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Oxygen diffusion studies in oxide scales thermally grown or deposited on mechanically loaded metallic surfaces (MS-P2)
Nuclear Instruments and Methods in Physics Research B 181 (2001) 382±388
www.elsevier.com/locate/nimb
Oxygen diusion studies in oxide scales thermally grown or deposited on mechanically loaded metallic surfaces (MS-P2) P. Berger b
a,*
, L. Gaillet b, R. El Tahhann b, G. Moulin b, M. Viennot
b
a Laboratoire Pierre Sue, CEA/CNRS, CE SACLAY, F-91191 Gif sur Yvette Cedex, France Laboratoire ROBERVAL Centre de Recherches de Royallieu, Universit e de Technologie de Compi egne, BP 20529, F-60205 Compi egne Cedex, France
Abstract The high temperature growth of oxides on metallic surfaces mechanically in loading is not well understood yet. The knowledge of the growth of oxides on static surfaces and of the mechanical behavior of the metal/oxide system does not give account of the synergetic eects between the load and the growth of the oxide. The formation of cracks on the oxide scales, their healing and the role of the load on the oxygen diusion processes have been studied on pure nickel and zirconium samples in creep. The use of oxygen-18 to study the oxygen diusion and the determination of local oxygen-18 diusion pro®les with use of the nuclear reaction 18 O(p, a)15 N show a sharp in¯uence of the load. The application of the load induces an increase of the oxygen diusion coecients until two orders of magnitude (typically from around 10 15 cm2 s 1 to around 10 13 cm2 s 1 in NiO thermally grown on nickel monocrystals). However, this enhancement decreases with the increase of the load. As the oxide scales are multilayered and as spalling and regrowth may occur, RBS mapping of the local thickness of the oxide stripes is also performed. This technique helps in understanding the formation of the microstructure and of the damaging process during the mechanical loading. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 81.65.Mq; 68.35.Fx; 81.70.Jb; 81.40.Np; 81.40.Lm; 07.79.)v Keywords: Oxidation; Diusion; Oxygen-18; Nickel; Nuclear microprobe; Creep; Periodic cracking
1. Introduction The trends in the ®eld of high temperature metallic corrosion are now to take into account the ``environmental eects'' which means that the corrosion sequences must be as representative as possible of the real aggressive environment. The
*
Corresponding author. Tel.: +33-1-6908-85-12; fax: +33-16908-69-23. E-mail address: [email protected] (P. Berger).
environment is no longer described by the atmosphere only, but by all the kinds of constraints a metallic part may undergo (mechanical, thermal, etc.). Those constraints are not independent. In particular, the eects of the coupling of high temperature oxidation and of a mechanical loading cannot be deduced from the mechanical behavior of bare samples and from classical corrosion studies in static conditions. Thanks to their high temperature creep resistance, metallic alloys based on elements such as nickel are widely employed in aeronautics (turbine
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 5 9 1 - 2
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
blades) or steam power plants (boilers, thermal exchangers) [1], but, from a fundamental point of view, the synergetic eects between the mechanical stresses and the oxidation are still not understood [2]. The presence of a thin oxide layer on the surface of a metallic sample aects its mechanical behavior [3,4]. When loaded in creep under oxygen, the rates of deformation measured for an oxidized sample dier from those observed for bare ones, although the thickness of the oxide scale is low compared to the whole section of the sample [5,6]. There is no doubt that this is not due to a mechanical eect but to a modi®cation of the creep mechanism, probably because of the modi®cation of the properties of the surface induced by the presence of the oxide (voids injection, dislocation blocking). Conversely, the growth of the oxide may be modi®ed upon the in¯uence of the mechanical loading, either because of the defects generated in the oxide layer (diusion enhancement, short circuits, etc.) [7] or because of a possible eect of the strain on the diusion coecients (oxygen or nickel). Since the oxidation mechanisms and the mechanical behavior are closely linked, synergetic eects may be observed. A fundamental study of the oxidation of pure nickel has been undertaken, especially when loaded in creep. The use of nickel monocrystals has been motivated by the results of preliminary investigations conducted on polycrystalline nickel samples [9]. It has been found that the oxygen diusion in the oxide layer was not uniform from one grain of nickel to another. Orientation eects were suspected. This paper presents the results obtained on the oxygen diusion in nickel oxide scales, grown on nickel monocrystals either thermally or deposited by laser ablation. 2. Experimental 2.1. Loading set-up The nickel samples have been loaded in a speci®c set up, described elsewhere [8], which enables
383
mechanical loading (creep in the present case), in temperature (here 550°C) and under controlled atmosphere (vacuum, oxygen-16 or oxygen-18). Both the load and elongation are recorded. An additional acoustic emission device enables in situ monitoring of the damaging of the oxide scales. 2.2. Test samples The nickel samples have been extracted from nickel (1 1 1) or (1 0 0) monocrystals containing less than 10 ppm of impurities (mainly iron and carbon). The test tubes have been machined in a ``dog bone'' shape with a rectangular useful length of 12 mm. 2.3. Experimental procedures It has been observed in the early experiments that the morphology of the oxide scales was not always the same when the load was applied from the beginning of the oxidation sequence [9]. In order to stabilize the microstructure, the samples have been ®rst preoxidized under oxygen-16 without load (4 h) and then loaded in creep in the range 10±60 MPa. The ®nal sequence, still under load, consists in replacing the oxygen-16 by an oxygen-18 atmosphere for the study of the oxygen diusion in the oxide. As the morphologies observed on the thermally grown oxides were slightly dierent according to the orientation of the nickel, a comparison has been made with oxide scales deposited by laser ablation, with a microstructure independent of the nickel orientation. 2.4. Nuclear microprobe measurement setup Two kinds of local data have been searched, oxygen-18 diusion pro®les and layer thickness. The oxygen-18 pro®les have been obtained with the use of the 18 O(p, a)15 N nuclear reaction (spot measurements) and the thickness with 4 He RBS mapping (beam scanning). 780±820 keV 1 H and 3 MeV 4 He beams were produced with the 3.75 MeV van de Graa accelerator located at the Labora (Saclay/France). Typical beam toire Pierre SUE sizes were 10 3 lm2 (non-symmetrical shape to
384
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
follow the features of the microstructure of the oxide, see below) or 6 4 lm2 . An annular detector (mean angle 170°) was used to collect the particles, without covering with an absorbent foil, with a large aperture for NRA (60 msr) and a small one (6 msr) for RBS.
3. Results 3.1. Microstructure The thermally grown oxides present a duplex morphology, an outer layer composed of large basaltic grains and an inner one with small equiaxe grains (two or three times smaller). The mean diameter of the surface grains in the outer layer is between 100 and 200 nm (cf. Fig. 1). Without load, the whole oxide thickness is nearly 0:6 lm on (1 1 1) monocrystals and nearly 1 lm on (1 0 0) monocrystals. The deposited oxides have a uniform microstructure with small grains. When loaded, periodic parallel cracks perpendicular to the strain axis are observed on thermally grown oxides, from a critical load of, respectively, 10 and 15 MPa for (1 1 1) and (1 0 0) orientations (Fig. 2(a)). This kind of periodic cracking has been already described for a few metal/oxide systems and modeled assuming a rigid layer coupled with a
Fig. 2. Morphology of the oxide scale after creep in oxygen at 550°C. Ni(1 1 1) loaded at 10 MPa (a) and Ni(1 0 0) loaded at 60 MPa (b).
plastically deforming substrate [10,11]. For the highest loads, some oxide spallings are observed on the surface, especially for the (1 1 1) orientation above a critical stress of 35 MPa (Fig. 2(b)). The loaded deposited oxides present periodic cracks too, but they are not continuous across the whole wideness of the samples. 3.2. Oxygen diusion pro®les
Fig. 1. Oxide scales on the cross-sections of Ni(1 0 0) monocrystal oxidized for 4 h at 550°C.
Local examination of the 18 O pro®les is performed on undamaged oxide stripes. The advantage of the nuclear microprobe is to enable to explore the strands of oxide between the cracks
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
(distance between two successive cracks from 2.5 to 10 lm. The measurement of the diusion parameters is then not aected by the presence of the cracks. An asymmetrical shape of the beam (10 3 lm2 , parallel to the cracks) was often chosen to keep a high beam intensity (typically 300 pA). Near 800 keV, the cross-section of the 18 O(p, 15 a) N reaction varies slowly with the energy of incident protons. The alpha spectrum re¯ects then the 18 O concentration pro®le. Two typical shapes are observed: a simple 18 O concentration decrease from the surface or a double distribution, with an indepth accumulation (cf. Fig. 3). The estimation of the depth of the over concentration shows it is located near the interface between the oxide and and the metal. The treatment of the spectra is made with the SIMNRA software [12] which enables the determination of the 18 O diusion pro®le, assuming that the oxide scale is composed of NiO. This point has been checked from XRD measurements. The apparent oxygen diusion coecients are deduced from the concentration pro®les with the help of a classical model assuming a constant surface 18 O2 concentration (imposed by the atmosphere). The shape of the diusion pro®les may be expressed as follows [13]:
385
Fig. 4. Eect of the load on the apparent oxygen diusion coecients measured, respectively, for thermally grown and deposited oxides.
Cs Cs
Cx x erf p ; C1 2 Dt
where Cx is the 18 O concentration at the depth x, Cs the surface 18 O concentration, C1 the natural level of 18 O, D the diusion coecient, t the time of diusion and erf is the error function. The oxygen diusion coecients, deduced form the slope of the straight line Argerf Cx =Cs f x, are shown in Fig. 4. The oxygen diusion coecients are always higher in the presence of the mechanical load (about two orders of magnitude), but for the thermally grown oxides, surprisingly, the diusion coecients decrease with the raise of the load. For the (1 1 1) orientation, above a critical load near 40 MPa, the diusion coecients increase again, as if a cyclic phenomenon was observed. For the deposited oxides, the diusion coecient is high and independent of the load. 3.3. RBS mapping
Fig. 3. Alpha energy spectrum of the 18 O(p, a)15 N nuclear reaction induced by 795 keV protons on an oxide layer developed on Ni(1 0 0) under a 60 MPaload.
The oxide thickness is usually estimated from scanning electron microscopy (SEM) investigations after transverse cut of the samples. The major drawback of this method is the destruction of the sample and the diculty to estimate the local oxide thickness in the areas where the scales have lost their integrity. Because of the duplex microstructure of the oxide layer, the removal of the oxide strands may concern either the whole
386
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
thickness of the oxide or the outer scale only. RBS mapping is an elegant way to answer this question. Fig. 5 shows a map obtained with 3.0 MeV 4 He on a thermally grown oxide on nickel (1 1 1), covering an area of 110 160 lm2 . The energy window selected to draw the map corresponds to the RBS step of the nickel in the oxide (1700±2300 keV). The areas covered with nickel oxide appear dark whereas the bright ones correspond to thin oxide layers or to bare nickel. In this example, the RBS spectra of the bright areas shows that the oxide has been completely removed. Another advantage of performing RBS on these oxide scales is that the information provided on
the thickness diers in nature (mass thickness) from that measured with SEM (geometrical thickness). Access to the density of the oxide is then possible. The oxide density seems to depend on the load and the observed tendency is an increase of the density with the load. 4. Discussion The most striking point on the eects of the load on the oxygen diusion is the decrease of the diusivity with the increase of the load, observed for the thermally grown oxides. Whereas the initial
Fig. 5. RBS beam scanning on a 10 MPa loaded NiO/Ni(1 1 1). The energy window of the backscattered used for the map corresponds to the nickel step of the oxide. The other frame corresponds to the optical view of the same area.
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
increase was predictable, the following decrease is not intuitive since an increase of the defects (and then an increase of diusivity) should be rather observed. A tentative answer to this question may be obtained from stress relaxation experiments [14] conducted on (1 1 1) samples. At the end of the creep sequence, the elongation is stopped and the temporal decrease of the load is recorded to follow the relaxation of the stress [5,15]. The whole load is not relaxed and a steady-state stress level is reached. The amplitude of the relaxation depends on the intensity of the load (cf. Table 1). When compared to relaxation under vacuum, the presence of the oxide favors the relaxation. The part of the applied load which may be relaxed is a reversible mechanical energy which can be used to activate a transport process (dislocation glide for instance) [16]. A plot of the logarithm of the oxygen diusion coecient as a function of the relaxed stress reveals a linear correlation (cf. Fig. 6). Starting from the Arrhenius form of the diusion coecient, D D0 exp Q=RT ; with D0 the pre-exponential factor, Q the activation energy for diusion and T the temperature, Q may be considered as the sum Q DH
sV ;
with DH the enthalpy for the activated diusion, s the relaxed stress and V is the activation volume concerned by the diusion process. The linear reTable 1 Evolution of the relaxed stress and of the apparent 18 O diusion coecient as a function of the applied stress for Ni(1 1 1) Applied stress r (MPa)
Relaxed stress Ds (MPa)
D18 O (cm2 s 1 )
0 10 15 25 35 45 57
0 10 8 4 1 30 30
3 10 16 2:3 10 14 8 10 15 1:4 10 15 8:2 10 16 1 10 13 4:1 10 14
387
Fig. 6. Evolutions of the apparent oxygen diusion coecient as a function of the relaxed stress.
lation between Ln D and s shows that the relaxed stress seems to ``activate'' the diusion process.
5. Conclusion The nuclear microanalysis technique perfectly suits to oxygen diusion studies in this kind of oxide scales. The diusion pro®les extend in a range of a few 100 nm, within the analytical capabilities of the 18 O(p, a)15 N nuclear reaction. In addition, the lateral resolution of the microprobe enables measurements in oxide stripes of a few micrometers wide without disruption due to extended defects (cracks, spallations, etc.). The dependence of oxygen diusion upon the intensity of the load and especially upon the relaxed stress (studied for the (1 1 1) orientation) indicate that the oxide stripes are still in mechanical relation with the substrate after the occurrence of the cracking (acoustic emission measurements reveal that the cracks are formed at the beginning of the creep, before the introduction of the oxygen-18). The mechanisms which favor the relaxation (void injection from the oxide to facilitate the movements of the dislocations?) and those which enhance the oxygen diusion under load are still unknown but have probably a common origin. The correlation observed between the oxygen diusion coecient and the relaxed stress seems then justi®ed. Lastly, the use of the micro-RBS for local mass thickness determination is an attractive method to study
388
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
possible densi®cation process. Further investigations are in progress. References [1] R.A. Rapp, High Temperature Corrosion 6, NACE Publ., San Diego, 1983. [2] M.I. Manning, Corrosion and Mechanical Stress at High Temperatures, Applied Science Publishers, London, 1981. [3] J.M. Davidson, K. Aning, J.K. Tien, Properties of High Temperature Alloys, Electrochem. Soc. Ed. Princeton, 1976, p. 197. [4] R.H Bricknell, D.A. Woodford, Met. Trans. A 12A (1981) 425. [5] L.Gaillet, Ph.D. thesis, Universite de Technologie de Compiegne, France, June 2000. [6] A.F. Gourges, E. Andrieu, J. Phys. 9 (1999) 297.
[7] M. Sch utze, in: Protective Oxide Scales and their Breakdown, Wiley, Chichester, 1997, p. 61. [8] G. Moulin, P. Berger, Mat. Sci. Forum 207 (1996) 809. [9] G. Moulin, P. Arevalo, A. Salleo, Oxid. Met. 45 (1996) 153. [10] V.C. Jobin, R. Raj, S.L. Phoenix, Acta Metall. 40 (1992) 2269. [11] P. Hancock, J.R. Nicholls, K. Mahmood, Corr. Sci. 24 (1993) 979. [12] M. Mayer, Simnra User's guide, Technical Report IPP 9/ 113, Max-Planck-Institute f ur Plasmaphysik, Garching, Germany, 1997. [13] J. Philibert, Diusion and Mass Transport in Solids, Ed. de Physique, Les Ulis, 1991, p. 7. [14] G. Baur, P. Lehr: RCP 244 CNRS Internal Report, Vitry, France, 1980. [15] L. Gaillet et al., Eect of NiO scales on the creep behavior of Ni single crystals, Mat. Sci. Eng. A, submitted. [16] P. Duong, Ph.D. thesis, Pierre et Marie Curie University, Paris, 1977.
www.elsevier.com/locate/nimb
Oxygen diusion studies in oxide scales thermally grown or deposited on mechanically loaded metallic surfaces (MS-P2) P. Berger b
a,*
, L. Gaillet b, R. El Tahhann b, G. Moulin b, M. Viennot
b
a Laboratoire Pierre Sue, CEA/CNRS, CE SACLAY, F-91191 Gif sur Yvette Cedex, France Laboratoire ROBERVAL Centre de Recherches de Royallieu, Universit e de Technologie de Compi egne, BP 20529, F-60205 Compi egne Cedex, France
Abstract The high temperature growth of oxides on metallic surfaces mechanically in loading is not well understood yet. The knowledge of the growth of oxides on static surfaces and of the mechanical behavior of the metal/oxide system does not give account of the synergetic eects between the load and the growth of the oxide. The formation of cracks on the oxide scales, their healing and the role of the load on the oxygen diusion processes have been studied on pure nickel and zirconium samples in creep. The use of oxygen-18 to study the oxygen diusion and the determination of local oxygen-18 diusion pro®les with use of the nuclear reaction 18 O(p, a)15 N show a sharp in¯uence of the load. The application of the load induces an increase of the oxygen diusion coecients until two orders of magnitude (typically from around 10 15 cm2 s 1 to around 10 13 cm2 s 1 in NiO thermally grown on nickel monocrystals). However, this enhancement decreases with the increase of the load. As the oxide scales are multilayered and as spalling and regrowth may occur, RBS mapping of the local thickness of the oxide stripes is also performed. This technique helps in understanding the formation of the microstructure and of the damaging process during the mechanical loading. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 81.65.Mq; 68.35.Fx; 81.70.Jb; 81.40.Np; 81.40.Lm; 07.79.)v Keywords: Oxidation; Diusion; Oxygen-18; Nickel; Nuclear microprobe; Creep; Periodic cracking
1. Introduction The trends in the ®eld of high temperature metallic corrosion are now to take into account the ``environmental eects'' which means that the corrosion sequences must be as representative as possible of the real aggressive environment. The
*
Corresponding author. Tel.: +33-1-6908-85-12; fax: +33-16908-69-23. E-mail address: [email protected] (P. Berger).
environment is no longer described by the atmosphere only, but by all the kinds of constraints a metallic part may undergo (mechanical, thermal, etc.). Those constraints are not independent. In particular, the eects of the coupling of high temperature oxidation and of a mechanical loading cannot be deduced from the mechanical behavior of bare samples and from classical corrosion studies in static conditions. Thanks to their high temperature creep resistance, metallic alloys based on elements such as nickel are widely employed in aeronautics (turbine
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 5 9 1 - 2
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
blades) or steam power plants (boilers, thermal exchangers) [1], but, from a fundamental point of view, the synergetic eects between the mechanical stresses and the oxidation are still not understood [2]. The presence of a thin oxide layer on the surface of a metallic sample aects its mechanical behavior [3,4]. When loaded in creep under oxygen, the rates of deformation measured for an oxidized sample dier from those observed for bare ones, although the thickness of the oxide scale is low compared to the whole section of the sample [5,6]. There is no doubt that this is not due to a mechanical eect but to a modi®cation of the creep mechanism, probably because of the modi®cation of the properties of the surface induced by the presence of the oxide (voids injection, dislocation blocking). Conversely, the growth of the oxide may be modi®ed upon the in¯uence of the mechanical loading, either because of the defects generated in the oxide layer (diusion enhancement, short circuits, etc.) [7] or because of a possible eect of the strain on the diusion coecients (oxygen or nickel). Since the oxidation mechanisms and the mechanical behavior are closely linked, synergetic eects may be observed. A fundamental study of the oxidation of pure nickel has been undertaken, especially when loaded in creep. The use of nickel monocrystals has been motivated by the results of preliminary investigations conducted on polycrystalline nickel samples [9]. It has been found that the oxygen diusion in the oxide layer was not uniform from one grain of nickel to another. Orientation eects were suspected. This paper presents the results obtained on the oxygen diusion in nickel oxide scales, grown on nickel monocrystals either thermally or deposited by laser ablation. 2. Experimental 2.1. Loading set-up The nickel samples have been loaded in a speci®c set up, described elsewhere [8], which enables
383
mechanical loading (creep in the present case), in temperature (here 550°C) and under controlled atmosphere (vacuum, oxygen-16 or oxygen-18). Both the load and elongation are recorded. An additional acoustic emission device enables in situ monitoring of the damaging of the oxide scales. 2.2. Test samples The nickel samples have been extracted from nickel (1 1 1) or (1 0 0) monocrystals containing less than 10 ppm of impurities (mainly iron and carbon). The test tubes have been machined in a ``dog bone'' shape with a rectangular useful length of 12 mm. 2.3. Experimental procedures It has been observed in the early experiments that the morphology of the oxide scales was not always the same when the load was applied from the beginning of the oxidation sequence [9]. In order to stabilize the microstructure, the samples have been ®rst preoxidized under oxygen-16 without load (4 h) and then loaded in creep in the range 10±60 MPa. The ®nal sequence, still under load, consists in replacing the oxygen-16 by an oxygen-18 atmosphere for the study of the oxygen diusion in the oxide. As the morphologies observed on the thermally grown oxides were slightly dierent according to the orientation of the nickel, a comparison has been made with oxide scales deposited by laser ablation, with a microstructure independent of the nickel orientation. 2.4. Nuclear microprobe measurement setup Two kinds of local data have been searched, oxygen-18 diusion pro®les and layer thickness. The oxygen-18 pro®les have been obtained with the use of the 18 O(p, a)15 N nuclear reaction (spot measurements) and the thickness with 4 He RBS mapping (beam scanning). 780±820 keV 1 H and 3 MeV 4 He beams were produced with the 3.75 MeV van de Graa accelerator located at the Labora (Saclay/France). Typical beam toire Pierre SUE sizes were 10 3 lm2 (non-symmetrical shape to
384
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
follow the features of the microstructure of the oxide, see below) or 6 4 lm2 . An annular detector (mean angle 170°) was used to collect the particles, without covering with an absorbent foil, with a large aperture for NRA (60 msr) and a small one (6 msr) for RBS.
3. Results 3.1. Microstructure The thermally grown oxides present a duplex morphology, an outer layer composed of large basaltic grains and an inner one with small equiaxe grains (two or three times smaller). The mean diameter of the surface grains in the outer layer is between 100 and 200 nm (cf. Fig. 1). Without load, the whole oxide thickness is nearly 0:6 lm on (1 1 1) monocrystals and nearly 1 lm on (1 0 0) monocrystals. The deposited oxides have a uniform microstructure with small grains. When loaded, periodic parallel cracks perpendicular to the strain axis are observed on thermally grown oxides, from a critical load of, respectively, 10 and 15 MPa for (1 1 1) and (1 0 0) orientations (Fig. 2(a)). This kind of periodic cracking has been already described for a few metal/oxide systems and modeled assuming a rigid layer coupled with a
Fig. 2. Morphology of the oxide scale after creep in oxygen at 550°C. Ni(1 1 1) loaded at 10 MPa (a) and Ni(1 0 0) loaded at 60 MPa (b).
plastically deforming substrate [10,11]. For the highest loads, some oxide spallings are observed on the surface, especially for the (1 1 1) orientation above a critical stress of 35 MPa (Fig. 2(b)). The loaded deposited oxides present periodic cracks too, but they are not continuous across the whole wideness of the samples. 3.2. Oxygen diusion pro®les
Fig. 1. Oxide scales on the cross-sections of Ni(1 0 0) monocrystal oxidized for 4 h at 550°C.
Local examination of the 18 O pro®les is performed on undamaged oxide stripes. The advantage of the nuclear microprobe is to enable to explore the strands of oxide between the cracks
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
(distance between two successive cracks from 2.5 to 10 lm. The measurement of the diusion parameters is then not aected by the presence of the cracks. An asymmetrical shape of the beam (10 3 lm2 , parallel to the cracks) was often chosen to keep a high beam intensity (typically 300 pA). Near 800 keV, the cross-section of the 18 O(p, 15 a) N reaction varies slowly with the energy of incident protons. The alpha spectrum re¯ects then the 18 O concentration pro®le. Two typical shapes are observed: a simple 18 O concentration decrease from the surface or a double distribution, with an indepth accumulation (cf. Fig. 3). The estimation of the depth of the over concentration shows it is located near the interface between the oxide and and the metal. The treatment of the spectra is made with the SIMNRA software [12] which enables the determination of the 18 O diusion pro®le, assuming that the oxide scale is composed of NiO. This point has been checked from XRD measurements. The apparent oxygen diusion coecients are deduced from the concentration pro®les with the help of a classical model assuming a constant surface 18 O2 concentration (imposed by the atmosphere). The shape of the diusion pro®les may be expressed as follows [13]:
385
Fig. 4. Eect of the load on the apparent oxygen diusion coecients measured, respectively, for thermally grown and deposited oxides.
Cs Cs
Cx x erf p ; C1 2 Dt
where Cx is the 18 O concentration at the depth x, Cs the surface 18 O concentration, C1 the natural level of 18 O, D the diusion coecient, t the time of diusion and erf is the error function. The oxygen diusion coecients, deduced form the slope of the straight line Argerf Cx =Cs f x, are shown in Fig. 4. The oxygen diusion coecients are always higher in the presence of the mechanical load (about two orders of magnitude), but for the thermally grown oxides, surprisingly, the diusion coecients decrease with the raise of the load. For the (1 1 1) orientation, above a critical load near 40 MPa, the diusion coecients increase again, as if a cyclic phenomenon was observed. For the deposited oxides, the diusion coecient is high and independent of the load. 3.3. RBS mapping
Fig. 3. Alpha energy spectrum of the 18 O(p, a)15 N nuclear reaction induced by 795 keV protons on an oxide layer developed on Ni(1 0 0) under a 60 MPaload.
The oxide thickness is usually estimated from scanning electron microscopy (SEM) investigations after transverse cut of the samples. The major drawback of this method is the destruction of the sample and the diculty to estimate the local oxide thickness in the areas where the scales have lost their integrity. Because of the duplex microstructure of the oxide layer, the removal of the oxide strands may concern either the whole
386
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
thickness of the oxide or the outer scale only. RBS mapping is an elegant way to answer this question. Fig. 5 shows a map obtained with 3.0 MeV 4 He on a thermally grown oxide on nickel (1 1 1), covering an area of 110 160 lm2 . The energy window selected to draw the map corresponds to the RBS step of the nickel in the oxide (1700±2300 keV). The areas covered with nickel oxide appear dark whereas the bright ones correspond to thin oxide layers or to bare nickel. In this example, the RBS spectra of the bright areas shows that the oxide has been completely removed. Another advantage of performing RBS on these oxide scales is that the information provided on
the thickness diers in nature (mass thickness) from that measured with SEM (geometrical thickness). Access to the density of the oxide is then possible. The oxide density seems to depend on the load and the observed tendency is an increase of the density with the load. 4. Discussion The most striking point on the eects of the load on the oxygen diusion is the decrease of the diusivity with the increase of the load, observed for the thermally grown oxides. Whereas the initial
Fig. 5. RBS beam scanning on a 10 MPa loaded NiO/Ni(1 1 1). The energy window of the backscattered used for the map corresponds to the nickel step of the oxide. The other frame corresponds to the optical view of the same area.
P. Berger et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 382±388
increase was predictable, the following decrease is not intuitive since an increase of the defects (and then an increase of diusivity) should be rather observed. A tentative answer to this question may be obtained from stress relaxation experiments [14] conducted on (1 1 1) samples. At the end of the creep sequence, the elongation is stopped and the temporal decrease of the load is recorded to follow the relaxation of the stress [5,15]. The whole load is not relaxed and a steady-state stress level is reached. The amplitude of the relaxation depends on the intensity of the load (cf. Table 1). When compared to relaxation under vacuum, the presence of the oxide favors the relaxation. The part of the applied load which may be relaxed is a reversible mechanical energy which can be used to activate a transport process (dislocation glide for instance) [16]. A plot of the logarithm of the oxygen diusion coecient as a function of the relaxed stress reveals a linear correlation (cf. Fig. 6). Starting from the Arrhenius form of the diusion coecient, D D0 exp Q=RT ; with D0 the pre-exponential factor, Q the activation energy for diusion and T the temperature, Q may be considered as the sum Q DH
sV ;
with DH the enthalpy for the activated diusion, s the relaxed stress and V is the activation volume concerned by the diusion process. The linear reTable 1 Evolution of the relaxed stress and of the apparent 18 O diusion coecient as a function of the applied stress for Ni(1 1 1) Applied stress r (MPa)
Relaxed stress Ds (MPa)
D18 O (cm2 s 1 )
0 10 15 25 35 45 57
0 10 8 4 1 30 30
3 10 16 2:3 10 14 8 10 15 1:4 10 15 8:2 10 16 1 10 13 4:1 10 14
387
Fig. 6. Evolutions of the apparent oxygen diusion coecient as a function of the relaxed stress.
lation between Ln D and s shows that the relaxed stress seems to ``activate'' the diusion process.
5. Conclusion The nuclear microanalysis technique perfectly suits to oxygen diusion studies in this kind of oxide scales. The diusion pro®les extend in a range of a few 100 nm, within the analytical capabilities of the 18 O(p, a)15 N nuclear reaction. In addition, the lateral resolution of the microprobe enables measurements in oxide stripes of a few micrometers wide without disruption due to extended defects (cracks, spallations, etc.). The dependence of oxygen diusion upon the intensity of the load and especially upon the relaxed stress (studied for the (1 1 1) orientation) indicate that the oxide stripes are still in mechanical relation with the substrate after the occurrence of the cracking (acoustic emission measurements reveal that the cracks are formed at the beginning of the creep, before the introduction of the oxygen-18). The mechanisms which favor the relaxation (void injection from the oxide to facilitate the movements of the dislocations?) and those which enhance the oxygen diusion under load are still unknown but have probably a common origin. The correlation observed between the oxygen diusion coecient and the relaxed stress seems then justi®ed. Lastly, the use of the micro-RBS for local mass thickness determination is an attractive method to study
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