Oxygen adsorption and oxide growth on Ni3Al single crystal surfaces

Oxygen adsorption and oxide growth on Ni3Al single crystal surfaces

Nuclear Ins|ruments and Methods in Physics Research B67 (1992) 350-354 North-Holland Nuclear Instruments & Methods in Physics Research Section B Oxy...

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Nuclear Ins|ruments and Methods in Physics Research B67 (1992) 350-354 North-Holland

Nuclear Instruments & Methods in Physics Research Section B

Oxygen adsorption and oxide growth on Ni3AI single crystal surfaces Yaogen Shen 1, D.J. O'Connor and R.J. MacDonald Department of Physics, Unitersity of Newcastle, Newcastle, NSW 2308, Australia

Ni3AI is a member of an interesting group of materials which form ordered alloys and which have useful properties in respect of strength and corrosion resistance. The kinetics of oxygen adsorption and oxide formation in a high temperature environment are then of particular interest. Low energy ion scattering has been applied to the study of oxygen adsorption and oxide formation under controlled exposure at 700°C. The results indicate that oxygen adsorption is accompanied by significant changes in the surface compositionl and that oxide growth proceeds by island growth, with the oxide layer disordered with respect to the underlying single crystal snbs~rate.

1. Introduction Ordered alloys such as Ni3AI have very useful properties, pat~ticularly in respect of their strength and corrosion resistance at high temperature [1]. The alloy which willl be studied in this work is Ni3AI, which crystailises in the face-centred cubic LI~ structure and which shows no order-disorder transition above room temperature. Previous studies using low energy ion scattering (LEIS) of He + and Li + have revealed the following [2]: (i) The surface of Ni3AI(001) and Ni3AI(011) can be cleaned by ion bombardment followed by subsequent annealing to 900°C; (ii) The surface composition of both surfaces, once established, remains constant over a wide temperature range; (iii) The surface composition of the (001) and (0l 1) faces determined by LEIS with both He + and Li + scattering are given in table 1. These results consistently point towards a surface layer composition in both cases of 50% AI, 50% Ni with some small percentage of Ni enhancement (perhaps 1-2%) and to a second layer composition which is basically Ni; (iv) Preferential sputtering occurs on both faces and in each case the AI atoms are preferentially removed from the top layer, with up to 2 / 3 of the available AI atoms being removed from the surface layer; (v) The surfaces of the Ni3Al(001) and Ni3AI(011) have been studied using the shadow cone method (Aono [3], Niehus [4]) and the results indicate that: (a) For the (001) surface the first layer Al atoms shift 0.06 ,A outwards from the first layer Ni and there is t Now at Surface Science Western, London, Ontario, Canada.

a contraction of 0.08 ,g, in the first to second layer spacing, (b) For the (011) surface there is no displacement of first layer AI relative to first layer Ni, but there is a contraction in the first to second layer spacing of 0.11 ,A. These results are compatible with interpretations of LEED observations [5].

2. Oxygen adsorption experiments The clean surface of the Ni3AI was exposed to oxygen at a constant pressure for a known time while the crystal was maintained at a temperature of 700°C. The LEIS spectra show that the growth of the oxygen peak is accompanied by a decreasing intensity in the A! and Ni peak. With increasing exposure to oxygen, the scattering from Ni atoms decreases much more rapidly than the scattering from the Al. atom (fig. la) and surface Ni is not detectable after an exposure to 10 L of oxygen. The AI scattered signal, however, remains readily detectable, as shown in fig. 2. This strongly supports the formation of an aluminium oxide film on the surface [6], though the mechanism for apparent interchange of Al atoms to the surface to replace the Ni is not clear. A similar result holds for the Ni3Al(011) surface. In order to locate the adsorption site for the oxygen uptake, experiments studying the variation of the scattered ion yield from the surface atoms as a function of the angle of incidence along low index azimuths were performed. The results for scattering from Ni on the clean surface and after some exposure to oxygen, are shown in fig. 3. These show a dimim~ion in the yield of ions scattered from surface Ni, but no change in the form of the variatien of yield with angle of incidence

0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

351

Y. Shen et aL / Oxygen adsorption and growth on N i s A I

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Fig. 2. The decrease in the ion scattering yield from A on the Ni3AI(001) surface, compared with the oxygen coverage on the surface. A similar result is found for oxygen adsorption on the Ni3AI(011) surface. D: experimental results, straight line: decrease in the signal if it were parallel to that shown for Ni (fig. lb), ×: the difference of the experimental results from this line, i.e. related to the enhanced A density on the surface.

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Fig. ]. (a) Experimental spectra showing the variation of AI and Ni signal with oxygen coverage increasing from curve (a) to curve (d). (b) The decrease in the ion scattering yield from Ni c,a the Ni3AI(00I) surface, compared with the oxygen coverage on the surface. A similar result is found for oxygen adsorption on the Ni3AI(I10) surface. The line is a fit to the experimental results.

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for the various azimuths. This has b e e n interpreted to m e a n that the oxygen adsorption does not affect the n e a r e s t n e i g h b o u r e n v i r o n m e n t of the Ni atom. T h e oxygen adsorption p r o m o t e s an a l u m i n i u m oxide growth in the form of islands. T h e Ni exists on the surface only

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ANGLE OF INEIOENEE ( d e g r e e ) Fig. 3. Angle of incidence scans for ions scattered from Ni on a clean surface and from a surface exposed to oxygen at 700°C. The lines are drawn to fit the experimental results. V. SURFACE PHENOMENA

Y. Shen et al. / Oxygen adsorption and growth on Ni3AI

352

Table 1 The surface composition of the Ni3AI(001) and (011) faces as determined by LEIS using He + and Li ÷ ions Azimuth and incident angle

Scattering angle [deg]

Composition of the (001) surface He + [ i 00]45° 90 He +[ 110135° 90 Li+ [100145° 90 Li + [ I i 0135° Composition of the (011) surface He + [ I]2130° 60 He + [I]2130 ° 90 • He + [I]01445~ 90 He + [ !]0145° 110 He + [001135° 70 Li + [I]2]30° 60 Li + [112130° 90 Li + [1i2130° 110 Li+[1i0145 ° 9O Li + [110145°

First layer composition [%] AI Ni

Second layer composition [%] AI Ni

47 +_2 48 -I-3 49+3 -

53 + 2 52 _+3 51 +3

-

47 + 2 46 + 3 49+3 52 + 3 49-+ 2 48 :t: 2 47 -+3 50 _+2

53 +_2 54 + 3 51 _+3 48 -+3 51 + 2 52 + 2 53 + 3 50 _+2 -

i !0

suggests that a two-stage process determines the surface layer composition. The first part corresponds to a low coverage of oxygen and its variation with coverage is close to parallel to the decrease in Ni signal intensity u n d e r the same circumstances. With increasing coverage, however, a second c o m p o n e n t of the Ai signal due to an increase in the n u m b e r of Al surface atoms can be seen. T h e latter correspond to the interchange of Ni and AI on the Ni3AI surface.

3. Azimuthal signal variations and island growth T h e experiments p e r f o r m e d above suggest a mechanism whereby oxygen adsorbs at a t e m p e r a t u r e of 700°C to form a thin oxide layer which, depending on the exposure, may be less than one monolayer. Those areas not covered with oxygen or oxide are clean. This is s u p p o r t e d by studies of the azimuthal dependence of the scattering of He + ions from the oxidised surface. This is shown in fig. 4. The oxygen signal strength shows no azimuthal orientation dependence similar to that observed from a clean surface. This indicates that the oxygen uptake .~he i~ es~ntia!ly dir,ordered, If the signals which are seen are the result of a combination of a clean surface (Ni3Al) area and an

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where O is the fraction of the surface covered by the oxide film. The azimuthal variation of the surface partially covered with oxygen is then fitted to the clean surface result to give a measure of O, the surface coverage. A measure of the applicability of the model will be the uniqueness of the fit. The Al and Ni results were separately fitted to the variations for the differing oxygen exposures shown in fig. 4. The results are shown in fig. 5 - the standard deviation on the fitting p a r a m e t e r (oxygen coverage) was less than + 10% for most of the experiments. The coverage O is independently derived from the azimuthal dependence of the Al and Ni signal strength and from the variation of the oxygen signal with coverage. All three determinations give the same result for O. This model assumes that there is no change in the neutralisation of the ions with oxygen adsorption. This assumption is always questionable, but it is supported

353

Y. Shen et aL / Oxygen adsorption and growth on Ni3AI

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Using LEIS it has been possible to obtain some interesting results on the oxygen adsorption and oxide growth on Ni3AI(001) and Ni3AI(011) surfaces. Two conclusions are strongly supported by the results. Firstly, oxygen adsorption activates a segregation of AI to the surface, with the oxygen apparently not bonding to the Ni in the surface plane at least. Secondly, an oxide film of aluminium grows on the surface, with apparently no order related to the Ni3Ai matrix and covering part of the surface - the uncovered part V. SURFACE PHENOMENA

354

Y. Shen et al. / Oxygen adsorption and growth on Ni 3 AI

retains all otl the surface compositional and structural characteristics of clean Ni.~AI.

Acknowledgment The Ion-Surface Interaction Group acknowledges with gratitude the support of the Australian Research Council without whose help this research would not have been possible.

References [1] T. Takasugi, N. Masakashi and O. Izumi, Acta Metall. 35 (19871 381. [2] Yaogen Shen, Ph,D. Thesis, University of Newcastle (1991). [3] M. Aono, Nucl. Instr. and Meth. 132 (1984) 374. [4] H. Niehus, J. Vac. Sci. Technol. A5 (1987) 751. [5] D. Sondericher, F. Jona and P.M. Marcus, Phys. Rev. 1333 (1986) 900, and 6775. [6] V. Bardi, A. Atrei and G. Rovida, Surf. Sei. 239 (1990) 1.511.