The effects of ion implantation upon nickel oxidation investigated by secondary ion mass spectrometry

Materials Science and Engineering, A 116 (1989) 111 - 117

Ill

The Effects of Ion Implantation upon Nickel Oxidation Investigated by Secondary Ion Mass Spectrometry* R J. GEORGE

Physics Department, Kurukshetra Universi~, Kun~kshetra, Haryana 132119 (India) M. J. BENNETT and H. E. BISHOP

Materials" Development Division, Harwell Laboratory, Didcot, OXI 10RA (U.K.) G. D E A R N A L E Y

Nuclear Physics Division, Harwell Laboratory, Didcot, OXI I ORA (U.K.) (Received September 16, 1988)

Abstract The influence of the ion implantation of several elements upon the thermal oxidation of polycrystalline, high purity nickel, in oxygen, at 500-1100 °C has been studied. The oxide thicknesses and the location of the implanted elements within the scales were determined by secondary ion mass spectrometry depth profiling. The study concentrated on the effect of yttrium implantation, with the derivation of the dependences upon ion dose, time and temperature. The possible role of the associated physical damage introduced by ion implantation was examined by comparison of the Y+ results with those for krypton implantation and also following post-implantation vacuum annealing. The effectiveness relative to yttrium implantation of the implantation of other reactive elements, cerium, lanthanum and ytterbium, as well as silicon, either alone or in combination with yttrium, was also established, 1. Introduction Since the early 1970s, ion implantation has been used increasingly, and with considerable success, to examine how the surface modification of metals and alloys affects their oxidation behaviour [1]. Several such studies have been undertaken on nickel [2-6] and have investigated the influence of a range of implanted ions. A topic of particular importance is the socalled "reactive element" effect by which ele*Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del

Garda, Italy,September 12-16, 1988. 0921-5093/89/$3.50

ments having a high affinity for oxygen, such as yttrium, cerium and other rare earths, improve significantly the high temperature corrosion behaviour of metals and alloys. Despite recent major advances in mechanistic understanding of the effect on technological alloys, in part originating from ion implantation studies such as those concerned with the 20Cr-25Ni-Nb (where the composition is in weight per cent) stabilized stainless steel [7], the complexity of the scale growth on technological alloys imposes limitations on the progress of understanding. There is a clear need, therefore, for this work to be augmented by cornparable studies on pure metals such as nickel. Since there have been only two previous investigations [5, 6] of the influence of the ion implantation of reactive elements upon nickel oxidation, a further programme was instigated. This paper describes the initial results obtained concerning the effect primarily of yttrium ion implantation, based on the analysis by secondary ion mass spectrometry (SIMS) of scale development and of the location of the reactive elements. 2. Experimental Specimens (10 m m X 1 0 ram×0.5 ram)were cut from polycrystalline nickel supplied by Johnson-Matthey and having as-measured impurities, 1 ppm of aluminium, iron, magnesium and silicon and less than 1 ppm of calcium, chromium and copper. The sheets were polished mechanically to a 1 /~m diamond surface finish and then annealed in a w e t N2/H _, atmosphere, for 1 h, at 900 °C. Finally, the specimen surfaces © Elsevier Sequoia/Printed in The Netherlands

112

were ion etched to remove a surface layer about 1 p m thick shown by SIMS analysis to contain a high silicon concentration, which was probably due to embedded SiC grinding residues, The ion implantations were carried out, at room temperature, using the Cockcroft-Walton accelerator at the Harwell Laboratory. The ions implanted were Y+, Ce +, La +, Yb + and Kr +. The maximum doses implanted, the respective beam energies, the calculated [8] depths of maximum ion concentration and implantation, together with the calculated maximum ion concentration for each ion, are detailed in Table 1. By masking, only one half of a main specimen face was implanted with a particular ion dose. This maximized the use of each specimen and enabled a direct comparison, during identical oxidation exposures, of the influence upon nickel oxidation either of up to three doses of the same ion or of up to four different ions. The specimens were oxidized at four temperatures, namely 500, 700, 900 and 1100 °C. Oxidations were undertaken in a closed end quartz tube, which was initially evacuated. Oxygen, dried by passage through a P205 dessicant, was introduced at a rate of 3 1 rain-l at a pressure of 40 mbar. The specimens were oxidized either-for 75 min or for successive periods of 15, 60, 240 and, in some instances, 420 min, such that the respective cumulative oxidation periods were 15, 75, 315 and 735 min. After each oxidation period the elemental profiles through different areas of the oxide scale were analyzed by SIMS using argon primary ions in a C A M E C A model IMS3F ion microscope,

gen secondary ion intensity, as shown in Fig. 1 for unimplanted nickel and nickel implanted with 2 X 1016 yttrium ions cm -2, following 75 min oxidation at 700 °C. The scale-substrate interface was assumed to have been reached when the oxygen ion intensity decreased to one half of the maximum value. The influence of the implantation variants was established by comparison of the ratios of the respective sputtering times reflecting the oxide thicknesses and normalized with respect to the maximum thickness within the particular experimental series of data. In this way uncertainties arising from slight differences in analytical conditions between experimental series were minimized. The effect of the implanted yttrium ion dose between 2 × 1015 and 5 × 1016 ions cm-2 was compared after 75 min oxidation at 700 °C. The thickness of the scale formed on unimplanted nickel was 6000 A and this was reduced to 3500 A by the implantation of 2 x 1015 ions cm -2. The thickness ratios of the scales formed on the nickel implanted with higher yttrium doses, shown in Fig. 2, were normalized to that of

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TABLE 1 Element implanted

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3. Results The thicknesses of the oxide scales formed w e r e determined from the SIMS profiles o f o x y -

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Fig. 1. SIMS depth profiles of the oxide formed on unimplanted and yttrium-implanted (2 x 10 j6 ions cm 2) nickel after 75 min oxidation in oxygen at 700 °C.

Implantation conditions and profiles Beam energy (keV)

Maximum dose (ions c m - 2)

250 250 250 250 230 120

5 × 1016 2 x 1016 1016 1016 l0 j6 2 x 1016

Depth (A) Peak concentration

Implantation

365 390 265 270 225 555

780 840 600 600 500 1080

Maximum atom concentration (at.%) 12.5 4.8 3.8 3.7 4.8 3.3

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Fig. 2. Influence of yttrium ion dose upon the oxide thickness formed on yttrium ion implanted nickel after 75 min oxidation in oxygen at 700 °C. (Ratio normalized to scale thickness formed on nickel implanted with 2 x 10 ~5 yttrium ions cm-2),

on unimplanted, krypton-, yttrium- and dual yttrium-andsilicon-implanted nickel during oxidation in oxygen at 700 °C. (All ion doses were 2 × l0 ~' cm 2. The oxide thickness ratios were normalized to that of unimplanted nickel oxidized for 735 min.)

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Fig. 3. Variation with time of the oxide thicknesses formed on unimplanted, krypton-, yttrium- and dual yttrium-andsilicon-implanted nickel during oxidation in oxygen at 500 °C. (All ion doses were 2 x 10 j6 cm -2. The oxide thickness ratios were normalized to that of unimplanted nickel oxidized for 735 min.)

the lowest yttrium ion dose (2 x 1015 ions cm-2). Increasing the dose from 2 x 1015 to 5 x 1015 ions cm- 2 further reduced the scale thickness to about 1400 A_. Higher doses apparently had little additional beneficial effect. As a consequence a yttrium dose of 2 x 1016 ions cm-2 was employed for studying the temperature-time dependence on nickel oxidation. In Figs. 3-6, for oxidation at 500, 700, 900 and 1100 °C respectively, the oxide thicknesses at each exposure period were normalized again to that of the thickest scale in the particular test series, i.e. that formed on unimplanted nickel following the longest exposure (after 735 min at 500 and 700 °C, and after

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Oxidation time (h) Fig. 5. Variation with time of the oxide thicknesses formed on unimplanted, krypton- and yttrium-implanted nickel during oxidation in oxygen at 900 °C. (All ion doses were 2 x 10 ~ cm -~.The oxide thickness ratios were normalized to that of unimplanted nickel oxidized for 315 min.',

315 min at 900 and 1100 °C). Although it was not possible to examine the oxidation kinetics in detail, the oxide growth rates on both unimplanted and yttrium-implanted nickel were fastest initially and decreased with increasing exposure, consistent with the anticipated parabolic kinetics. At all temperatures yttrium implantation significantly inhibited the scale growth rate throughout the exposures. The reduction in the scale thickness at the completion of oxidation appeared to be slightly greater at 500 °C than at the higher temperatures.

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0"2

and yttrium-implanted nickel in both an unannealed and an annealed condition. (The yttrium dose was 2 x 1 0 ~6 ions cm 2. The annealing conditions were 1 h in vacuum at

t 2

I 6 Oxidation time (h)

0

I 4

I 8

900 °C).

Fig. 6. Variation with time of the oxide thicknesses formed on unimplanted, krypton- and yttrium-implanted nickel during oxidation in oxygen at 1100 °C. (All ion doses were 2 x l016 cm 2 The oxide thickness ratios were normalized to that of unimplanted nickel oxidized for 315 min.)

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Fig. 7. SIMS analysis of the yttrium depth profiles through the oxides formed on yttrium-implanted nickel during between15 and 735 min oxidation in oxyg enat700°C,

The yttrium profiles through the adherent oxide scales were analyzed by SIMS and consequently start from the gas-scale interface. By comparison with the corresponding oxygen profiles (e.g. Fig. 1 ), and in particular the designation of the oxide-nickel interface, it was shown (Fig. 7) that in all the oxide scales, irrespective of the temperature or duration of oxidation, the yttrium was located within the scale near the interface with the underlying nickel. Unfortunately, quantitative comparison between these profiles was not possible due to slight differences between the sputtering conditions employed for these analyses. However, it is probable that the yttrium

distribution was broader after the longest exposure.

Krypton ion implantation was carried o u t to study whether the physical defects introduced into the nickel surface during implantation influenced its subsequent oxidation. Although krypton implantation could have reduced the initial scale growth rate at 500, 700 and 900 °C, its effect diminished with time and it was without influence after longer exposures (Figs. 3 - 5 ) . A t the higher temperature (1100 °C), however, krypton implantation continued to reduce oxidation throughout the exposure by an extent which decreased with time but was less than that resulting from yttrium implantation (Fig. 6). The possible involvement of the physical effects of ion implantation upon the subsequent oxidation behaviour of nickel was investigated further by annealing out the damage after irradiation. For this purpose, nickel implanted with 2 x 1 0 ~6 yttrium ions and with 2 x 10 t6 krypton ions cm-2, as well as unimplanted nickel, was annealed in vacua (less than 1 0 -6 Torr) for 1 h at 900 °C. This annealing had no effect on the oxide scale thicknesses, which developed after 75 min oxidation at 700 °C on either the unimplanted or krypton-implanted nickel surfaces (Fig. 8). Also, it did not influence the extent of inhibition resulting from yttrium ion implantation (Fig. 8). The influence of the implantation of other potential reactive elements (cerium, lanthanum and ytterbium) was compared with that of yttrium (Fig. 9). Doses of 1 0 ]6 ions cm -2 of all these elements inhibited scale development on nickel by comparable extents, at least following 75 min oxidation at 700 °C.

115

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Fig. 9. Comparison of the effect of the implantation of 10 ~6 silicon, lanthanum, cerium, ytterbium and yttrium ions cm- 2 upon 75 rain oxidation of nickel in oxygen at 700 °C. (The oxide thickness ratios were normalized to that of the yttrium implanted nickel.)

The implantation of 2 x 1016 silicon ions cm-2 in conjunction with 2 x 1016 yttrium ions cm-2 further slightly reduced the nickel oxide scale thicknesses formed at 500 and 700 °C compared with those obtained following yttrium ion implantation alone (Figs. 3 and 4). Silicon implantation (at a dose of 1016 ions c m -2) also proved to be equally as effective as the other reactive elements in reducing the oxidation of nickel following 75 min exposure at 700 °C (Fig. 9). SIMS analysis indicated that within the scale silicon tended to accumulate at a similar location to that of yttrium (i.e. near to the metal interface). Lanthanum concentrated nearer to the gas interface, while cerium and ytterbium were distributed over a broader band within the centre of the scale, 4. Discussion Up to 1000 °C, the oxidation of nickel is considered to be dominated by nickel diffusion along grain boundaries with the major diffusing species being cation vacancies [9]. Above this temperature scale growth becomes controlled by lattice diffusion. In both regimes, at least over the temperature range 500-1100 °C, yttrium ion implantation, while not affecting the oxidation kinetics, significantly reduced the oxide growth rate. This agrees with the results of Pivin [6]. The current study has demonstrated a dependence on dose, at least at 700 °C, for the effectiveness of yttrium ion implantation increased up to 5 × 1 0 1 5 ions cm -2, while higher doses up to 5 × 1016 i o n s cm -2 provided no further significant improvement. The ion implantation results also agree in part with previous observations [10] that alloy additions of 0.1 and 0.3 wt.% Y retard the oxidation of nickel at 700-900 °C. By contrast, at

lower temperatures (500 and 600 °C), the alloy additions had an opposite effect to ion implantation and increased the oxidation rate of nickel, which was attributed to a reduction in oxide grain size. The inhibition of nickel oxidation by yttrium ion implantation could be chemical in origin and/ or derive from the physical damage to the nickel surface resulting from the ion implantation. The role of the latter has been assessed by examining the influence of a comparable dose of a chemically inert element krypton, having a similar mass to that of yttrium and also by vacuum annealing the radiation damage prior to oxidation. Krypton ion implantation did not influence the long-term oxidation of nickel up to 900 °C, but reduced the scale growth rate at 1100 °C, although not so effectively as yttrium implantation. Vacuum annealing at 900 °C had no effect at 700 °C on either the oxidation behaviour of unimplanted and krypton-implanted nickel or the inhibition provided by yttrium ion implantation. Therefore, at least over the temperature range 500-900 °C, where scale growth is controlled by grain boundary diffusion any physical effects of ion implantation are insignificant. SIMS analysis confirmed, and extended previous observations [6], that the implanted yttrium was located within all scales near the interface with the underlying metal. As this also provided a marker of the original metal surface it would suggest that, irrespective of any modification by the yttrium ion implantation of the initial nucleation and transient stages of oxidation, continuing scale growth on yttrium ion implanted nickel still occurred by outward cation movement. These observations are entirely consistent with those on a 2 0 C r - 2 5 N i - N b stainless steel [7] on which scale growth is also dominated by cation movement, via grain boundaries. In contrast, recent work [11] has indicated that for oxide development on yttrium-ion-implanted chromium in oxygen at 900 °C the growth mechanism was changed and occurred by inward oxidant movement. Even so, the important common factor between all these studies was that yttrium ion implantation blocked outward cation movement. Based on the more detailed microstructural examinations, including analytical electron microscopy, undertaken on these systems and also on related CeO2-coated nickel [12], a probable mechanism can be advanced by which yttrium reduced the scale growth rate of nickel at

116 500-900 °C within the temperature range of grain boundary diffusion control. On exposure to the oxidant the implanted yttrium probably became oxidized as Y203 particles and also segregated along grain boundaries within the initially formed NiO scale. The formation of this yttriumpacked grain boundary network was facilitated by the high concentration of small Y203 particles produced within the shallow (780 A) surface implantation layer. This network hindered diffusion due to their attractive interaction with cation vacancies, such that bulk lattice diffusion became the preferred transport mechanism. The size and spacing of the reactive element oxide particles governed the crucial segregation in the oxide grain boundaries and as a consequence, the oxide growth rate inhibition [12]. The observed pattern of the relationship between improved oxidation resistance and yttrium ion dose was identical with the corresponding dependence of nickel oxidation on the amount of ceria surface coating [ 12]. Lanthanum and ytterbium ion implantation, as well as cerium [5] and silicon [6] implantation, in agreement with previous observations, all reduced nickel oxidation rates. Like yttrium, these elements form thermodynamically more stable oxides than nickel. The implanted atoms probably become incorporated as oxides and grain boundary segregants within the nickel oxide scales to affect grain boundary diffusion in a comparable manner tc~that by yttrium. Ytrrium ion implantation also inhibited nickel oxidation at the higher temperature of 1100 °C, at which grain boundary diffusion continues to occur but is dominated by lattice diffusion, which then governs scale growth. Substitutional impurities in solid solution in an oxide can affect the mobile defect concentration (the Wagner-Hauffe effect) and is believed to be the primary cause of the effects of chromium and lithium implantation upon the oxidation of nickel in oxygen at 950-1150 °C [3, 4]. However, this is unlikely to account for the influence of yttrium, as this element is probably relatively insoluble in the NiO lattice and in any case would increase the majority defect concentration, whereas yttrium ion implantation decreased scale growth rates, Another possible explanation is also unlikely. For Y203 particles to reduce the cross-sectional area of lattice oxide available for ion transport would undoubtedly require a significantly higher particle concentration than would be present in the scale. A more probable explanation for the

growth rate inhibition is that the small well grouped yttria particles within the scale acted as sinks for cation vacancies [13]. This reduced the vacancy flux through the scale and as a consequence lattice diffusion. The grain boundary diffusion component was also reduced, as at the lower temperatures, by the segregated yttrium within the oxide grain boundaries. The effectiveness of krypton ion implantation in inhibiting nickel oxidation at 1100 °C, albeit by not the same extent as yttrium ion implantation, does not accord with previous observations [3, 4] in which inert elements such as argon and nickel enhanced the growth rate. No reason can be advanced at present for this discrepancy. It is also of interest in this context that implantation of the same krypton dose (1017 ions cm -2) had no effect on the oxidation behaviour of the 20Cr-25Ni-Nb stainless steel in CO2 at 900 and 950 °C, but did reduce the rate of attack at 1000 °C [7].

5. Conclusions (1) Yttrium ion implantation inhibited the extent but did not affect the kinetics of oxidation of nickel, in oxygen, at temperatures between 500 and 1100 °C. At least at 700 °C the scale growth rate decreased progressively with increasing dose up to 5x1015 ions cm -2. Higher doses up to 5 × 1016 ions c m - : provided no further significant improvement. (2) The yttrium was located in all scales near to the interface with the metal. (3) Comparison of the effect of yttrium ion implantation with that of krypton implantation and also following post-implantation vacuum annealing established that the effect was chemical and not physical in origin. The implanted atoms probably became incorporated as Y203 particles and grain boundary segregants within the NiO scale. Over the temperature range (500-1000 °C) controlled by grain boundary diffusion, these blocked the cation transport responsible for scale growth. At l l00°C, where nickel oxidation becomes dominated by lattice diffusion, it is probable that scale growth was reduced also by the yttria particles acting as sinks for cation vacancies. (4) Lanthanum, ytterbium, cerium and silicon implantation were as effective as yttrium ion implantation in inhibiting nickel oxidation at 700 °C, probably by the same mechanism.

117

Acknowledgments The work described in this report is part of the longer term research carried out within the Underlying Programme of the UKAEA. The first author is grateful to the U K A E A for Vacation Associateship to carry out the investigations. Valuable discussions with Professor N. Nath (Kurukshetra University) and Dr. D. R Moon (Harwell Laboratory) are gratefully acknowledged.

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Ion Implantation into Metals, Pergamon, Oxford, 1982, p. 231. 4 F. H. Stott, Zhou Peide, W. A. Grant and R. P. M. Procter, Corros. Sci.,22(1982) 305. 5 N. Nath, N. Eyre and G. Dearnaley, Nucl. lnstrum. MethodsB, 10/11(1985)580. 6 J. C. Pivin, Proc. 9th Int. Congr. Metall. (orr., Vol. 4, National Research Council of Canada, Ottawa, 1984,

p. 10. 7 M. J. Bennett, H. E. Bishop, P. R. Chalker and A. T. Tuson, Mater. Sci. Eng., 90 (1987) 177. 8 J. Lindhard, M. Scharff and H. E. Schiott, Mat-Fys. Medd. K. Dan. Vidensk. Selsk., 33(1963) 14. 9 A. Atkinson, R. I. Taylor and A. E. Hughes, Philos. Mag. A, 45 (1982) 823. 10 A. A. Moosa and S. J. Rothman, Oxid. Met., 24 (1985) 133. 11 C.M. Cotell, G. J. Yurek, R. J. Hussey, D. F. Mitchell and M.J. Graham, J. Electrochem. Soc., 134 (1987) 1871. 12 D. P. Moon and M. J. Bennett, UKAEA Rep. AERER12757, 1987, (Harwell, Didcot); to be published in Mater. Sci. Forum. 13 A. Galerie, M. Caillet and M. Pons, Mater. Sci. Eng., 69 (1985) 329.