......._
s IL-i 5”
_---
1000
2000
Sputtering Time(s) Fig. 3. The depth concentration profiles of thin oxide films formed on Fe-Cr alloys by heating at 873 K for 3ooO s. (a) Fe-30%Cr, (b) Fe-SO%Cr. (c) Fe-7O%Cr, (d) Fe-90%Cr.
T. Kosaka et al. /Applied Surface Science IO3 (1996) 55-61
58
Cd)
r\ 600
Binding Energy (eV)
w
Binding Energy (eV)
n
Fig. 4. The (a) Fe 2p and (b) Cr 3p XPS spectra from oxide film formed on Fe-30%Cr heated at 873 K for 3000 s (above: before sputtering, below: sputter for 480 s).
It should also be noted, as shown in Fig. 2 and Fig. 3, that the oxide/metal interface is not flat enough. The concentration broadening at the oxide/metal interface may be tentatively estimated using a method for describing the depth resolution definition, in which the broadening is taken between 84 and 16% of signals in a profile [I I]. For this reason, the error bars in Fig. 4 denote the interface roughness corresponding to the sputtering interval between 84% and 16% of the oxide concentration in the oxide films. The present authors maintain the view that the roughness of the oxide/metal interface is attributed to the formation of polycrystalline oxides, inducing some fluctuation of the interface. Although argon ion sputtering during depth profiling appears to produce roughness in the flat surface even if the composition is kept step-like at the interface,
I
loo-
, 0
SO-
z +
60-
1 e
40- -
,
,
,
60.3@-lO%Cr
0
300s
0
3000s
1
d
I
,
I
f-----.
(b)
10
IS
30
Fig. 6. The diffracted intensity patterns of thin oxide films formed on Fe-Cr alloys heated at 873 K for 300 s measured by the GIXS geometry. (a) Fe-lO%Cr, (b) Fe-30%Cr, (c) Fe40%Cr. (d) Fe-70%Cr and (e) Fe-90%Cr.
insignificance contribution of such factor is confirmed by obtaining the depth profiles of reference films of iron oxide and chromium oxide deposited on Si wafer. 3.2. GIXS
only)
4
Fig. 6 and Fig. 7 show the diffracted intensity of five Fe-Cr alloys heated at 873 K for 300 and 3000 s, respectively. They were measured by GIXS with the incidence angle of 0.16”. When the incidence angle of MO K cy radiation is fixed at 0.16”, the e ~ ’ penetration depth, T(n), is calculated to be 88 nm for Cr,O, based on the following equation [12]. T( fY) = A/47rq, q=-+{~(a.a1)+4p’
Fig. 5. The thickness of the oxide films estimated from the XPS depth profiles as a function of the bulk chromium concentration. Solid circles: samples heated for 300 s. and open circles: samples heated for 3000 s.
(1) -(&a,?),
(2)
where A is the wave length of incident X-rays, (Y is the incidence angle, (Y, the critical angle, and p is
T. Kosaka et al./Applied
110 l
200
10
15
20
25
59
Surface Science 103 (1996155-61
30
cP(degN
Fig. 7. The diffracted intensity patterns of thin oxide films formed on Fe-0 alloys heated at 873 K for 3000 s are measured by the GIXS geometry. (a) Fe-lO%Cr, (b) Fe-30%Cr. (c) Fe-SO%Cr, (d) Fe-70%Cr and fe) Fe-90%Cr.
the absorptive term of the refractive index. Then, the results illustrated in Fig. 6 and Fig. 7 provide the structural information in the range of 90 N 100 nm from the surface of each sample. The peaks observed in the results of Fig. 6 and Fig. 7 identify the oxide films with corundum type (Fe,O, or Cr,O,) structure as well as b.c.c. structure (ferritic phase) of Fe-Cr matrix. It is difficult to distinguish the diffracted peaks between Fe,O, and Cr,O, due to the limited angular resolution. Nevertheless, these results are consistent with the XPS spectra showing Fe3+ and Cr3+. In Fig. 7, five peaks or more are detected in the diffracted intensity pattern of the oxide film formed on alloy surface by heating at 873 K for 3000 s and they are coincident with positions suggesting similar behavior is also detected in Fig. 6 for alloy heated at 873 K for 300 s, although the variation is rather blurred. As the chromium concentration increases, the intensity of some of peaks is reduced and only a few peaks are clearly observed in alloys with chromium more than 50 mass%. Such variation can be interpreted by
considering that crystalline oxide film formed on alloys with chromium more than 50 mass% includes, more or less, the preferential crystallographic orientation. In order to investigate the preferential crystallographic orientation of the oxide film, the intensity pattern of the oxide film formed on the Fe-70%Cr alloy surface heated at 873 K for 3000 s is measured by the Seemann-Bohlin geometry (Fig. l(b)) at the same incidence angle of 0.16“. The result given in Fig. 8, together with that of the GIXS geometry. In the GIXS geometry, the 104 peak is not detected but the 110 peak is clearly observed. The opposite variation is found in the Seemann-Bohlin geometry. Since the structural information of the three dimensional crystal lattice inclined against the sample surface can be detected by the Seemann-Bohlin geometry, the 110 peak is expected to be observed if the oxide film is grown without the crystallographic orientation. On the other hand, only the reflections of the 110 and 300 plane, which are perpendicular to the sample surface, are observed in GIXS geometry as shown in Fig. 7 and Fig. 8. Consequently, these results imply that the crystalline oxide film formed on this alloy surface is described by the preferential orientation of [OOl] (Fig. 9). These results suggest that the growth direction of the oxide film is not dominated by the epitaxy between the oxide film and the matrix. It is rather coincident with the oxidation direction. The thickness of the oxide films can also be estimated from the angular dependence of the
I
’
11
0
11
II
‘I
1
’
GIXSgeometry
_
-
-
------. S.B. geometry
15
110
16
17
$NkwN Fig. 8. The intensity pattern of the oxide film formed on the Fe-70%Cr alloy surface heated at 873 K for 3ooO s measured by the Seemann-Bohlin geometry and together with that of the GIXS geometry.
T. Kosaka et al. /Applied &&ace Science 103 (1996) 55-61
60
Slde Vlew
The unit cell of the oxide film a b
Top View
Fig. 9. Schematic on Fe-Cr alloys.
diagrams
of the structure of oxide film formed
diffracted peak as reported previously [8,13]. Solid circles in Fig. 10 show the angular dependence of the (110) peak intensity for the corundum structure of the oxide film formed on the Fe-SO%Cr by heating at 873 K for 3000 s. On the other hand, the theoretical variation in intensity for a film of thickness, t,, with the concentration, C, is calculated by the following equations.
I( a) a G( cx)IEi( a)ll;:“Ccxp
- 6 i
i
dr
(3)
where
l.g( a)1 =
0
4a
(a+py+q?
0.1
Incidence
0.2
0.3
angle(degree)
Fig. 10. The angular dependence of the I 10 peak intensity for the corundum structure of the oxide film formed on the Fe-50%Cr alloy by heating at 873 K for 3000 s.
Fig. I I. (a) The thickness, to, and (b) the effective linear absorption coefficient, I*, of the oxide films estimated from the GIXS data as a function of the bulk chromium concentration.
and p=$/(a’af)i4P2
+(a?+)
(5)
where E, is the electric field at the surface and G(a) the geometrical factor. When we assume that the film is of Cr,O, and the oxide/metal interface is perfectly sharp, the values of thickness and effective linear absorption coefficient of the oxide film can be estimated from comparison with the experimental data using the nonlinear least-squares variational method, as exemplified by the results of Fig. 10 as an example. The resultant values for the oxide films for alloys heated at 873 K for 3000 s versus the chromium concentration are given in Fig. 1 l(a) and (b). It is rather stressed here that the values of thickness estimated from the GIXS method are small compared to those obtained from the XPS depth profiles by a factor of more than five, and the effective linear absorption coefficients obtained in this method are considerably larger than 142.7 cm-’ for the bulk of Cr,O,. A part of the reason for such discrepancy results from the assumption that the oxide/metal interface is perfectly sharp. The homogeneous film samples are known to show the characteristic interface oscillations in their reflection curves [8]. However, such interference oscillations are not detected in the present case, suggesting that the oxide/metal interface is not so sharp when oxidizing Fe-Cr alloys by heating in air. It is, therefore, concluded that the oxide thickness obtained by GIXS
T. Kosaka et al. /Applied Suqtace Science 103 (1996) 55-61
may be underestimated. Small crystalline oxide grains are likely to be nucleated on surface at the initial stage of thermal oxidation, whereas amorphous oxide films are formed mainly by anodic reaction at room temperature [8]. The thermal oxidation appears to cause the surface roughness and make its scattering intensity reduced and this results in best fit only when the smaller values of thickness are given. It is difficult to predict the depth dependence of peak intensity above the first maximum due to the effects such as texture in the sample and variations in incident intensity from the sample [14]. Then, further theoretical effort so as to include the interface roughness is need, but that is outside the scope of the present work. Accordingly, the thickness of oxide film estimated in this work is, in the present author’s view, rather acceptable the value of XPS than that of GIXS.
61
(3) The thickness of the oxide films was estimated from the angular dependence of the intensity of the 110 diffracted peak in the GIXS measurements, assuming the oxide/metal interface is perfectly sharp. Similar variation versus the chromium concentration is found, but the GIXS results are small compared with those obtained from the XPS depth profiles by a factor of more than five. Such discrepancy may be improved by taking account of the roughness of the oxide/metal interface.
Acknowledgements The authors are grateful to Mr. T. Sato and Ms. F. Yasuda for their help in operation of XPS apparatus.
References 4. Conclusion XPS and GIXS have been used for obtaining the structural information of thin oxide films formed on Fe-10 h 90mass%Cr alloys by heating at 873 K for 300 s and 3000 s in air. The results are summarized as follows: (1) The oxide film formed on Fe-30mass%Cr alloy surface consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in inner layer. A chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50 mass%. The thickness of oxide films on these alloys are independent of the bulk chromium concentration more than 30 mass% under the present oxidation condition. (2) It is found from the GIXS results that the oxide films are identified by the corundum type (Fe,O, or Cr203) structure within the range of about 100 nm from the sample surface. These oxide films formed on Fe-Cr alloys grow with the preferential orientation, particularly in the initial stage of oxidation.
[I] G. Betz, G.K. Wehner, L. Toth and A. Joshi, J. Appl. Phys. 45 (1974) 5312. [2] J.C. Langevoort, I. Sutherland, L.J. Hanekamp and P.J. Gelling& Appl. Surf. Sci. 28 (1987) 167. 131 D.F. Mitchell and M.J. Graham. Surf. Interf. Anal, 10 (1987) 259. [4] G. Lorang, M. Da Cunha Belo and J.P. Langeron, J. Vat. Sci. Technol. A 5 (1987) 1213. [5] S. Suzuki and K. Suzuki, Surf. Interf. Anal. 17 (1991) 551. 161 N. Birks and G.H. Meier, Introduction of High Temperature Oxidation of Metals (Arnold, London, 1983) p. 110. 171 P. Kofstad, High Temperature Corrosion (Elsevier Applied Science, London, 1988) p. 361. [8] M. Saito, T. Kosaka, E. Matsubara and Y. Waseda, Mater. Trans. JIM 36 (1995) 1. 191 F. Moulder, W.F. Stickle, P.E. Sobol and K.D. Bombem, Handbook of X-ray Photoelectron Spectroscopy (Physical Electronics Division, Perkin-Elmer. MN, 1982) p. 80. [IO] S. Suzuki, T. Kosaka, H. Inoue and Y. Waseda, Mater. Trans. JIM, submitted. [l 11 S. Hofmann, in: Practical Surface Analysis, 2nd ed., Vol. 1, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1992). [12] G.H. Vineyard, Phys. Rev. B 26 (1982) 4146. [13] M.F. Toney, T.C. Huang. S. Brennan and Z. Rek, J. Mater. Res. 3 (1988) 351. [14] M.F. Doerner and S. Brennan, J. Appl. Phys. 63 (1988) 126.
.‘.~.:.:.~.::::::::::~:~(i:::~:::~;~~,.~
/.../(,,,,,,,~ ,.,(/, ,((,
“” “‘.A’................/,. “’ .............,.:i.,...,.. ,I::::~,~,~,::~~:~:::~:::~:.:.:.:.~ .~.‘:::~:~~,x~:* ....... .//, ., I
..,,,.,.,.
>
_,_;,,,;,
~y.$
.:.:.:......
applied
surface science ELSEVIER
Applied Surface Science 103 (1996) 55-61
XPS/GIXS
studies of thin oxide films formed on Fe-Cr alloys
T. Kosaka a3*, S. Suzuki a, H. Inoue a, M. Saito a, Y. Waseda a, E. Matsubara b ’ Institute of Admnced Materials Processing, Tohoku University, Sendui 980.77, Japan b Graduate School, Kyoto Unic,ersi&, Kyoto 606, Japan Received 9 November
1995; accepted 27 November
1995
Abstract X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) have been used for characterizing thin oxide films formed on Fe- 10 _ 90mass%Cr alloys by heating at 873 K in air. The XPS depth profiles indicate that the oxide film of Fe-lOmass%Cr alloy consists of mainly an iron oxide, and a chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50mass%. In Fe-30mass%Cr alloy, the oxide film consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in the inner layer. The thickness of the oxide films appears to be insensitive to the bulk chromium concentration in the range between 30 and 90 mass% under the present oxidation condition. The GIXS results identify the main crystallographic structure of the oxide film with corundum (Fe,O, or Cr,O,) type structure, although it includes the preferential orientation and depends on the bulk chromium concentration.
1. Introduction Addition of chromium to iron or steels is very effective in improving heat-resistance and and therefore oxidation of corrosion-resistance, iron-chromium alloys have been widely investigated by surface analytical methods such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) [l-5]. In these works, changes in thickness and compositional distribution in oxide films formed on the surface were investigated for discussing the role of chromium or other alloying elements in oxidation-resistance. On the other hand, the layered structure of thick over a few micrometers
* Corresponding 217 521 I.
author. Tel.: +81 22 217 5168; fax: +81
0169-4332/96/$15.00 Copyright SSDI 0169-4332(95)00596-X
0 1996 Published
22
oxides formed on Fe-Cr alloys due to high temperature oxidation has also been investigated by analyzing their cross section using electron microprobe analysis [6,7]. In spite of further progress in surface analytical methods such as low energy electron diffraction (LEED) and reflective high energy electron diffraction (RHEED), information on the crystallographic structure of oxide films for Fe-Cr alloys still far from complete, because these methods are applicable to the structural analysis of thin films formed or deposited on single crystal samples. Saito et al. [8] have recently developed in-house grazing incidence X-ray scattering (GIXS) method, which utilizes total reflection of X-rays on the surface of materials occurred at small incidence angles less than the critical angle. This method enables us to give information on the depth profile of the surface crystallographic structure, since X-ray penetration depth changes from
by Elsevier Science B.V. All rights reserved
56
T. Kosaka et al./Applied
&face
several hundred nanometers to a few nanometers by varying the incidence angle. Using this method, structural features of oxide films formed on stainless steels [8] were provided from both X-ray reflection and diffraction curves. This prompts us to carry out systematic studies on the structure of thin oxide film grown on Fe-Cr alloys. The purpose of this work is to describe the compositional and structural data of thin oxide films formed on five Fe-Cr alloys obtained by XPS with argon ion sputtering and GIXS in order to clarify the growth mechanism of oxide films in this alloy system.
2. Experimental 2.1. Sample preparation Iron-chromium binary alloys with the chromium concentration of 10.0, 30.1, 50.7, 70.0 and 89.0 mass% were prepared by vacuum melting. They are referred to as Fe-lO%Cr, Fe-30%Cr, Fe-SO%Cr, Fe-70%Cr, Fe-90%Cr, respectively. Samples for XPS were shaped to plates of 3 mm thickness and 10 mm square, and samples for GIXS were plates of 3 mm thickness and 20 mm square. They were polished with emery papers in steps down to No. 4000 and with buffing with 1 pm alumina to obtain flat and smooth surface. The samples were heated at 873 K in air for 300 s or 3000 s for oxidation except Fe-lO%Cr, and then cooled to room temperature. 2.2. Measurement XPS analysis was carried out by using an apparatus of PHI Model 5600, in which monochromated Al Ka radiation was used. The angle between the axis of input lens of the analyzer and the surface normal was 45”. In order to estimate the thickness of the oxide layers and the elemental distribution in the oxide layers, XPS depth profiles of Fe 2p, Cr 2p, 0 1s and C Is were measured by following argon ion sputtering. The argon ion gun was operated with 3 kV and ion current was about 9 nA. The ion beam of about 1 mm diameter was rastered over an area of about 4 mm square, and the area of 0.8 mm diameter was analyzed. The thicknesses of the oxide layers
Science 103 (1996155-61 DifTractedBeam
4:
:?FY+z+__
(a)
Fig.
(b)
I (a) GIXS geometry and (b) Seemann-Bohlin
geometry.
were estimated by taking the oxide/metal interface as the depth reaching a half of the oxygen concentration in the oxide layer. The sputtering depth was estimated from the sputtering rate for films of iron oxide (Fe,O,) and chromium oxide (Cr,O,) of about 100 nm thickness deposited on Si wafer, of which the sputtering rates were determined to be 0.023 rim/s and 0.038 rim/s under the present condition, respectively. GIXS apparatus consists of an 18 kW rotating anode X-ray generator, a flat Ge(l11) single crystal monochromator and crossed double-axis diffractometers. In this measurement, MO-target was used for reducing the fluorescent X-rays from samples. Surface diffraction profiles of an oxide film on the samples were measured with the GIXS geometry (Fig. l(a)), where the incidence angle (Y and the take-off angle cr’ are selected so as to keep a value near the critical angle and the direction of scattering vector almost parallel to the sample surface. Therefore, the diffraction intensities from a crystallographic plane almost perpendicular to the sample surface can be detected. In order to determine the crystallographic orientation of the oxide film, measurements with the Seemann-Bohlin geometry (Fig. l(b)) were also carried out.
3. Results and discussion 3.1. XPS depth profiles Fig. 2(a) shows the XPS depth profile of FelO%Cr heated at 873 K for 60 s. This alloy is shortly oxidized, since its oxide film grows easily compared with other higher chromium alloys. Fig. 2(b)-(e) give the XPS depth profiles of Fe-30%Cr, 50%Cr, 70%Cr and 90%Cr heated at 873 K for 300 s,
51
T. Kosaka et al. /Applied Surface Science 103 (1996155-61
-.0
Cr in the oxide films for these two cases are mainly allocated to Fe3+ and Cr3+ [9]. A small amount of Fe*+ component is detected and this may be attributed to reduction of the oxides induced by argon ion sputtering. The thickness of the oxide films formed on the sample surface is plotted as a function of bulk chromium concentration in Fig. 5. Again the result for Fe-IO%Cr is only given for the thickness of the oxide film formed by heating for 1 min. These results indicate that the thickness of the oxide layers is almost unchanged in Fe-Cr alloys with chromium more than 30 mass%, although it is known to be reduced by the addition of chromium up to 20 mass%. It is interesting to note that the similar concentration independence of the thickness of the oxide films on Fe-Cr alloys has been observed in the oxidation at room temperature by using angle-resolved XPS [lo]. Then, it may safely be suggested that the oxidation of Fe-Cr binary alloys proceeds non-linearly against the chromium concentration both at room temperature and higher temperatures.
- -. 500
1000
1500
Sputtering Time(s) Fig. 2. The depth concentration profiles of thin oxide films formed on Fe-G alloys by heating at 873 K for 300 s. (a) Fe-lO%Cr heated at 873 K for 1 min only in this case, (b) Fe-30%Cr, (c) Fe-508Cr. (d) Fe-70%Cr, (e) Fe-90%Cr.
5” -_
_________._ ..--___.. ____ ..____.----<-
,y” CC ,,,,\ ,.____._______-/__.______/ I --.-_. 0
I&
respectively. Similar depth profiles for these four Fe-Cr alloys heated at 873 K for 3000 s were also obtained and the results are summarized in Fig. 3(a)-(d). In Fe-30%Cr heated for 300 s, the oxide film consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in the inner layer. On the other hand, a chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50% and it is almost independent of heating time. The similar effect of the chromium concentration on the layered structure has also been observed for relatively thick oxide formed on iron alloys with chromium less than 20% [6,7]. Fig. 4(a) and (b) show Fe 2p and Cr 2p XPS spectra from the oxide film formed on Fe-30%Cr heated at 873 K for 3000 s, respectively. The measurements were carried out prior sputtering and after sputtering for 480 s. The chemical states of Fe and
,_-+_--
1
(b)
”
”
I
Cr ---”
‘_ 1
-I
L
t .=
1
-
,111
e z
___-_--__A
--------
2 5o ___..___ ...____ ___.__......._
s IL-i 5”
_---
1000
2000
Sputtering Time(s) Fig. 3. The depth concentration profiles of thin oxide films formed on Fe-Cr alloys by heating at 873 K for 3ooO s. (a) Fe-30%Cr, (b) Fe-SO%Cr. (c) Fe-7O%Cr, (d) Fe-90%Cr.
T. Kosaka et al. /Applied Surface Science IO3 (1996) 55-61
58
Cd)
r\ 600
Binding Energy (eV)
w
Binding Energy (eV)
n
Fig. 4. The (a) Fe 2p and (b) Cr 3p XPS spectra from oxide film formed on Fe-30%Cr heated at 873 K for 3000 s (above: before sputtering, below: sputter for 480 s).
It should also be noted, as shown in Fig. 2 and Fig. 3, that the oxide/metal interface is not flat enough. The concentration broadening at the oxide/metal interface may be tentatively estimated using a method for describing the depth resolution definition, in which the broadening is taken between 84 and 16% of signals in a profile [I I]. For this reason, the error bars in Fig. 4 denote the interface roughness corresponding to the sputtering interval between 84% and 16% of the oxide concentration in the oxide films. The present authors maintain the view that the roughness of the oxide/metal interface is attributed to the formation of polycrystalline oxides, inducing some fluctuation of the interface. Although argon ion sputtering during depth profiling appears to produce roughness in the flat surface even if the composition is kept step-like at the interface,
I
loo-
, 0
SO-
z +
60-
1 e
40- -
,
,
,
60.3@-lO%Cr
0
300s
0
3000s
1
d
I
,
I
f-----.
(b)
10
IS
30
Fig. 6. The diffracted intensity patterns of thin oxide films formed on Fe-Cr alloys heated at 873 K for 300 s measured by the GIXS geometry. (a) Fe-lO%Cr, (b) Fe-30%Cr, (c) Fe40%Cr. (d) Fe-70%Cr and (e) Fe-90%Cr.
insignificance contribution of such factor is confirmed by obtaining the depth profiles of reference films of iron oxide and chromium oxide deposited on Si wafer. 3.2. GIXS
only)
4
Fig. 6 and Fig. 7 show the diffracted intensity of five Fe-Cr alloys heated at 873 K for 300 and 3000 s, respectively. They were measured by GIXS with the incidence angle of 0.16”. When the incidence angle of MO K cy radiation is fixed at 0.16”, the e ~ ’ penetration depth, T(n), is calculated to be 88 nm for Cr,O, based on the following equation [12]. T( fY) = A/47rq, q=-+{~(a.a1)+4p’
Fig. 5. The thickness of the oxide films estimated from the XPS depth profiles as a function of the bulk chromium concentration. Solid circles: samples heated for 300 s. and open circles: samples heated for 3000 s.
(1) -(&a,?),
(2)
where A is the wave length of incident X-rays, (Y is the incidence angle, (Y, the critical angle, and p is
T. Kosaka et al./Applied
110 l
200
10
15
20
25
59
Surface Science 103 (1996155-61
30
cP(degN
Fig. 7. The diffracted intensity patterns of thin oxide films formed on Fe-0 alloys heated at 873 K for 3000 s are measured by the GIXS geometry. (a) Fe-lO%Cr, (b) Fe-30%Cr. (c) Fe-SO%Cr, (d) Fe-70%Cr and fe) Fe-90%Cr.
the absorptive term of the refractive index. Then, the results illustrated in Fig. 6 and Fig. 7 provide the structural information in the range of 90 N 100 nm from the surface of each sample. The peaks observed in the results of Fig. 6 and Fig. 7 identify the oxide films with corundum type (Fe,O, or Cr,O,) structure as well as b.c.c. structure (ferritic phase) of Fe-Cr matrix. It is difficult to distinguish the diffracted peaks between Fe,O, and Cr,O, due to the limited angular resolution. Nevertheless, these results are consistent with the XPS spectra showing Fe3+ and Cr3+. In Fig. 7, five peaks or more are detected in the diffracted intensity pattern of the oxide film formed on alloy surface by heating at 873 K for 3000 s and they are coincident with positions suggesting similar behavior is also detected in Fig. 6 for alloy heated at 873 K for 300 s, although the variation is rather blurred. As the chromium concentration increases, the intensity of some of peaks is reduced and only a few peaks are clearly observed in alloys with chromium more than 50 mass%. Such variation can be interpreted by
considering that crystalline oxide film formed on alloys with chromium more than 50 mass% includes, more or less, the preferential crystallographic orientation. In order to investigate the preferential crystallographic orientation of the oxide film, the intensity pattern of the oxide film formed on the Fe-70%Cr alloy surface heated at 873 K for 3000 s is measured by the Seemann-Bohlin geometry (Fig. l(b)) at the same incidence angle of 0.16“. The result given in Fig. 8, together with that of the GIXS geometry. In the GIXS geometry, the 104 peak is not detected but the 110 peak is clearly observed. The opposite variation is found in the Seemann-Bohlin geometry. Since the structural information of the three dimensional crystal lattice inclined against the sample surface can be detected by the Seemann-Bohlin geometry, the 110 peak is expected to be observed if the oxide film is grown without the crystallographic orientation. On the other hand, only the reflections of the 110 and 300 plane, which are perpendicular to the sample surface, are observed in GIXS geometry as shown in Fig. 7 and Fig. 8. Consequently, these results imply that the crystalline oxide film formed on this alloy surface is described by the preferential orientation of [OOl] (Fig. 9). These results suggest that the growth direction of the oxide film is not dominated by the epitaxy between the oxide film and the matrix. It is rather coincident with the oxidation direction. The thickness of the oxide films can also be estimated from the angular dependence of the
I
’
11
0
11
II
‘I
1
’
GIXSgeometry
_
-
-
------. S.B. geometry
15
110
16
17
$NkwN Fig. 8. The intensity pattern of the oxide film formed on the Fe-70%Cr alloy surface heated at 873 K for 3ooO s measured by the Seemann-Bohlin geometry and together with that of the GIXS geometry.
T. Kosaka et al. /Applied &&ace Science 103 (1996) 55-61
60
Slde Vlew
The unit cell of the oxide film a b
Top View
Fig. 9. Schematic on Fe-Cr alloys.
diagrams
of the structure of oxide film formed
diffracted peak as reported previously [8,13]. Solid circles in Fig. 10 show the angular dependence of the (110) peak intensity for the corundum structure of the oxide film formed on the Fe-SO%Cr by heating at 873 K for 3000 s. On the other hand, the theoretical variation in intensity for a film of thickness, t,, with the concentration, C, is calculated by the following equations.
I( a) a G( cx)IEi( a)ll;:“Ccxp
- 6 i
i
dr
(3)
where
l.g( a)1 =
0
4a
(a+py+q?
0.1
Incidence
0.2
0.3
angle(degree)
Fig. 10. The angular dependence of the I 10 peak intensity for the corundum structure of the oxide film formed on the Fe-50%Cr alloy by heating at 873 K for 3000 s.
Fig. I I. (a) The thickness, to, and (b) the effective linear absorption coefficient, I*, of the oxide films estimated from the GIXS data as a function of the bulk chromium concentration.
and p=$/(a’af)i4P2
+(a?+)
(5)
where E, is the electric field at the surface and G(a) the geometrical factor. When we assume that the film is of Cr,O, and the oxide/metal interface is perfectly sharp, the values of thickness and effective linear absorption coefficient of the oxide film can be estimated from comparison with the experimental data using the nonlinear least-squares variational method, as exemplified by the results of Fig. 10 as an example. The resultant values for the oxide films for alloys heated at 873 K for 3000 s versus the chromium concentration are given in Fig. 1 l(a) and (b). It is rather stressed here that the values of thickness estimated from the GIXS method are small compared to those obtained from the XPS depth profiles by a factor of more than five, and the effective linear absorption coefficients obtained in this method are considerably larger than 142.7 cm-’ for the bulk of Cr,O,. A part of the reason for such discrepancy results from the assumption that the oxide/metal interface is perfectly sharp. The homogeneous film samples are known to show the characteristic interface oscillations in their reflection curves [8]. However, such interference oscillations are not detected in the present case, suggesting that the oxide/metal interface is not so sharp when oxidizing Fe-Cr alloys by heating in air. It is, therefore, concluded that the oxide thickness obtained by GIXS
T. Kosaka et al. /Applied Suqtace Science 103 (1996) 55-61
may be underestimated. Small crystalline oxide grains are likely to be nucleated on surface at the initial stage of thermal oxidation, whereas amorphous oxide films are formed mainly by anodic reaction at room temperature [8]. The thermal oxidation appears to cause the surface roughness and make its scattering intensity reduced and this results in best fit only when the smaller values of thickness are given. It is difficult to predict the depth dependence of peak intensity above the first maximum due to the effects such as texture in the sample and variations in incident intensity from the sample [14]. Then, further theoretical effort so as to include the interface roughness is need, but that is outside the scope of the present work. Accordingly, the thickness of oxide film estimated in this work is, in the present author’s view, rather acceptable the value of XPS than that of GIXS.
61
(3) The thickness of the oxide films was estimated from the angular dependence of the intensity of the 110 diffracted peak in the GIXS measurements, assuming the oxide/metal interface is perfectly sharp. Similar variation versus the chromium concentration is found, but the GIXS results are small compared with those obtained from the XPS depth profiles by a factor of more than five. Such discrepancy may be improved by taking account of the roughness of the oxide/metal interface.
Acknowledgements The authors are grateful to Mr. T. Sato and Ms. F. Yasuda for their help in operation of XPS apparatus.
References 4. Conclusion XPS and GIXS have been used for obtaining the structural information of thin oxide films formed on Fe-10 h 90mass%Cr alloys by heating at 873 K for 300 s and 3000 s in air. The results are summarized as follows: (1) The oxide film formed on Fe-30mass%Cr alloy surface consists of a two layered structure; iron rich oxide in the outer layer and chromium rich oxide in inner layer. A chromium oxide is predominant in the oxide film formed on alloys with chromium more than 50 mass%. The thickness of oxide films on these alloys are independent of the bulk chromium concentration more than 30 mass% under the present oxidation condition. (2) It is found from the GIXS results that the oxide films are identified by the corundum type (Fe,O, or Cr203) structure within the range of about 100 nm from the sample surface. These oxide films formed on Fe-Cr alloys grow with the preferential orientation, particularly in the initial stage of oxidation.
[I] G. Betz, G.K. Wehner, L. Toth and A. Joshi, J. Appl. Phys. 45 (1974) 5312. [2] J.C. Langevoort, I. Sutherland, L.J. Hanekamp and P.J. Gelling& Appl. Surf. Sci. 28 (1987) 167. 131 D.F. Mitchell and M.J. Graham. Surf. Interf. Anal, 10 (1987) 259. [4] G. Lorang, M. Da Cunha Belo and J.P. Langeron, J. Vat. Sci. Technol. A 5 (1987) 1213. [5] S. Suzuki and K. Suzuki, Surf. Interf. Anal. 17 (1991) 551. 161 N. Birks and G.H. Meier, Introduction of High Temperature Oxidation of Metals (Arnold, London, 1983) p. 110. 171 P. Kofstad, High Temperature Corrosion (Elsevier Applied Science, London, 1988) p. 361. [8] M. Saito, T. Kosaka, E. Matsubara and Y. Waseda, Mater. Trans. JIM 36 (1995) 1. 191 F. Moulder, W.F. Stickle, P.E. Sobol and K.D. Bombem, Handbook of X-ray Photoelectron Spectroscopy (Physical Electronics Division, Perkin-Elmer. MN, 1982) p. 80. [IO] S. Suzuki, T. Kosaka, H. Inoue and Y. Waseda, Mater. Trans. JIM, submitted. [l 11 S. Hofmann, in: Practical Surface Analysis, 2nd ed., Vol. 1, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1992). [12] G.H. Vineyard, Phys. Rev. B 26 (1982) 4146. [13] M.F. Toney, T.C. Huang. S. Brennan and Z. Rek, J. Mater. Res. 3 (1988) 351. [14] M.F. Doerner and S. Brennan, J. Appl. Phys. 63 (1988) 126.