SURFACE AND INTERFACE ANALYSIS, VOL. 25, 948È951 (1997)

SIMS Image Analysis of Migration inD2O ZrO

2Thin Films

A. H. Clarke1 and N. S. McIntyre2,*1 Celestica Inc., 844 Don Mills Road, North York, Ontario M3C 1V7, Canada2 Surface Science Western, Western Science Centre, The University of Western Ontario, London, Ontario N6A 5B7, Canada

The di†usion of into and through oxide Ðlms on Zr–Nb alloys was investigated using secondary ion massD2O

spectrometry (SIMS) image analysis. In preparation for this, the microscopic structures of the oxide were studiedin relation to the grain structures in the underlying alloy. On alloys containing low concentrations of niobium (1–2.5wt.% ), the oxide was found to exhibit more localized growth, particularly above grain boundaries in the alloy. Suchoxide regions appeared to be considerably more porous to ingress. By contrast, the oxide Ðlm on the Zr 20D

2O

wt.% Nb alloy was found to be the most resistant to ingress ; no local regions of higher porosity could beD2O

found. 1997 John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 948È951 (1997)No. of Figures : 1 No. of Tables : 0 No. of Refs : 15

KEYWORDS: SIMS; thin Ðlm; imaging ; di†usion

INTRODUCTION

The surface oxides formed on the ZrÈ2.5% Nb alloy,used for pressure tubes in CANDU nuclear reactors, arebelieved to be beneÐcial to the integrity of the materialas a whole. In particular, these thin Ðlms haveZrO2appeared to be important in retarding the movement ofhydrogen, probably as water, from the exterior D2OÑuid into the bulk of the alloy, where they can formbrittle zirconium hydrides.1

In an attempt to provide further information on theprotective properties of the thin Ðlms on pressureZrO2tubes, several studies have monitored hydrogen di†u-sion into oxides formed on zirconium alloys.2h5 Some ofthese studies have involved alloys with compositionsranging from 1 wt.% to 20 wt.% Nb, which bound twoimportant metallurgical phases found in the ZrÈ2.5%Nb pressure tube : a-Zr (D1 wt.% Nb) and b-Zr (D20wt.% Nb). The results of hydrogen di†usion from earlierstudies have shown signiÐcant di†erences based onniobium concentration in the alloy.6 From this, it isproposed that the two a and b metallurgical phasesproduce oxides with entirely di†erent permeabilities towater. Such permeability could result from di†usionthrough inter- or intragranular pores whose dimensionsand frequency could be a†ected by factors such as oxidegrain size and grain boundary crystallographic orienta-tions. For a thin oxide growing primarily by aniontransport, one would expect that the grain structure andcomposition of the underlying metal would have astrong e†ect on the grain structures of the oxide phasesthat grow from them.

Secondary ion mass spectrometry has been used pre-viously to study hydrogen migration through oxide

* Correspondence to : N. S. McIntyre, Surface Science Western,Western Science Centre, The University of Western Ontario, London,Ontario N6A 5B7, Canada.

Contract grant sponsor : Technology Ontario.

Ðlms on pressure tube surfaces.7 The technique has alsodemonstrated unique strengths when applied to theinvestigation of metallurgical structures at or near thesurface.8,9 The high dynamic range of the secondary ionsignal imparts a capability to measure phase composi-tions involving a wide range of elements, includinghydrogen. Combined with excellent depth resolutionand very low detection limits, analysis of phase com-position within microscopic areas is possible. Secondaryion mass spectrometry is therefore an excellent tool forstudies of the relationships between oxide and metalgrain structures and should be able to identify thoseareas of the oxide grain microstructure that are mostporous to water ingress.

The objective of this work was to measure deuteriumdistribution within the oxide and metal phases follow-ing exposure to vapour at temperatures approx-D2Oimating reactor operating conditions. Of particularinterest was the e†ect of the underlying alloy grainstructure on deuterium distribution in the adjacentoxide structure. Grain structure for the alloys of interestin this study was previously well-characterised metallur-gically.10,11

EXPERIMENTAL

Specimens

Two series of ZrÈNb alloys were used throughout thisstudy : annealed (0.5, 1.0 and 20% Nb) and quenched(1.0, 2.5, 5.0 and 20% Nb). Use of these two di†erentheat treatments allowed for investigation of di†erentgrain sizes and stabilities. While the annealing processmore closely matched the pressure tube heat-treatingprocess, the quenched alloys permitted investigation oflarge grained microstructures. The samples were pol-ished to a 1 km Ðnish and then oxidized in steam at 673K for 24 h. Following oxidation, the samples were

CCC 0142È2421/97/100948È04 $17.50 Received 20 March 1997( 1997 by John Wiley & Sons, Ltd. Accepted 31 July 1997

SIMS OF D2O MIGRATION IN ZrO2 949

exposed to gaseous at 300 ¡C for 6 h and a partialD2Opressure of 1.3 ] 10~3 Pa (1.0] 10~5 Torr).

Secondary ion mass spectrometry conditions

A Cameca IMS-3f secondary ion mass spectrometer wasused with a 60 lm, 0.8È1.0 lA caesium primary beamrastered over a sample area of 250 ] 250 lm2. Negativesecondary ions of 1H, 2D, 90Zr and were col-138ZrO3lected from a 150 lm diameter area. No voltage o†setwas used during the analysis so that the maximum sec-ondary ion yield could be imaged. Details of the SIMSdepth proÐling method and imaging instrumentationhave been reported elsewhere.12,13 Typically, the samplewas proÐled to a depth in the metal where the

signal was low and had reached a steady138ZrO3~state. Although this molecular ion is used as a highlysensitive indicator of oxidized zirconium, it does notimply the presence of such a structure in the solid.Several image depth proÐles were taken on each sampleto determine reproducibility.

The image depth proÐle is composed of a series of ionimages taken at regular intervals during sputtering.Qualitative information is achieved in each ion imageby the use of a computer-generated false colour scale.This colour scale can represent 16-bit intensity data(red/green scale) or 8-bit intensity data (yellow/orangescale). Selection of the colour scale is done to optimizecontrast in the data. Quantitative depth information isobtained from sputter rates.

For ease of understanding, the three-dimensionalSIMS images are presented as cross-sections. The sec-tions pass through a circular locus near the periphery ofthe image. In this way, a relatively large section can bemade, which is representative of as much of the near-surface structure as possible. The sections intersectgrain boundaries at di†erent angles ; thus, some bound-aries appear wider than others.

These sputter rates for (0.11] 10~1 nm nA~1ZrO2s~1) and Zr (1.59] 10~1 nm nA~1 s~1) were deter-mined using an oxidised ZrÈ2.5% Nb standard and aDektak proÐlometer.

RESULTS AND DISCUSSION

Prior to deuterium exposure, the oxide Ðlm on each ofthe samples was investigated. Figure 1 is a SEM photo-micrograph of the quenched ZrÈ2.5% Nb alloy follow-ing oxidation in steam. Two visibly di†erent forms ofoxide were identiÐed : a dark-grey oxide and a light-greyoxide. The dark-grey oxide was found most predomi-nantly on the grain surfaces, while the light-grey oxidewas observed on certain grain and twin boundaries.This light-grey phase-appears to be a less adherentporous oxide, often formed as “breakawayÏ oxidation.15Similar light-grey oxides were also observed on thequenched 1% and 5% Nb alloys but not on the 20%Nb alloy. Analysis of the oxide regions on the annealedalloys could not be veriÐed due to the small grain sizes.

Figure 1. The SEM image of an oxidized Zr–2.5% Nb alloy(magnification ¼150Ã). The curved lines at the periphery of theimage are from a marking tool used prior to oxidation. Enhancedoxidation is observed on some of the grain boundaries.

Quenched alloys

Plate 1 illustrates a two-dimensional vertical recon-struction of the quenched 1%, 2.5% and 5% Nb alloysfollowing exposure to The top layer of each imageD2O.represents the Ðrst image acquired and is thus near theoutermost surface. Subsequent layers are stackedsequentially beneath the top layer to create a visualdepth distribution of the ion analysed. The greenregions indicate the higher 8-bit intensities while the redrepresents the lower 8-bit intensities. From these verti-cal reconstructions, a number of observations can bemade. The oxide thickness of the three alloys varied sys-tematically with niobium content, i.e. increased oxidethickness occurred with increased Nb content in alloy.Further, the image for the three alloys shows anZrO3~uneven interface. “FingersÏ of localized ionZr/ZrO2intensity appear to extend into the bulk Zr alloy partic-ularly for the 2.5% and 5% Nb images. These localizedregions are consistent with grain boundary locations inthe underlying alloy. These Ðngers are believed to bedue to locally thickened oxides that grow immediatelyabove some grain boundaries in the metal (see Fig. 1).They appear to extend into the metal because a Ñatouter surface had been assumed in the image analysis.Surface roughness measurements using a Dektak proÐ-lometer prior to depth proÐling conÐrmed that theoxide was thicker over grain boundaries. However, thegrain boundary heights measured were smaller than theÐnger depths illustrated in Plate 1. A second likely con-tribution to the Ðngers could be that the light-greyoxide has a lower sputter rate than the dark-grey oxideregions. This di†erence in sputter rates could exacerbatethe uneven interfaces seen, for example, in image (c).Consequently, the presence of oxide Ðngers in the

image is most likely a combination of elevatedZrO3~grain boundaries prior to analysis in addition to di†er-ential sputtering. It is therefore not possible to infer anyoxide penetration into the alloy in this case.

The D~ images show di†erent degrees of penetrationof the oxide by water. The D~ signal is most intenseand penetrates further in those regions that correspond

( 1997 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 948È951 (1997)

950 A. H. CLARKE AND N. S. MCINTYRE

to the Ðngers. These high D~ regions are associ-ZrO3~ated with grain boundaries ; these oxides could be moredefective than those produced by the bulk grain phase,thus they could be more permeable to water. In the caseof oxide not associated with the Ðngers, the resistance towater ingress appears to be considerably greater. Thedegree of penetration of water appears to be consider-ably greater for the 1% alloy than the 2.5% or 5%alloys ; in no case does it appear that the water pen-etrates the entire thickness of this oxide phase andreaches the metal interface. However, it appears thatthere is evidence for a considerable amount of localizeddeuterium beneath the oxide in the 1% alloy. For thetwo regions on the left in Plate 1(a) and (b), whichextend below the interface, it cannot be determined withcertainty whether these regions are, in fact, part of aremnant of slow-sputtering oxide above the interface orare part of an oxidized grain boundary within the metal.Both oxide and deuterium signals are detected withinthe same region. However, in the case of the two local-ized deuterium signals on the right side of Plate 1(b), nocorresponding oxide signal was observed. This suggeststhat the deuterium observed represents a localizedmetal deuteride, which perhaps occurred by di†usionfrom a nearby grain boundary. In any case, this showsthat localized deuterium penetration of the metal inter-face has occured. It would not be unreasonable topropose that such penetration occurred via the morepermeable oxide on the grain boundary. In the case ofthe 2.5% and 5% alloys, some deuterium uptake by themetal is evident from the occasional D~ regions. Again,it is likely that such “breakthroughÏ of the interface thatdoes occur, does so via the more permeable oxide onthe grain boundaries.

Annealed alloys

Secondary ion mass spectrometry results for the oxida-tion and reactions of the annealed alloys (0.5%D2Oand 1.0% Nb) are illustrated in Plate 2. Oxide thick-nesses of these annealed alloys were found to be muchthinner than those on the quenched alloys ; also, no evi-dence of localized outward oxide growth was observedduring the SEM examination. In the SIMS analysis,however, Ðngers of oxide were noted for both the 0.5%and 1% Nb alloys. In contrast to the quenched alloys,there is good reason to believe that the Ðngers shown inPlate 2 actually represent localized extensions of theoxide into the alloy itself, possibly following grainboundaries. Our primary proof of this is that the Ðngersin this case do not extend vertically below the oxide, butrather trace paths of di†ering curvatures and sub-tending di†erent angles to the oxide ; this is what wouldbe expected for boundaries of submicron grain struc-tures that would intersect the surface at many di†erentangles. By contrast, the Ðngers for the annealed alloysall extend vertically downward from the oxide. For boththe 0.5% and 1% Nb annealed alloys, the penetration ofwater, as measured by the D~ signal, is seen to not haveextended to the metal/oxide interface, nor is there evi-dence of any signiÐcant D~ signal within the metal. Itis, however, clear that there is some localized penetrat-ion of water into the oxide, and that the regions wherethis occurs seem to be located above those features thatwe suggest are associated with grain boundaries. Thus,

just as was observed for the annealed alloys, thereappears to be more porous oxide regions associatedwith areas where extensive grain boundary oxidationhas occurred.

Plate 3 shows SIMS analysis of the oxidation andwater reactions of the annealed 20% Nb alloy. In con-trast to the previous alloys illustrated, the verti-ZrO3~cal reconstruction in part (a) does not show distinctlocalized regions at the oxide/metal interface. Distribu-tion of deuterium in the 20% niobium sample was alsomarkedly di†erent from any of the other alloys present-ed here. Image (b) shows a small localized amount ofdeuterium at the very surface of the oxide, but no sig-niÐcant penetration of the oxide. Some deuterium,however, is found in the metal itself. This can be attrib-uted to the uptake of water during the autoclave oxida-tion process. Although this water is primarily H2O,there is still a natural deuterium isotope abundance of0.015% that becomes detectable when large amounts ofthe H isotope are present. When the unoxidized alloy isÐrst exposed in the autoclave, there is only a thin nativeoxide surface Ðlm to prevent reaction of water vapourwith the metal at the interface. Thus, such reactions dooccur, to some extent, and they result in the productionof metal hydrides (and deuterides). As the anodicallyproduced oxide grows, the di†usion pathways of waterto the interface are cut o† and no further hydride entersthe metal. The oxide formed on the 20% alloy underthese conditions appears to have very low permeabilityto water ingress and to exhibit no localized growth fea-tures.

Do the above results support the suggestion thatoxides on the b-Nb phase (typiÐed by the 20% alloy)have a di†erent resistance than those on a-Zr to hydro-gen (deuterium) uptake by the alloy? The much higherresistance of the oxide on 20% Nb to permeabilityD2Ocompared to that on the 1% Nb alloy seems to suggestthat this is so. The nature of the oxide on those alloyscontaining primarily a-Zr alloys seems more complexthan that on the b-Nb alloy. These alloys low in Nbconcentration have complex oxidation mechanismsinvolving strong localized nucleation and growth atgrain boundaries. These locally generated oxides seemto have di†erent porosities than the oxides growing onthe grain face itself. As the Nb content increases (as inPlate 1), the extent of localized oxidation behaviourdiminishes, probably because of an increase in the con-centration of the b-Nb phase. The generalized oxide onthe b-Nb phase grows more rapidly than that on thea-Zr and clearly involves oxygen anion ingress ratherthan water to promote such growth. By contrast, wateringress may be involved in the localized growth ofoxides on a-Zr.

Exaggerated oxidation occurs on all quenched alloys,compared to their annealed analogues. This is probablyrelated to the increase in defects introduced by theformer procedure.

CONCLUSIONS

(1) Oxides grown on low Nb/Zr alloys (typifying ana-Zr phase structure) have higher permeabilities towater than those grown on higher Nb/Zr alloys,where the b-Nb phase becomes more important.

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 948È951 (1997) ( 1997 John Wiley & Sons, Ltd.

SIMS OF D2O MIGRATION IN ZrO2 951

(2) The low Nb alloys have more complex oxidationmechanisms. At least two di†erent surface oxidestructures are identiÐed : one is localized above grainboundaries and is more permeable to water ingress,presumably because of a more defective structure.

(3) Low Nb/Zr alloys produced by quenching havehigher oxidation rates and undergo more localizedgrowth than equivalent annealed alloys.

(4) Steam oxidation at 400 ¡C of the 20% Nb/Zr alloyinitially allows some permeation of water into the

alloy, but continued oxidation results in an oxideÐlm that is not permeable to water.

Acknowledgements

The authors wish to acknowledge the Ðnancial support provided bythe Ontario government through the University Research IncentiveFund, a component of Technology Ontario. Dr V. Urbanic of TheChalk River Laboratory of AECL Research is thanked for thesamples used in this study and for useful discussions.

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( 1997 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 948È951 (1997)