The initial oxidation of ε-Fe2N1−x: an XPS investigation

Applied Surface Science 136 Ž1998. 238–259

The initial oxidation of ´-Fe 2 N1yx : an XPS investigation P.C.J. Graat a

a,),1

, M.A.J. Somers

a,2

, E.J. Mittemeijer

a,1

Laboratory of Materials Science, Delft UniÕersity of Technology, Rotterdamseweg 137, NL-2628 AL Delft, Netherlands Received 20 April 1998; accepted 14 July 1998

Abstract The initial oxidation of a-Fe and ´-Fe 2 N1yx , subjected either to a sputter cleaning pretreatment or a sputter cleaning plus additional annealing pretreatment, was investigated with XPS. The samples were oxidised at pO 2 s 8 P 10y5 Pa and temperatures ranging from 300 to 600 K. From the Fe 2p and O 1s spectra the thickness and composition of the oxide film was determined. The composition of the oxide films formed on a-Fe and on ´-Fe 2 N1yx was only a function of oxidation temperature and film thickness and was independent of the composition or pretreatment of the substrate. Analysis of the N 1s spectra provided information on the electric charge on nitrogen atoms and the depth distribution of nitrogen. A linear relation was found between the N 1s electron binding energy and the nitrogen concentration in the substrate. Upon oxidation of the iron nitride, nitrogen atoms accumulated underneath the oxide film. If the nitrogen concentration at that location exceeded the maximum solubility of nitrogen in ´-Fe 2 N1yx an additional N peak appeared in the N 1s spectrum, which indicated the formation of a nitrogen containing phase other than ´-Fe 2 N1yx at the nitride–oxide interface. For the oxygen exposures applied, the oxide-film thickness decreased with increasing nitrogen concentration in the substrate. The effect of nitrogen in the substrate on the initial oxidation was evaluated from the results. q 1998 Elsevier Science B.V. All rights reserved. PACS: 81.65.M; 75.50.B; 81.05.J; 79.60 Keywords: Oxidation; Iron; Iron nitride; Iron oxide; Nitrogen; XPS

1. Introduction The presence of nitrogen in iron and iron alloys has beneficial effects on the mechanical and electro-

) Corresponding author. Tel.: q49-711-2095-339; Fax: q49711-2095-320; E-mail: [email protected]. 1 Present address: Max Planck Institute for Metals Research, Seestraße 92, D-70174 Stuttgart, Germany. 2 Present address: Department of Manufacturing Engineering, The Technical University of Denmark, Building 204, DK-2800 Lyngby, Denmark.

chemical properties Žsee, e.g., Refs. w1–11x.. A technique which is often applied to introduce nitrogen in the surface region of ironrsteel workpieces is thermochemical nitriding, which brings about an iron nitride based compound layer, typically composed of ´-Fe 2 N1yx and gX-Fe 4 N Žsee Fig. 1.. In several commercial nitriding treatments, nitrided workpieces are subjected to a post-oxidation treatment to form an iron oxide layer at the surface. The iron oxide layer significantly enhances the corrosion resistance of the workpiece, in contrast with iron oxide layers on ironrsteel Žsee, e.g., Refs. w12–17x..

0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 4 5 - 6

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239

Fig. 1. Light-optical micrograph Žinterference contrast. of a cross section of the nitrided sample, showing the ´-Fe 2 N1yx layer on top of the X a-Fe substrate with an intermediate layer of g -Fe 4 N.

Various mechanisms have been proposed to explain the effect of nitrogen on the oxidation behaviour and the resulting electrochemical properties of iron and iron nitrides. Ži. Upon oxidation of iron nitrides, iron atoms of the nitride are incorporated in the developing oxide and consequently nitrogen atoms become abundant. It was shown that nitrogen enrichment occurred directly beneath the oxide layer on nitrogen-containing stainless steel w8x, on gX-Fe 4 N w18x and on ´-Fe 2 N1yx w19,20x. For the case of stainless steel this nitrogen enrichment was concluded to be responsible for the improvement of the resistance to Žpitting. corrosion w8x. Žii. The oxide-growth kinetics at room temperature for several Fe–N phases showed a decrease of the initial oxidation rate with increasing nitrogen content, which was ascribed to an electrostatic potential difference over the oxide film Žsurface potential minus interface potential., becoming less negative for increasing nitrogen content in the substrate region adjacent to the oxide film. In a later stage the oxidation rate increased with increasing nitrogen content, which was ascribed to a defect concentration becoming larger with increasing nitrogen content in the substrate region adjacent to the oxide film w21x. Žiii. The development of an oxidised form of N, probably NO, during oxidation of iron–nitrogen

phases w10,11,22x was suggested to promote the presence of Fe 3q, relative to Fe 2q, in the oxide film w10x, which would result in a more stable oxide film w10,11x. Živ. An iron nitride layer on a ferrite substrate was proposed to serve as an intermediate layer that bridges the misfit between the oxide and the substrate, and thereby reduces the growth strains in the oxide layer w9x. The opposite signs of the growth strains in the magnetite ŽFe 3 O4 . and hematite Ž a-Fe 2 O 3 . sublayers were reported to contribute to the buckling observed for an oxide layer on pure a-Fe w23x. Thus, the absence of buckling observed for the oxide layer on ´-Fe 2 N1yx , ascribed to the relatively small growth strains in the magnetite and hematite sublayers and to a relatively small grain size of the iron nitride ‘substrate’, was concluded to be a prerequisite for the improved corrosion resistance of nitrided and subsequently oxidised ferritic workpieces w23x. Žv. The observed finer microstructure of a nitrided iron surface that is composed of austenite Ž g-Fe. and martensite Ž aX-Fe. after nitriding, as compared to untreated surfaces could lead to a larger number of oxide nucleation sites and consequently to a finer oxide-grain size. Further, upon oxidation of a nitrided surface, vacancies, which arise as a result of oxide growth by outward diffusion of iron cations, were shown not to form voids at the metal–oxide

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interface, as in the case of an untreated surface, but at some depth into the metal. Both effects would lead to improved adhesion of the oxide layer to the metal substrate, and thus to a better corrosion resistance w6,24x. Further, it is remarked that differences in the effect of nitrogen on the oxidation behaviour and electrochemical properties can result from differences in the nitriding treatment Žsee, e.g., Ref. w16x., the pretreatment of the nitrided sample, and the conditions of the oxidation or electrochemical treatment. The present work focuses on the initial oxidation of ´-Fe 2 N1yx , subjected to either a sputter cleaning pretreatment or a sputter cleaningq annealing pretreatment, at a low oxygen pressure Ž pO 2 s 8 P 10y5 Pa. and at temperatures ranging from 300 K to 600 K. From the XPS Fe 2p and O 1s spectra the thickness and composition Žamounts of Fe 2q, Fe 3q and O 2y . of the oxide film were determined. Analysis of the N 1s spectra provided information on the Žre.distribution of nitrogen underneath the oxide layer. The results were compared with similar oxidation experiments applied to a-Fe.

mm thick polycrystalline ´-Fe 2 N1yx layer, with an average grain diameter of 4 mm, was formed on top of a 1.5 mm thick polycrystalline gX-Fe 4 N layer Žsee Fig. 1.. The nitrogen content in the ´-Fe 2 N1yx layer was determined at 27.0 at% Ži.e., x s 0.26, corresponding with an N concentration of 26.2 N atoms nmy3 . from the a and c lattice parameters of the hexagonal ´ phase as determined with X-ray diffraction Žsee Ref. w25x for the dependence of the a and c lattice parameters on the N content.. The a-Fe sample was cut from a sheet of polycrystalline a-Fe Ž- 0.03 at% impurities., ground with P1200 sand paper, chemically polished in Kawamura’s reagent, and mechanically polished with a suspension of 0.05 mm Al 2 O 3 grains in distilled water. In between and after these preparation steps the sample was ultrasonically cleaned in ethanol andror 2-propanol. Prior to oxidation the a-Fe sample was annealed for 30 min at 1100 K in the UHV preparation chamber Žsee below.. Light-optical microscopy Žplanar view. indicated an average grain diameter of 80 mm.

2. Experimental

The samples were introduced in the UHV equipment, which consisted of a preparation chamber with a UHV connection to the XPS chamber Žbase pressure in both chambers - 10y7 Pa.. Oxidation and annealing treatments described below were performed in the preparation chamber; sputter cleaning and XPS measurements were performed in the XPS chamber. Prior to oxidation of the ´-Fe 2 N1yx sample one of the following two pretreatments was applied in UHV: Ži. sputter cleaning with 2 keV Arq ions; Žii. sputter cleaning with 2 keV Arq ions q 15 min annealing at 573 K. During sputtering, the ion beam, with a flux of f 1.5 mArmm2 , was rastered over an area of 6 = 6 mm2 at an angle of incidence of 408 with respect to the surface normal. The sputter time was varied from 15 min to 60 min depending on the thickness of the previously formed oxide film that had to be removed. Annealing in pretreatment Žii. was performed to remove sputter-induced damage. The sample was

2.1. Sample preparation The ´-Fe 2 N1yx sample was prepared from a 7 mm thick sheet of polycrystalline a-Fe Ž- 0.15 at% impurities., which was cold rolled to a thickness of 1 mm and subsequently annealed at 923 K in N2 at 1 atm for 90 min. A 10-mm outer diameter disc was cut from the sheet and ground with P1200 sand paper and chemically polished in Kawamura’s reagent Ž80 vol% H 2 O 2 , 15 vol% H 2 O, 5 vol% HF. for 90 s. The sample was nitrided in a 71.8 vol% NH 3 q28.2 vol% H 2 gas mixture at 838 K for 3 h. After nitriding the sample was mechanically polished with a suspension of 0.05 mm Al 2 O 3 grains in distilled water. In between and after these preparation steps the sample was ultrasonically cleaned in ethanol andror 2-propanol. In a cross section of the sample Žprepared after the oxidation experiments. it was observed with light-optical microscopy that a 10

2.2. Pretreatment

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

heated by light, which was focused on the back side of the sample by a parabolic mirror. Typically, the annealing temperature was reached within 2 min. During cooling the temperature of the sample, expressed in degrees centigrade, decreased typically with a factor 2 within 15 min. The temperature of the sample was measured with a NiAlrNiCr thermocouple, which was spot welded to the top side of the sample. The annealing temperature was chosen relatively low to prevent the development and subsequent desorption of N2 at the ´-Fe 2 N1yx surface which was suggested to occur at temperatures exceeding 673 K w25x. The same pretreatments were applied to the a-Fe sample prior to oxidation, except that an annealing temperature in pretreatment Žii. was chosen at 700 K. 2.3. Oxidation treatment After the pretreatment, a series of oxidation experiments was performed using the two samples. They were oxidised Žone at a time. in pure O 2 at a total pressure of 8 P 10y5 Pa, as measured with a Balzers IMG300 pressure gauge, and at temperatures, Tox , ranging from 323 to 573 K. Heating to the oxidation temperature was performed as described above for the annealing in pretreatment Žii.. The oxidation time was varied from 5 to 60 min. 2.4. XPS measurements Both after pretreatment and after oxidation, the Fe 2p, N 1s and O 1s XPS spectra were recorded with a Perkin-Elmer PHI 5400 ESCA, applying unmonochromatised Al K a radiation Ž hn s 1486.6 eV.. A step size of 0.10 eV and a constant pass energy of 35.75 eV were applied. The energy scale of the hemispherical spectrometer was calibrated according to the procedure described in Ref. w26x. The intensities were corrected for the spectrometer function as provided by the manufacturer. In general the detection angle, Q , as measured with respect to the surface normal, was 458; occasionally spectra were recorded at Q s 158, 358 and 608. The angle between the incident photons and the detected electrons was 548. The analysed area was 0.95rcosŽQ . mm2 .

241

2.5. Quantification of XPS spectra The experimental XPS spectra contain a contribution due to the instrumental noise and the tails of peaks at higher kinetic energies. It is assumed that this contribution can be removed adequately by subtracting the minimum intensity value of each spectrum from all intensity values measured. The remaining intensities of the Fe 2p spectra measured after pretreatment were corrected for the background due to inelastically scattered Fe 2p electrons according to Tougaard’s formalism Žsee, e.g., Refs. w27–29x.. Assuming a homogeneous depth distribution of iron, and using the universal cross section for inelastic electron scattering, with the socalled parameters B and C taken as 2866 eV 2 and 1643 eV 2 , respectively w30x, this background was calculated and subtracted in the binding energy range of 693.0 to 748.0 eV. The concentrations of Fe atoms in a-Fe and ´-Fe 2 N1yx were taken 85 Fe atoms nmy3 Žobtained from the lattice parameter of a-Fe w31x. and 71 Fe atoms nmy3 Žobtained from the a and c lattice parameters of ´-Fe 2 N1yx with x s 0.26 w25x., respectively. The inelastic mean free path ŽIMFP. of Fe 2p electrons in a-Fe and ´-Fe 2 N1yx were taken 1.36 and 1.42 nm, respectively Žsee Appendix A.. The peak area was subsequently determined as the sum of the remaining intensities in the energy range considered. The position of the Fe 2p peak was determined by fitting a Doniach–Sunjic line-shape function w32x to the top of the Fe 2p 3r2 peaks in a range of "2.0 eV around the maximum intensity. The inaccuracy in the peak position as determined by this procedure was estimated to be "0.05 eV. After oxidation the Fe 2p spectrum was composed of overlapping peaks from Fe atoms in the substrate and Fe cations in the oxide film. The Fe 2p spectra were unravelled according to the procedure presented in Refs. w33–35x as follows. Fe 2p reference spectra of the different Žoxidised. states of Fe, i.e., Fe 0 , Fe 2q and Fe 3q, were obtained from the cleaned substrates, from an FeO sample and from an a-Fe 2 O 3 sample, respectively. The background due to inelastically scattered electrons was subtracted from these reference spectra as described above for the Fe 2p spectra of the cleaned substrates. The concentration of Fe cations in the FeO and a-Fe 2 O 3 reference

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oxides are 40.0 Fe 2q cations nmy3 and 46.6 Fe 3q cations nmy3 , respectively w35x. The average value of the IMFP of Fe 2p electrons in FeO, Fe 3 O4 and a-Fe 2 O 3 , i.e., 1.63 nm Žsee Appendix A., was taken as the IMFP in the reference oxides. Then, the contributions of Fe 0 , Fe 2q and Fe 3q Žincluding their backgrounds. to an overall spectrum of oxidised iron or iron nitride were calculated, in the binding energy range of 693.0 to 748.0 eV, from the corresponding intrinsic reference spectra applying the Žgeneralised; see Ref. w36x. method of Tougaard. Assuming that an oxide film with constant thickness and a homogeneous distribution of Fe 2q and Fe 3q had formed, three parameters are unknown in this calculation, i.e., the oxide-film thickness, d ox , and the concentrations of Fe 2q and Fe 3q cations in the oxide film, C Fe 2q and C Fe 3q. Optimal values of these parameters were found by minimising the squared difference, x 2 , between the measured Fe 2p spectrum and the reconstructed spectrum, i.e., the sum of the calculated contributions of Fe 0 , Fe 2q and Fe 3q. As an estimate for the statistical error of the values of d ox , C Fe 2q and C Fe 3q the deviations from the optimal values were taken of which the corresponding reconstructed spectra resulted in values of x 2 which deviated less than 10% from the minimum value of x 2 Žsee also Ref. w35x.. Since the N 1s and O 1s spectra had a relatively small contribution of inelastically scattered electrons, the intensities of these spectra were insensitive to the method of background calculation. Therefore, a simple Shirley type background w37x was subtracted from the N 1s spectra in the binding energy range of 390.0 to 405.0 eV, and from the O 1s spectra in the binding energy range of 525.0 to 540.0 eV. The peak area was subsequently determined as the sum of the remaining intensities in the energy range considered. The position of the N 1s peak and of the O 1s peak was determined by fitting a Doniach–Sunjic lineshape function w32x to the top of the peak in a range of "1.0 eV around the maximum intensity. The inaccuracy in the peak position as determined by this procedure was estimated to be "0.05 eV. In the determination of the amount of an element present in the sample from a corresponding XPS peak the usual expression was adopted for the exponential decrease of the number of photoelectrons reaching the surface without energy loss with dis-

tance travelled by the photoelectrons along a straight line from the depth of origin of the photoelectron to the surface Žsee, e.g., Ref. w38x.. Taking the depth dependence of the inelastic mean free path ŽIMFP. of the electrons into account w36,39x, the intensity of the XPS spectrum is given by: `

I s Fs

ž

d zX

z

Hzs0C Ž z . exp yHz s0 lŽ z . cos Ž Q . X

X

/

dz

Ž 1. where I is the peak area of the XPS peak considered; C Ž z . is the concentration of the corresponding element as a function of depth z below the surface; lŽ z . is the IMFP of the photoelectrons corresponding to the XPS peak as a function of depth; s is the photoemission cross section, i.e., the probability for emission of a photoelectron per incident photon, and F is a factor depending on the instrument settings Žincident photon flux, analysed area, etc...

3. Results and interpretation 3.1. The pretreated substrates Fe 2p and N 1s spectra as recorded from the a-Fe and ´-Fe 2 N1yx samples prior to oxidation provide information on the influence of the pretreatment on the composition of the surface region of the two substrates. 3.1.1. a-Fe The areas of the Fe 2p peaks recorded from a-Fe after the sputter cleaning pretreatment and after the sputter cleaningq annealing pretreatment did not differ significantly and were independent of the detection angle, Q , within experimental accuracy. Hence, within the analysed volume, the Fe concentration was the same after both pretreatments and Fe was distributed homogeneously. The average positions of the Fe 2p 3r2 peaks after the two different pretreatments differed slightly: 706.89 eV after sputter cleaning and 706.95 eV after annealing. These positions fall within the range of binding energies reported for a-Fe in the literature Že.g., see the overview in Ref. w40x.. The slightly larger binding energy of the Fe 2p electrons after annealing may be

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

related to the presence of segregated nitrogen at the surface Žcf. effect of N in Fe–N phases on the Fe 2p binding energy as discussed below.. Annealing of sputter cleaned a-Fe resulted in the appearance of a N 1s peak in the recorded spectra Žsee Fig. 2., most probably due to segregation of nitrogen from the bulk to the surface upon annealing Žsee, e.g., Refs. w22,41–46x.. The average position of the N 1s peak after sputter cleaningq annealing was 397.07 eV, which is well within the range of binding energies reported for the N 1s peak of nitrogen segregated to the surface of a-Fe Ž397.0 eV w45x to 397.2 eV w22,43x.. The area of the N 1s peak increased with increasing detection angle Q Žsee Fig. 2. indicating that the nitrogen atoms were indeed located relatively close to or at the surface. Considering the segregated N atoms to be concentrated in a layer of thickness d seg on a flat Fe surface, the area of the N 1s peak, I N 1s , is given by: IN 1s I N 1s ,ref

s

C N ,seg lseg Ž N 1s . C N ,ref l ref Ž N 1s .

ž

= 1 y exp y

d seg

lseg Ž N 1s . cos Ž Q .

/ Ž 2.

where I N 1s,ref is the N 1s peak area of a reference sample; C N,seg and C N,ref are the concentrations of

Fig. 2. N 1s spectra of ´-Fe 2 N1yx measured, at two detection angles ŽQ ., after sputter cleaning and after sputter cleaningq annealing and of a-Fe after sputter cleaningqannealing.

243

nitrogen in the layer and in the reference sample, respectively, and lseg ŽN 1s. and lref ŽN 1s. are the inelastic mean free paths ŽIMFP. of emitted N 1s electrons in the layer and in the reference sample, respectively. Eq. Ž2. was derived from Eq. Ž1., assuming a homogeneous distribution of N in the segregated layer and assuming that the photoemission cross section, s , of N 1s electrons and the instrument factor, F, was the same for the layer of segregated N and for the reference sample. The N 1s peak area and N concentration of ´-Fe 2 N1yx Žeither sputter cleaned or sputter cleanedq annealed; see below. were used for I N 1s,ref and C N,ref , respectively; the IMFP of N 1s electrons in ´-Fe 2 N1yx , i.e., 1.82 nm Žsee Appendix A., was used for l ref ŽN 1s.. The peak areas, I N 1s , were determined experimentally at Q s 158, 358 and 458. Three parameters remain to be determined, C N,seg , lseg and d seg . Eq. Ž2. does not allow to determine these param eters independently. However, d segrlseg ŽN 1s. and C N,seg P lseg ŽN 1s. can be determined by fitting Eq. Ž2. to the N 1s peak areas obtained at the three detection angles. The average values obtained, using both sputter cleaned and sputter cleanedq annealed ´-Fe 2 N1yx as references, were d segrlseg ŽN 1s. s 0.044 and C N,seg P lseg ŽN 1s. s 242 N atoms nmy2 . Hence, as follows by multiplication of these factors, the surface concentration of segregated nitrogen atoms, C N,seg P d seg , was 10.6 N atoms nmy2 . Segregation of N to a-Fe Ž100. single crystal surfaces was reported to result in a cŽ2 = 2. structure of N w41x, which would correspond to a surface concentration of 6.1 N atoms nmy2 Žas calculated from the lattice parameter of a-Fe, i.e., 0.28665 nm w31x.. The larger concentration of nitrogen in the segregated layer as found in the present work may be attributed to a larger amount of available sites per unit area for nitrogen at the surface due to surface roughness and grain boundaries intersecting the surface. 3.1.2. ´-Fe2 N1y x The areas of the Fe 2p spectra recorded from ´-Fe 2 N1yx after sputter cleaning only Žpretreatment Ži.. and after sputter cleaningq annealing Žpretreatment Žii.. were practically equal and were also independent of the detection angle. Hence, within the

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analysed volume, the Fe concentration was the same after both pretreatments and Fe was distributed homogeneously. The average positions of the Fe 2p 3r2 peaks after the two different pretreatments differed slightly: 707.01 eV after sputter cleaning and 707.13 eV after annealing. No data for the Fe 2p 3r2 binding energies of clean ´-Fe 2 N1yx have been reported before. The difference between the two pretreatments in position of the Fe 2p 3r2 peak may be related to the corresponding difference in nitrogen concentration in the surface adjacent region of the substrate Žsee below.. The N 1s spectra measured at two detection angles are given in Fig. 2 for the two pretreatments. The intensity after sputter cleaningq annealing is about twice the intensity after sputtering only. Annealing at 573 K for 5 min and for 30 min, instead of the usual 15 min, yielded approximately the same

intensity for the N 1s spectra. The areas of the N 1s peaks recorded at Q s 158 and Q s 458 did not differ significantly, suggesting a homogeneous distribution of N in the volume analysed. The average position of the N 1s peak after only sputter cleaning was 397.34 eV; the average position of the N 1s peak after sputter cleaningq annealing was 397.81 eV Žexperimental uncertainty in peak position is "0.05 eV; see Section 2.. These positions fall within the range of binding energies reported for the N 1s peak of ´-Fe 2 N1yx Ž397.3 eV w10x to 398.1 eV w43x.. The positions of the N 1s and Fe 2p 3r2 peaks of ´-Fe 2 N1yx are given as a function of the area of the N 1s peak, relative to the Žaverage. N 1s peak area of sputter cleanedq annealed ´-Fe 2 N1yx , in Fig. 3. The binding energy of the N 1s electrons, E N 1s , and of the Fe 2p 3r2 electrons, E Fe 2p , was found to increase approximately linear with the relative area

Fig. 3. Binding energy of N 1s Žleft axis. and Fe 2p 3r 2 Žright axis. electrons as a function of the intensity of the N 1s peak, relative to the average intensity of sputter cleanedq annealed ´-Fe 2 N1yx . The solid line is a least-squares fit through the N 1s data points pertaining to ´-Fe 2 N1yx and indicates the linear relation between the N concentration Žtop axis. and the N 1s binding energy given by Eq. Ž3.. The dotted line through the Fe 2p data serves as a guide for the eye. The error bar indicates the estimated error of the N 1s and Fe 2p 3r 2 binding energies.

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of the N 1s peak. Additional data obtained from sputter cleaned a-Fe, sputter cleaned g-FewNx Žaustenite; f 9 at% N., sputter cleaned gX-Fe 4 N1yx Ž19.8 at.%N., and during sputtering of sputter cleanedq annealed ´-Fe 2 N1yx , are shown as well. The linear dependence of the N 1s peak position on the N 1s peak area allows calculation of the N concentration, C N , from the N 1s peak position after the following calibration. It is assumed that the highest N 1s binding energy measured, 397.88 eV Žafter oxidation of ´-Fe 2 N1yx for 30 min at 523 K; cf. Section 3.4., corresponds with the maximum nitrogen concentration of ´-Fe 2 N1yx , i.e., 33.8 N atoms nmy3 Žas determined from the relation between the N content and the a and c lattice parameter of ´-Fe 2 N1yx , for x s 0 w25x.. Extrapolation of the straight line through the N 1s data pertaining to ´-Fe 2 N1yx in Fig. 3 Žleast-squares fit to the filled circles and triangles in Fig. 3. to I N 1s s 0 yields a N 1s binding energy of 396.90 " 0.07 eV, which corresponds to C N s 0. Then, the relation between the N 1s binding energy, E N 1s , and the N concentration, C N , is given by: EN 1s s Ž 2.90 " 0.25 . P 10y2 C N q Ž 396.90 " 0.07 . eV

Ž 3.

with C N expressed in N atoms nmy3 . It is noted that the N concentration of sputter cleanedq annealed a-Fe as determined from its N

245

1s peak position using Eq. Ž3. Ž397.07 eV ™ 5.9 N atoms nmy3 ; see above and Fig. 3. does not correspond with the N concentration as determined from its N 1s peak area Ž10.6 N atoms nmy2 Žsee above. which, assuming a segregated layer thickness - 0.5 nm, would correspond to ) 20 N atoms nmy3 .. This is probably caused by Ž1. the difference in electronic state between N atoms chemisorbed at the iron surface and N atoms dissolved in the octahedral interstices of an Fe host lattice and Ž2. the inhomogeneity of the distribution Ži.e., strongly localised at the surface. of the segregated nitrogen. Using Eq. Ž3., for sputter cleaned ´-Fe 2 N1yx and sputter cleanedq annealed ´-Fe 2 N1yx nitrogen concentrations of 15.2 N atoms nmy3 and 31.4 N atoms nmy3 , respectively, were obtained. It follows that Ži. sputtering causes a reduction of the N concentration in the analysed volume of ´-Fe 2 N1yx Žas compared to the bulk concentration of 26.2 N atoms nmy3 ; cf. Section 2.; Žii. subsequent annealing leads to replenishment of N from the bulk of the ´-Fe 2 N1yx layer Žto a concentration even larger than the bulk concentration.. The effect of annealing on the N concentration in the sputter-affected region can be calculated according to Appendix B. Concentration–depth profiles of N in the sputter-affected region as calculated with Eq. ŽB1. for anneals of 100 and 1000 s at several temperatures are shown in Fig. 4. For temperatures as high as 573 K the replenishment of N in the sputter-affected region by diffusion from the bulk

Fig. 4. N concentration as a function of depth in the sputter-affected region calculated with Eq. ŽB1. ŽAppendix B. for Ža. 100 s and Žb. 1000 s anneals at several temperatures.

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Fig. 5. Reconstruction of an experimental Fe 2p spectrum Žof sputter cleaned a-Fe, oxidised for 12 min at room temperature. from Fe 2p reference spectra of Fe 0 , Fe 2q and Fe 3q. The individual contributions of Fe 0 , Fe 2q and Fe 3q are shown as well as the experimental and reconstructed spectra. Lower part shows the difference between experimental and reconstructed spectrum on the same scale as the spectra Žsee further Section 2..

is accomplished within a few seconds. This explains why the N 1s intensities observed after annealing at 573 K for 5, 15 and 30 min do not differ Žsee above..

and on ´-Fe 2 N1yx after oxidation for 16 min and for 30 min are shown in Fig. 6 as a function of oxidation temperature, Tox .

3.2. The oxide-film thickness

3.2.1. a-Fe The thickness of the oxide film on a-Fe oxidised for 16 min increases with oxidation temperature up to 523 K Žcf. Fig. 6a.. The smaller film thickness at Tox s 573 K, as compared to the film thickness at Tox s 523 K, may be caused by a sticking coefficient of oxygen on the Žoxidised. iron surface that is smaller at 573 K than at 523 K. For all temperatures the oxide film on sputter cleaned a-Fe is slightly thicker than on sputter cleanedq annealed a-Fe. This difference may be caused by the presence of segregated nitrogen on the sputter cleanedq annealed a-Fe surface Žsee Section 3.1.. Initially, the segregated N may occupy surface sites otherwise available for the adsorption of O 2 molecules. As soon as the segregated N has disap-

The thickness of the oxide film, d ox , Žand also the Fe and Fe 3q concentrations, C Fe 2q and C Fe 3q . was obtained, as described in Section 2, by reconstruction of the Fe 2p spectra recorded after oxidation. 3 A typical example of an experimental and reconstructed Fe 2p spectrum is shown in Fig. 5. The thickness values of the oxide films formed on a-Fe 2q

3

For the reconstruction of the Fe 2p spectra measured after oxidation of sputter cleaned ´-Fe 2 N1yx , the metal ŽFe 0 . reference spectrum was shifted 0.1 eV to account for the influence of the increased nitrogen concentration underneath the oxide film Žsee Section 3.4. on the binding energy of the Fe 2p electrons Žsee Section 3.1..

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

247

nitrogen andror sputter-induced defects as discussed in Ref. w47x.

Fig. 6. Thickness of oxide-films on a-Fe and ´-Fe 2 N1yx after Ža. 16 min and Žb. 30 min of oxidation as a function of oxidation temperature. The thickness values were obtained by reconstruction of the Fe 2p spectra as described in Section 2. The error in the value for the film thickness depends on the actual thickness value, as has been indicated for a few thickness values Žsee also Appendix C..

peared from these surface sites the oxide film grows with the same rate as the oxide film on sputter cleaned a-Fe. This explanation accounts for the absence of a N 1s peak after oxidation of sputter cleanedq annealed a-Fe. An alternative explanation for the thicker oxide film on sputter cleaned a-Fe as compared to sputter cleanedq annealed a-Fe would be a change of the work functions of the metal–oxide interface or the oxide surface due to segregated

3.2.2. ´-Fe2 N1y x The thickness of the oxide film on sputter cleaned q annealed ´-Fe 2 N1yx , after 16 min and 30 min of oxidation, increases with oxidation temperature up to 523 K Žcf. Fig. 6a and b.. As for a-Fe, the smaller film thickness at Tox s 573 K, as compared to the film thickness at Tox s 523 K, may be caused by a sticking coefficient of oxygen on the Žoxidised. surface that is smaller at 573 K than at 523 K. For all temperatures the oxide film on sputter cleaned ´-Fe 2 N1yx is thicker than on sputter cleaned q annealed ´-Fe 2 N1yx . Although the sputter-induced defects may bring about a thicker oxide film on the sputter cleaned substrate as compared to the sputter cleanedq annealed substrate, as suggested above for a-Fe, it is made likely below that for ´-Fe 2 N1yx the differences in nitrogen concentration in the interface adjacent region of the substrate Žcf. Section 3.1. are largely responsible for the differences observed in the oxide-film thickness values: the higher the N concentration in the surface adjacent region the thinner is the oxide film formed upon oxidation. The oxide-film thickness on sputter cleaned ´Fe 2 N1yx , after 16 and 30 min of oxidation, increases with oxidation temperature up to f 450 K, and then decreases. Beyond about 550 K the oxide-film thicknesses for sputter cleaned ´-Fe 2 N1yx and sputter cleanedq annealed ´-Fe 2 N1yx are almost equal. The decrease of the oxide-film thickness for sputter cleaned ´-Fe 2 N1yx beyond Tox f 450 K can be caused by an increase of the N content in the surface adjacent region of the ´-Fe 2 N1yx substrate due to supply of N from the bulk of the ´-Fe 2 N1yx layer to the sputter-affected region during heating the sample to the oxidation temperature and during oxidation: at Tox ) 450 K the N concentration in the sputter-affected region reaches the bulk concentration within 16 min Žsee Appendix B and Fig. 4.. Thus, above 450 K the oxidation behaviour of sputtered ´Fe 2 N1yx approaches that of sputteredq annealed ´Fe 2 N1yx . Note that the decrease of the oxide-film thickness already above Tox s 450 K was not observed for sputter cleaned a-Fe Žcf. Fig. 6a., which is consistent with this interpretation.

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248

3.3. The oxide-film composition The concentrations of Fe 2q and Fe 3q cations in the oxide film, C Fe 2q and C Fe 3q, were obtained, as described in Section 2, by reconstruction of the Fe 2p spectra recorded after oxidation. The concentration of O 2y anions in the oxide film was determined from the area of the O 1s peak using: IO 1s IO 1s ,ref

s

CO l ox Ž O 1s . CO ,ref lref Ž O 1 s .

ž

= 1 y exp y

d ox

lox Ž O 1s . cos Ž Q .

/

Ž 4.

where IO 1s and IO 1s,ref are the peak areas of the O 1s peak of the oxidised sample and of a reference sample, respectively; CO and CO,ref are the concentrations of oxygen anions in the oxide film and the reference sample, respectively; lox ŽO 1s. and l ref ŽO 1s. are the inelastic mean free paths ŽIMFP. of O 1s electrons in the oxide film and in the reference sample, respectively, and d ox is the oxide film thickness. Eq. Ž4. was derived from Eq. Ž1., assuming a homogeneous distribution of O 2y in the oxide film and assuming that the photoemission cross section, s , of O 1s electrons and the instrument factor, F, are equal for the oxide film and for the reference sample. For the IMFP of the O 1s electrons in the oxide film and in the reference oxide the average of the IMFP’s in FeO, Fe 3 O4 and a-Fe 2 O 3 was taken Ži.e., 1.90 nm; see Appendix A.. The oxide-film thickness values obtained from the reconstruction of the Fe 2p spectra were used for d ox . A reference sample was obtained by oxidation of a-Fe for 22 h at 673 K in pure O 2 at a total pressure of f 10 5 Pa. It was verified with X-ray diffraction that a thick oxide layer was formed containing a-Fe 2 O 3 as well as Fe 3 O4 . In UHV this sample was sputter cleaned and subsequently oxidised for 20 min at 1.6 P 10y5 Pa. Reconstruction of the Fe 2p peak of this oxide reference sample, as described in Section 2, yielded a composition of 20.2 Fe 2q cations nmy3 and 22.8 Fe 3q cations nmy3 . Then, assuming a Fe:O stoichiometry in the oxide film that yields charge neutrality, the O 2y concentration of the reference sample is 54.4 O 2y anions nmy3 . The compositions of the oxide films on a-Fe and on ´-Fe 2 N1yx are shown in Fig. 7 as a function of

Fig. 7. Ža. The fraction of Fe 2q cations in the oxide film and Žb. the ratio of the Fe 2q qFe 3q cation and the O 2y anion concentrations in the oxide film as a function of the thickness of the oxide film on a-Fe and on ´-Fe 2 N1yx , oxidised at several indicated temperatures. No difference was observed between the results for the two different pretreatments applied. The compositions of FeO, Fe 3 O4 and a-Fe 2 O 3 bulk oxides have also been indicated. The errors of the Fe 2q cation fraction and the Ž C Fe 2q qC Fe 3q .r CO 2y ratio have been indicated for two data points Žsee Appendix C.. The dashed lines in Ža. are drawn as a guide for the eye.

the oxide-film thickness for several temperatures. The compositions of the FeO, Fe 3 O4 and a-Fe 2 O 3 bulk oxides have been indicated likewise in Fig. 7. 3.3.1. a-Fe No difference was observed between the compositions of the oxide films on sputter cleaned a-Fe and on sputter cleanedq annealed a-Fe. For both pre-

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249

treatments the fraction of Fe 2q cations ŽFig. 7a. and the ratio of the Fe and O concentrations in the oxide film ŽFig. 7b. decrease with increasing film thickness, suggesting a change of the oxide film from an FeO like composition to an Fe 3 O4 like composition. For thickŽer. oxide layers it is expected that the oxide most rich in oxygen resides at the outer surface. Indeed, quantitative analysis of the Fe 2p spectra measured at a detection angle of 158 yielded a smaller amount of Fe 3q than the spectra measured at 458, which indicates that the concentration of Fe 3q close to the surface is higher than close to the metal–oxide interface, suggesting the presence of Fe 3 O4 near the outer surface as was confirmed by high-resolution transmission electron microscopy w48x. Further, for a constant film thickness, the fraction of Fe 2q cations increases with increasing oxidation temperature ŽFig. 7a.. This could be interpreted as a consequence of FeO being Žmore. stable at elevated temperature Žcf. Fe–O phase diagram w49x.. 3.3.2. ´-Fe2 N1y x No difference was observed between the composition of the oxide film on sputter cleaned ´-Fe 2 N1yx and on sputter cleanedq annealed ´-Fe 2 N1yx . The dependence of the fraction of Fe 2q cations on oxide-film thickness and on oxidation temperature ŽFig. 7a. and the dependence of the Fe and O concentration ratio on film thickness and on oxidation temperature ŽFig. 7b. is the same as for the oxide film on a-Fe: the compositions of oxide films grown on a-Fe and ´-Fe 2 N1yx do not depend on the type of substrate. 3.4. The redistribution of nitrogen in the ´-Fe2 N1y x substrate A selection of N 1s spectra measured after oxidation of ´-Fe 2 N1yx is presented in Fig. 8. The intensity of the N 1s peak decreases with increasing oxidation time, and thus with oxide-film thickness, for both sputter cleaned and for sputter cleanedq annealed ´-Fe 2 N1yx . Further, the intensity of the N 1s peak was observed to decrease with increasing detection angle. Both observations indicate that the majority of the N 1s electrons originates from underneath the oxide film.

Fig. 8. N 1s peaks measured after oxidation of Ža. sputter cleaned ´-Fe 2 N1yx and Žb. sputter cleanedqannealed ´-Fe 2 N1yx at Tox s 400 K for various oxidation times. The corresponding oxide-film thickness values as obtained from reconstruction of the Fe 2p spectrum Žsee Section 2. are indicated in brackets. The spectra in Ža. and Žb. are plotted on the same scale.

Two ways were considered for assessing the N concentration, C N , in the substrate after oxidation, from the N 1s spectrum: 1. Using the position of the N 1s peak, E N 1s , applying Eq. Ž3.. 2. Using the area of the N 1s peak, I N 1s , applying: I N 1s I N 1s ,ref

s

C N lsub Ž N 1s . C N ,ref l ref Ž N 1s .

ž

=exp y

d ox

lox Ž N 1s . cos Ž Q .

/

Ž 5.

250

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

where I N 1s,ref is the area of the N 1s peak of a reference sample; C N,ref is the N concentration in the reference sample, and lsub ŽN 1s., lref ŽN 1s., and lox ŽN 1s. are the inelastic mean free paths ŽIMFP. of the N 1s electrons in the substrate Žafter oxidation., in the reference sample, and in the oxide film, respectively. Eq. Ž5. was derived from Eq. Ž1., assuming that nitrogen is distributed homogeneously in the probed part of the substrate underneath the oxide film and assuming that the photoemission cross section, s , of N 1s electrons and the instrument factor, F, are equal for the oxidised sample and for the reference sample. The N 1s peak of the ´-Fe 2 N1yx substrate prior to oxidation was used as reference. The values for lsub ŽN 1s. and lref ŽN 1s. were taken equal; for lox ŽN 1s. the average value for the IMFP’s in FeO, Fe 3 O4 and a-Fe 2 O 3 was taken, i.e., 2.09 nm Žsee Appendix A.. The oxide-film thickness values obtained from the reconstruction of the Fe 2p spectra were used for d ox . The N concentrations underneath the oxide film after oxidation of ´-Fe 2 N1yx , as obtained from the N 1s peak positions and peak areas, are given in Fig. 9 as a function of the corresponding oxide-film thickness.

3.4.1. ´-Fe2 N1y x , To x G 523 K For ´-Fe 2 N1yx oxidised at Tox G 523 K the binding energy of the N 1s electrons, and thus the N concentration in the ´ phase below the oxide film, is approximately constant and independent of the pretreatment Žcf. bold data points in Fig. 9a and b.. The N concentrations obtained from the peak area according to Eq. Ž5. are, within the experimental error, equal to the N concentrations obtained from the peak position. The values for the N concentrations can be explained by a rearrangement of the N atoms by solid-state diffusion during oxidation at temperatures G 523 K as follows. For sputter cleanedq annealed ´-Fe 2 N1yx , nitrogen atoms which are released upon oxidation of ´-Fe 2 N1yx , because Fe is incorporated in the oxide film, at these temperatures diffuse rapidly into the bulk of the ´-Fe 2 N1yx layer Žsee Appendix B and Fig. 4.. Consequently, for sputter cleanedq annealed ´-Fe 2 N1yx the N concentration underneath the oxide film does not differ significantly from the N concen-

Fig. 9. The nitrogen concentration underneath the oxide film, calculated from the position and from the area of the N 1s peak, as a function of the oxide-film thickness for Ža. sputter cleaned ´-Fe 2 N1yx and Žb. sputter cleanedqannealed ´-Fe 2 N1yx . The initial N concentration of the substrate Ž C N Ž0.. and the maximum N concentration of ´-Fe 2 N1yx Ž C N,max . have also been indicated. The error of the N concentration has been indicated for a few data points Žsee Appendix C..

tration of sputter cleaned q annealed ´-Fe 2 N1yx prior to oxidation. For sputter cleaned ´-Fe 2 N1yx , in addition to the inward diffusion of nitrogen atoms released upon oxidation of iron atoms, nitrogen atoms from the bulk of the ´-Fe 2 N1yx layer diffuse into the sputteraffected region Žcf. discussion in Section 3.2.. This replenishment of nitrogen atoms from the bulk is

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

accomplished within a few seconds at T G 523 K Žsee Appendix B and Fig. 4.. Consequently, the N concentration of sputter cleaned ´-Fe 2 N1yx is practically equal to the N concentration of sputter cleaned q annealed ´-Fe 2 N1yx , shortly after the start of oxidation at Tox G 523 K. 3.4.2. Sputter cleaned ´-Fe2 N1y x , To x F 423 K For sputter cleaned ´-Fe 2 N1yx oxidised at Tox F 423 K the binding energy of the N 1s electrons, and thus the associated N concentration ŽEq. Ž3.. below the oxide film, increases with increasing film thickness and approaches the values for the maximum concentration of N in ´-Fe 2 N1yx ŽFig. 9a.. In general, the peak area of the N 1s spectrum yielded a lower value for the N concentration than the peak position. An increase of the N concentration in the part of the substrate adjacent to the oxide film is expected, since the incorporation of Fe atoms from the substrate in the growing oxide film causes a release of N atoms, which can accumulate in the relatively nitrogen poor sputter-affected region of the ´-Fe 2 N1yx substrate. The mobility of nitrogen atoms at these temperatures is relatively low Žsee Appendix B and Fig. 4. and as a consequence an inhomogeneous distribution of nitrogen underneath the oxide film will result, such that the concentration of N is highest close to the nitride–oxide interface. Since a homogeneous distribution of nitrogen atoms in the substrate is assumed in the application of Eq. Ž5., the above indicated discrepancy between the N concentrations obtained from the peak position and the peak area may be understood.

3.4.3. Sputter cleanedq annealed ´-Fe2 N1y x , To x F 423 K In the N 1s spectra recorded after the oxidation of sputter cleanedq annealed ´-Fe 2 N1yx at Tox F 423 K a N 1s peak at f 396.4 eV appeared, in addition to the peak at f 397.8 eV ŽFig. 8b.. The additional peak shifts the position determined for the total N 1s peak to a lower binding energy. Consequently, the N 1s peak position is not an accurate measure for the N concentration in this case. The N concentrations calculated from the area of the Žtotal. N 1s spectrum

251

are presented in Fig. 9b as a function of oxide-film thickness Žopen circles.. The N concentration increases approximately linearly with the thickness of the oxide film to values considerably larger than the maximum solubility of nitrogen in ´-Fe 2 N1yx Ž C N,max s 33.8 N atoms nmy3 .. The relatively large N concentrations can be explained as follows. As discussed above for sputter cleaned ´-Fe 2 N1yx at Tox F 423 K nitrogen diffusion is negligible. Thus, N atoms which are released because Fe of the substrate is incorporated in the oxide film, accumulate in the part of the substrate adjacent to the oxide film. In contrast with sputter cleaned ´-Fe 2 N1yx , the N concentration in the sputter cleanedq annealed ´-Fe 2 N1yx substrate is already close to C N,max prior to oxidation Žsee Section 3.1.. Then, the N concentration beneath the oxide film can exceed C N,max relatively soon after the start of oxidation. The appearance of the additional peak in the N 1s spectrum suggests that then a nitrogen containing phase other than ´-Fe 2 N1yx is formed. The N 1s peak was analysed in more detail by unravelling it in two peaks. For the shape of the high binding energy peak Žreferred to as peak I. a N 1s peak of sputter cleanedq annealed ´-Fe 2 N1yx measured prior to oxidation was taken and the peak position was set to the position corresponding to C N,max , i.e., 397.88 eV. To isolate the additional N 1s peak at relatively low binding energy Žreferred to as peak II. a N 1s spectrum was used which contains a relatively large contribution of peak II Žsputter cleanedq annealed ´-Fe 2 N1yx oxidised for 36 min at 400 K.. The contribution of peak I was subtracted from this spectrum with a weight factor such that the envelope of the remaining peak was minimised. Subsequently, all Žtotal. N 1s spectra measured after oxidation of sputter cleanedq annealed ´-Fe 2 N1yx were decomposed in peak I and peak II Žsee Fig. 10 for a few typical examples.. The N concentration of ´-Fe 2 N1yx below the oxide film calculated with Eq. Ž5. from the area of the high binding energy peak Žpeak I. is given in Fig. 9b as a function of oxide-film thickness. The N concentration values thus obtained are approximately constant and agree well with the maximum N concentration of ´-Fe 2 N1yx , thereby validating the assumption that peak I originates from nitrogen atoms in saturated ´-Fe 2 N1yx .

252

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

derneath the oxide film, the ratio of the area of peak II, I N 1s,II , and the area of peak I, I N 1s,I , is given by: I N 1s ,II I N 1s ,I

s

C N ,II l II Ž N 1s . C N ,max lsub Ž N 1s .

= exp

Fig. 10. N 1s peaks measured at different detection angles Žrelative to the surface normal. after oxidation of sputter cleanedq annealed ´-Fe 2 N1yx for 50 min at 400 K. The peaks were fitted with reference peaks of the high binding energy component Žpeak I. and the additional low binding energy component Žpeak II.; see Section 3.4.

To locate the origin of the N 1s electrons associated with the low binding energy N 1s peak Žpeak II., the N 1s spectrum of sputter cleanedq annealed ´-Fe 2 N1yx oxidised for 50 min at 400 K was measured at several detection angles and unravelled in peak I and peak II as described above Žsee Fig. 10.. The results show that Ž1. the intensity of peak II decreases with increasing detection angle, which implies that the corresponding nitrogen atoms are located at some depth below the surface, and Ž2. the contribution of peak II increases with increasing detection angle, relative to the contribution of peak I, which indicates that the nitrogen atoms corresponding to peak II are located closer to the surface than the nitrogen atoms corresponding to peak I. From these results it appears probable that the nitrogen leading to the additional N 1s peak Žpeak II. is present close to the substrate–oxide interface. Considering the nitrogen atoms pertaining to peak II to be concentrated in a layer of thickness d II , which is sandwiched in between the oxide film and the remaining part of the substrate, and the nitrogen atoms pertaining to peak I to be distributed homogeneously in the Žremaining part of the. substrate un-

ž

d II

lII Ž N 1s . cos Ž Q .

/

y1

Ž 6.

where C N,II is the concentration of nitrogen in the sandwiched layer, and l II ŽN 1s. and lsub ŽN 1s. are the inelastic mean free paths ŽIMFP. of N 1s electrons in the sandwiched layer and in the substrate, respectively. Eq. Ž6. was derived from Eq. Ž1. by assuming a homogeneous distribution of nitrogen atoms in the sandwiched layer and in the probed part of the remaining substrate, and by assuming that the photoemission cross section, s , of N 1s electrons and the instrument factor, F, is the same for the sandwiched layer and the substrate. The IMFP of N 1s electrons in ´-Fe 2 N1yx , i.e., 1.82 nm Žsee Appendix A., was used for lsub ŽN 1s.. The peak areas I N 1s,I and IN 1s,II were determined experimentally Žsee above. at Q s 158, 458 and 608 for sputter cleanedq annealed ´-Fe 2 N1yx oxidised for 50 min at 400 K ŽFig. 10.. Three parameters remain to be determined, C N,II , lII and d II . Eq. Ž6. does not allow to determine these parameters independently. However, d II rlII ŽN 1s. and C N,II P lII ŽN 1s. can be determined by fitting Eq. Ž6. to the N 1s peak areas obtained at the three detection angles. Good fits were only obtained for a relatively large value of C N,II P lII Ž) 10 3 N atoms nmy2 . and a relatively small value of d II rlII Ž10y2 .. ŽThe product of C N,II P lII and d IIrlII was practically independent of the individual values of C N,II P l II and d IIrlII : C N,II P d II s 22.7 N atoms nmy2 for the data obtained from the spectra shown in Fig. 10.. Thus, recognising that l II is not expected to be larger than a few nanometers, the assumption of a very thin layer with a high N concentration is consistent with the measurements. 3.4.4. Nitrogen atom balance for oxidation of ´Fe2 N1y x at To x F 423 K The number of N atoms released upon incorporation of Fe in the oxide film can be compared to the

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

number of the released N atoms taken up in the ´-Fe 2 N1yx substrate underneath the oxide film, and the number of released N atoms incorporated in the Žunknown. N containing phase at the substrate–oxide interface, in the cases where this phase develops Ži.e., for the sputter cleanedq annealed substrate.. On transforming iron nitride into iron oxide, the number of released N atoms per unit area, n N,rel , is given by: n N ,rel s C N ,sub Ž 0 .

C Fe ,ox C Fe ,sub

d ox

253

from Eqs. Ž7. and Ž8. are plotted in Fig. 11a as a function of oxide-film thickness. Upon oxidation of sputter cleanedq annealed ´Fe 2 N1yx at Tox F 423 K part of the released N atoms is incorporated in the Žunknown. nitrogen containing phase that develops at the substrate–oxide interface, in addition to the part of the released N atoms that is

Ž 7.

where C N,sub Ž0. is the N concentration in the substrate prior to oxidation; C Fe,sub and C Fe,ox are the Fe concentrations in the substrate and in the oxide film, respectively, and d ox is the oxide-film thickness. The change Žincrease. of the number of N atoms per unit area in the ´-Fe 2 N1yx substrate underneath the oxide film, n N,acc , is given by: n N ,acc s C N ,sub Ž d ox . y C N ,sub Ž 0 . d N

Ž 8.

where C N,sub Ž d ox . is the N concentration in the substrate after development of an oxide film of thickness d ox , and d N is the extent of the region in which the released N atoms are incorporated in the ´-Fe 2 N1yx substrate. To calculate n N,rel and n N,acc for sputter cleaned ´-Fe 2 N1yx oxidised at Tox F 423 K, the values of d ox and C Fe,ox ŽFe 2qq Fe 3q . as obtained from the reconstruction of the Fe 2p spectra were used Žsee Sections 3.2 and 3.3, respectively.; C Fe,sub was taken 71 Fe atoms nmy3 Žsee Section 2., and C N,sub Ž0. was taken 15.2 N atoms nmy3 Žsee Section 3.1.. For C N,sub Ž d ox . the N concentrations underneath the oxide film calculated from the peak position Žsee Fig. 9a and above discussion. were used. Considering N diffusion in a semi-infinite medium with constant surface concentration C N,sub Ž d ox . and bulk concentration C N,sub Ž0., an estimate for d N is given by 2 D P trp w50x, where D is the diffusion coefficient and t is the diffusion time. Values for d N were calculated using the oxidation time for t and taking the values of D, applying to a concentration range of 15.2 atoms nmy3 to 28.1 N atoms nmy3 Ži.e., the average value of C N,sub Ž d ox .., from Table 2 ŽAppendix B.. The values for d N were taken at least 0.4 nm. The values thus obtained for n N,rel and n N,acc

'

Fig. 11. The numbers of nitrogen atoms per unit area, released, accommodated in the ´-Fe 2 N1yx lattice, and accommodated at the substrate–oxide interface, upon oxidation at Tox F 423 K of Ža. sputter cleaned ´-Fe 2 N1yx and Žb. sputter cleanedqannealed ´-Fe 2 N1yx as function of the oxide-film thickness. The dotted lines in Ža. and Žb. indicate the amount of released nitrogen as a function of oxide-film thickness; they have been calculated from Eq. Ž7. using the average value of the experimental oxide-film compositions, i.e., C Fe,ox s 43 Fe cations nmy3 . Error bars have been indicated for a few data points Žsee Appendix C..

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

254

taken up in the remaining part of the ´-Fe 2 N1yx substrate. With respect to the results discussed below Eq. Ž6., the unknown N containing phase Žresponsible for peak II. is considered to be present in an infinitesimally thin layer at the substrate–oxide interface. Then the depth-distribution of nitrogen atoms in this phase is given by a delta function: n N,int d Ž z y d ox ., where n N,int is the nitrogen concentration in this phase per unit area at the substrate–oxide interface. Assuming that the nitrogen atoms taken up in the ´-Fe 2 N1yx substrate Žresponsible for peak I. are distributed homogeneously over the analysed depth in the substrate, n N,int follows from the ratio of the intensities of peak I, I N 1s,I , and peak II, I N 1s,II : I N 1s ,II I N 1s ,I

s

n N ,int C N ,sub Ž d ox . lsub Ž N 1s . cos Ž Q .

.

Ž 9.

Eq. Ž9. was derived from Eq. Ž1., assuming that the photoemission cross section, s , of N 1s electrons and the instrument factor, F, are equal for the layer and the substrate. To calculate n N,rel , n N,acc and n N,int for sputter cleanedq annealed ´-Fe 2 N1yx oxidised at Tox F 423 K, the IMFP of N 1s electrons in ´-Fe 2 N1yx , i.e., 1.82 nm Žsee Appendix A., was used for lsub ŽN 1s.. C N,sub Ž0. was taken 31.4 N atoms nmy3 Žsee Section 3.1.. For C N,sub Ž d ox . the N concentration corresponding to the position of peak I was used, i.e., C N,max Žsee above.. Values for d N Žcf. Eq. Ž8.. were calculated from 2 D P trp Žsee discussion above. taking the values of D, applying to a concentration range of 31.4 atoms nmy3 to 33.8 N atoms nmy3 , from Table 2 ŽAppendix B.. The values for d N were taken at least 0.4 nm. Other parameters in Eqs. Ž7. and Ž8. were obtained similarly as described above for sputter cleaned ´-Fe 2 N1yx . The values thus obtained for n N,rel , n N,acc and n N,int from Eqs. Ž7. – Ž9. are plotted in Fig. 11b as a function of oxide-film thickness. The values of n N,rel are larger than the values of n N,acc for sputter cleaned ´-Fe 2 N1yx ŽFig. 11a. and larger than the values of n N,acc q n N,int for sputter cleanedq annealed ´-Fe 2 N1yx ŽFig. 11b.. Note that the straight lines which could be drawn through the n N,acc data in Fig. 11a and n N,acc q n N,int data in Fig. 11b appear to cut the abscissa at oxide-film thicknesses larger than zero. This is interpreted as a

'

consequence of loss of released nitrogen to the outer atmosphere, particularly in the beginning of oxidefilm growth. A discrepancy between the amount of released nitrogen and the amount of nitrogen accommodated in the nitride substrate was also observed for the oxidation Žat 673 K. of gX-Fe 4 N; in that case it was suggested that the missing nitrogen was in particular present as N2 in pores in the substrate w51x.

4. Discussion 4.1. ElectronegatiÕity of the nitrogen atoms The binding energy of the N 1s and Fe 2p electrons increases with increasing N concentration Žcf. Fig. 3.. These binding energy shifts can be discussed in terms of the transfer of negative charge from Fe atoms to N atoms, as is expected from the difference in electronegativity between nitrogen and iron. The more negative Žexcess. charge is present on the nitrogen atoms, the smaller the binding energy for the N 1s electrons will be. Then the observed increase of the N 1s electron binding energy for increasing N content would imply that the negative charge per N atom decreases with increasing N content. At the same time it is observed that the binding energy of the Fe 2p electrons increases with increasing nitrogen concentration as well, which indicates that per Fe atom in the nitride more negative charge is removed when more N atoms are dissolved in the nitride. Thus, the total amount of negative charge taken from the Fe atoms increases when more N atoms are dissolved in the nitride, but the increase of total amount of negative charge transferred from Fe atoms to N atoms cannot keep up with the increase of the N concentration, which results in a decrease of the amount of negative charge added per N atom. 4.2. Nature of the additional, unknown N containing phase The binding energy of 396.4 eV of the additional N 1s peak Žpeak II. of the N 1s electrons of these N atoms at the nitride–oxide interface is clearly outside the range of possible binding energies described by

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

Eq. Ž3. Žsince C N ) 0.. This binding energy implies a relatively strong electronegativity: the excess charge per N atom would correspond to y0.62 P e, according to a relation between the N 1s binding energies and the calculated excess charge on the N atoms as given for a variety of organic nitrogen compounds in Ref. w52x. Regarding the identity of the nitrogen atoms in the unknown phase at the substrate–oxide interface, reference is made to the binding energy of the N 1s peak associated with segregated N atoms on an a-Fe surface Ž397.07 eV; Section 3.1., which would correspond to an excess charge of y0.49 P e per N atom according to the forementioned relation from Ref. w52x. Since nitrogen atoms at a surface Žas segregated N atoms. cannot experience octahedral coordination by Fe atoms, the nitrogen atoms associated with peak II could be less than octahedrally coordinated by Fe atoms Žas in ´-Fe 2 N1yx ., suggesting that they may be ‘adsorbed’ at the nitride side of the nitride–oxide interface. It is noted that the development of Fe–N phases with nitrogen contents higher than 33.3 at% has been reported to occur with techniques as glow discharge or plasma nitriding w53x and laser nitriding w54x, which realise states far from equilibrium at the surface. It is unlikely that such a high nitrogen Fe–N phase can form in the present low temperature gaseous oxidation experiments, which establish a near equilibrium state at the surface, and that such a phase could be responsible for the appearance of peak II in the N 1s spectra. Moreover, such a phase cannot be responsible for the appearance of peak II in the N 1s spectra at a binding energy value of 396.4 eV. As follows from Fig. 3 the N 1s binding energy for N in Fe–N phases increases linearly with the nitrogen concentration. Accordingly, the N 1s binding energy of a high nitrogen Fe–N phase Ž) 33.3 at% N. would be expected at binding energy values higher than 397.9 eV. Neither can the additional N 1s peak be ascribed to the presence of NO or N2 , because the corresponding N 1s peaks were reported to occur at about 400 eV w10,11,22x and above 400 eV w55,56x, respectively. The creation of the required sites for the N atoms, ‘adsorbed’ at the nitride–oxide interface, may be explained as follows. Growth of the oxide film on ´-Fe 2 N1yx is likely controlled by transport of Fe cations through the oxide layer, as is the case for the

255

oxidation of a-Fe Žsee, e.g., Refs. w57,58x.. Then, the conversion of an iron atom in the Fe Žsub.lattice of the substrate into an iron cation in the cation sublattice of the oxide layer is associated with the creation of a vacancy at the substrate–oxide interface in the Fe Žsub.lattice of the substrate. At the moment the nitrogen concentration in the nitride substrate exceeds the maximum solubility, nitrogen atoms released upon oxidation may lead to ‘nitrogen precipitation’ at locations nearrat the nitride–oxide interface where vacancies are injected as described above. 4.3. The role of nitrogen in the oxidation of ´Fe2 N1y x The present work shows that, for the substrates considered, the composition of the initial oxide film depends only on temperature and oxide-film thickness Žcf. Fig. 7.. Thus, the differences in oxidation behaviour of a-Fe and ´-Fe 2 N1yx may be ascribed to the nitrogen atoms accumulated in the ´-Fe 2 N1yx substrate and, in case of sputter cleanedq annealed ´-Fe 2 N1yx , at the substrate–oxide interface. The influence of nitrogen can be explained as follows. Ži. Nitrogen atoms at the substrate–oxide interface may block the incorporation of iron cations in the oxide film. Žii. Nitrogen atoms in the interface adjacent region of the ´-Fe 2 N1yx substrate and at the substrate–oxide interface may reduce the electrostatic potential difference over the oxide film, which is a driving force for cation transport through thin oxide films Žsee, e.g., Refs. w47,59,60x.. Both effects lead to a reduction of the growth rate of the oxide film on ´-Fe 2 N1yx as compared to a-Fe Žcf. Fig. 6 and Section 3.2.. The effect of nitrogen on the electrostatic potential difference Žsee Žii. above. can be explained as follows. The nitrogen atoms are negatively charged and the iron atoms are positively charged Žsee above.. Upon oxidation of already positively charged Fe atoms to Fe 2q or Fe 3q cations, less electrons are released than upon oxidation of neutral Fe atoms. As a consequence less oxygen atoms adsorbed at the oxide surface are reduced to oxygen anions, which leads to a lower Žless negative. surface potential and thereby, taking that the potential at the nitride–oxide interface is constant, the potential difference over the

256

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layer becomes smaller. Furthermore, the presence of negatively charged N atoms ‘adsorbed’ at the substrate–oxide interface may bring about an additional decrease of the potential difference over the oxide film. Now, with respect to the mechanisms reported in the literature for the influence of nitrogen on the oxidation behaviour Žsee survey in Section 1., it can be concluded that mechanism Ži. and mechanism Žii. apply to the present work: the presence of nitrogen in the substrate is held responsible for a reduction of the electrostatic potential difference over the oxide film, which causes a decrease of the initial oxidation rate Žmechanism Žii. in Section 1.; this effect is enhanced during oxidation by the accumulation of nitrogen atoms, which are released upon the incorporation of Fe atoms in the growing oxide film, underneath the oxide film Žmechanism Ži. in Section 1.. No evidence was found that could support mechanism Žiii.. To understand the observed significant improvement of the corrosion resistance of ´Fe 2 N1yx , mechanisms Živ. and Žv. in Section 1 can be of importance. These mechanisms refer to the role of the microstructure of the oxide layer, which is beyond the scope of this paper Žbut see Ref. w48x..

the nitride–oxide interface into the bulk of the ´Fe 2 N1yx layer. - For ´-Fe 2 N1yx oxidised at temperatures F 423 K the released nitrogen atoms accumulate in the interface adjacent region of the ´-Fe 2 N1yx substrate. If the nitrogen concentration underneath the oxide film exceeds the maximum solubility of nitrogen in ´-Fe 2 N1yx released nitrogen atoms ‘adsorb’ at the nitride–oxide interface. - The amount of released nitrogen atoms is larger than the amount of nitrogen atoms incorporated in the ´-Fe 2 N1yx substrate and at the substrate–oxide interface. The difference is ascribed to loss of nitrogen to the outer atmosphere. Živ. The oxide films formed on a-Fe and on ´-Fe 2 N1yx have a composition that is dependent on oxide-film thickness and oxidation temperature, but that is independent of the type of substrate and the type of pretreatment. Žv. The presence of nitrogen in the substrate leads generally to the formation of thinner oxide films on ´-Fe 2 N1yx than on a-Fe. This may be ascribed to the stronger electronegativity of nitrogen as compared to iron, which leads to a reduction of the electrostatic potential difference over the oxide film, that provides a driving force for the cation transport through the oxide film.

5. Conclusions Ži. A linear relation exists between the N concentration in an iron based substrate and the binding energy of the N 1s electrons and, similarly, the Fe 2p electrons. Žii. The type of pretreatment influences the surface composition of the substrates. - Annealing of sputter cleaned a-Fe leads to segregation of nitrogen from the bulk to the surface. - Sputter cleaning of ´-Fe 2 N1yx leads to a reduction of the N concentration in the surface region; by subsequent annealing the N concentration is restored by suppletion of N from the bulk. Žiii. Upon oxidation of ´-Fe 2 N1yx iron atoms are incorporated in the oxide film and nitrogen atoms are released. Part of the released nitrogen atoms are taken up by the ´-Fe 2 N1yx substrate. - For ´-Fe 2 N1yx oxidised at temperatures G 523 K the released nitrogen atoms diffuse rapidly from

Acknowledgements The authors thank ing. N. Geerlofs for provision of the nitriding furnace. Dr. ir. W.G. Sloof, ing. E.J.M. Fakkeldij and C.G. Borsboom are acknowledged for the provision of the XPS and oxidation equipment.

Appendix A. The IMFP of electrons in a-Fe, ´-Fe 2 N1I x and iron oxides The inelastic mean free paths ŽIMFP. of electrons in the Fe–N and Fe–O phases in this work were calculated according to Tanuma et al. w61x. The parameters needed for the calculation and the resulting IMFP’s are presented in Table A.1. Values for the lattice parameter a of a-Fe, FeO and Fe 3 O4 , and

P.C.J. Graat et al.r Applied Surface Science 136 (1998) 238–259

257

Table A.1 Inelastic mean free path ŽIMFP. of the Fe 2p, O 1s and N 1s photoelectrons Žexcited with Al K a radiation. in a number of Fe–N and Fe–O phases Phase

a Žnm.

a-Fe ´-Fe 2 N0.74 a-Fe 2 O 3 Fe 3 O4 FeO

0.2865 0.4719 0.5034 0.8396 0.4326

c Žnm. 0.4390 1.3747

M Žg moly1 .

r Žg cmy3 .

n ve

Eg ŽeV.

lŽFe 2p. Žnm.

lŽO 1s. Žnm.

lŽN 1s. Žnm.

55.85 121.92 159.69 231.54 71.85

7.87 7.18 5.27 5.20 5.89

8.0 19.7 34.0 48.0 14.0

0.0 0.0 2.2 0.1 2.3

1.36 1.42 1.67 1.65 1.58

1.59 1.65 1.94 1.92 1.83

1.75 1.82 2.14 2.12 2.02

the lattice parameters a and c of a-Fe 2 O 3 were taken from Ref. w31x; values for the lattice parameters a and c of ´-Fe 2 N1yx Žwith 27 at% N, i.e., x s 0.26. were obtained from the relationships given in Ref. w25x. The density, r , was calculated from the molecular mass, M, and the lattice parameters. The number of valence electrons, n ve , is the total number of electrons in the outer electron shells of the atoms. The band gap, Eg , of the iron oxides was taken from Ref. w62x. Using these data, the IMFP’s, l, were calculated for electron kinetic energies of 780 eV ŽFe 2p., 957 eV ŽO 1s., and 1089 eV ŽN 1s..

Appendix B. N diffusion in the sputter-affected region The concentration of a diffusing species at constant temperature as a function of time and position

in a layer of thickness L and fixed boundary concentrations of the species, is given by w50x: C Ž z ,t . s C0 q Ž CL y C0 . 1 y `

=

Ž y1.

Ý ns0

=cos

p

n

2nq1

žŽ

4

ž

exp y

2 n q 1. p z 2L

2 Ž 2 n q 1 . p 2 Dt

4 L2

/

/

Ž B1.

where C Ž z,t . is the concentration of the species in the layer as a function of time t and distance z from the boundary at z s 0; C0 and CL are the concentrations of species at z s 0 and z s L, respectively, and D is the diffusion coefficient of the species in the layer. To calculate the N concentration in the sputter-affected region after an annealing treatment, the

Table B.1 The diffusion coefficients in the sputter-affected region calculated according to Table 5 in Ref. w63x, for different temperatures and different boundary concentrations, C0 and CL Žgiven in N atoms nmy3 . Temperature

Diffusion coefficient Žm2 sy1 .

ŽK.

C0 s 15.2; CL s 26.2

C0 s 15.2; CL s 28.1

C0 s 31.4; CL s 33.8

323 373 400 423 473 523 573

1.9 P 10y26 7.2 P 10y2 4 9.7 P 10y2 3 6.8 P 10y2 2 2.4 P 10y2 0 4.4 P 10y1 9 4.7 P 10y1 8

2.2 P 10y26 8.4 P 10y24 1.1 P 10y22 7.9 P 10y22 2.8 P 10y20 5.1 P 10y19 5.5 P 10y18

6.7 P 10y26 2.6 P 10y23 3.4 P 10y22 2.4 P 10y21 8.5 P 10y20 1.5 P 10y18 1.7 P 10y17

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boundary concentrations, C0 and CL , of nitrogen were taken as 15.2 N atoms nmy3 and 26.2 N atoms nmy3 , respectively Žcf. Sections 2 and 3.1.. The diffusion coefficients, D, of nitrogen as calculated for different temperatures and different N concentration ranges using the data collected in Ref. w63x are given in Table B.1. The thickness, L, of the sputteraffected region was taken as 4.0 nm, which is the penetration depth of the 2.0 keV Arq ions at an angle of incidence of 408 in ´-Fe 2 N1yx as calculated with the TRIM program Žversion 96.01. w64x. The differences between the concentration profiles shown in Fig. 4, calculated using Eq. ŽB1. for different temperatures, are mainly determined by the temperature dependence of the diffusion coefficient Žcf. Table B.1.. Variation of the boundary concentrations affects the results only slightly.

Appendix C. Estimation of the errors of the results For a number of data points in Figs. 6, 7, 9 and 11 error bars have been given. The errors were estimated as follows. Errors of d o x , CF e 2 q and CF e 3 q (Figs. 6 and 7). Optimal values of d ox , C Fe 2q and C Fe 3q were obtained from the reconstruction of the Fe 2p spectra. The corresponding value of the squared difference, x 2 , of the measured and reconstructed Fe 2p spectra 2 is xopt . Values of d ox , C Fe 2q and C Fe 3q were also 2 determined for the case that x 2 s 1.1 P xopt . Differences of these last values of d ox , C Fe 2q and C Fe 3q and the optimal ones have been used as the error estimates Žsee also Ref. w35x.. Error of CO 2 y (Fig. 7b). The error of the O 2y concentration was calculated by partial differentiation of Eq. Ž4. with respect to IO 1s and d ox . The error of IO 1s was taken as 1%; the error of d ox was obtained as described above. The resulting error of the O 2y concentration is largely dominated by the error of d ox . Error of CN (Fig. 9). For the N concentration derived from the N 1s peak position the error was calculated by partial differentiation of Eq. Ž3. with respect to E N 1s . The error of EN 1s was estimated at 0.05 eV Žsee Section 2..

The error of the N concentration derived from the N 1s peak area was calculated by partial differentiation of Eq. Ž5. with respect to I N 1s and d ox . The error of I N 1s was taken as 1%; the error of d ox was obtained as described above. The error of the N concentration is largely dominated by the error of d ox . Errors of n N,r el , n N,acc , and n N,i n t (Fig. 11). The error of the amount of released nitrogen, n N,rel , were calculated by partial differentiation of Eq. Ž7. with respect to C Fe,ox and d ox . The errors of C Fe,ox and d ox were obtained from the reconstruction of the Fe 2p spectra Žsee above.. The error of the amount of nitrogen taken up in the ´-Fe 2 N1yx substrate underneath the oxide film, n N,acc , was calculated by partial differentiation of Eq. Ž8. with respect to C N,sub Ž d ox .. The error of C N,sub Ž d ox . was obtained as described above for the N concentration derived from the N 1s peak position. The error of the amount of nitrogen incorporated in the unknown N containing phase at the substrate–oxide interface, n N,int , was calculated by partial differentiation of Eq. Ž9. with respect to I N 1s,I and I N 1s,II . The errors of I N 1s,I and IN 1s,II were assumed to be 1%, which yields error bars smaller than the size of the data-point symbols in Fig. 11b. References ¨ w1x F. Hanaman, Uber Rostversuche mit nitriertem Eisen, PhD Thesis, Technische Hochschule Berlin, Berlin, 1913. w2x H.H. Uhlig, Trans. Am. Soc. Met. 30 Ž1942. 947. w3x M.A. Streicher, J. Electrochem. Soc. 103 Ž1956. 375. w4x O. Steensland, Iron and Steel 42 Ž1969. 104. w5x T. Bell, Heat Treat. Met. 2 Ž1975. 39. w6x A. Hendry, Corros. Sci. 18 Ž1978. 555. w7x K. Sachs, D.B. Clayton, Heat Treat. Met. 6 Ž1979. 29. w8x Y.C. Lu, R. Bandy, C.R. Clayton, R.C. Newman, J. Electrochem. Soc. 130 Ž1983. 1774. w9x E.J. Mittemeijer, P.F. Colijn, Harterei-Tech. Mitt. 40 Ž1985. ¨ 77. w10x D.L. Cocke, M. Jurcik-Rajman, S. Veprek, J. Electrochem. Soc. 136 Ž1989. 3655. w11x V. Brusic, G.S. Frankel, B.M. Rush, A.G. Schrott, C. Jahnes, M.A. Russak, T. Petersen, J. Electrochem. Soc. 139 Ž1992. 1530. w12x H. Kunst, Harterei-Tech. Mitt. 33 Ž1978. 21. ¨ w13x G. Wahl, Fachber. Huttenpraxis Metallweiterverarb. 19 ¨ Ž1981. 1076. w14x G. Werner, J. Ziese, Harterei-Tech. Mitt. 39 Ž1984. 156. ¨ w15x C. Dawes, D.F. Tranter, Heat Treat. Met. 12 Ž1985. 70.

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