XPS, FTIR, EDX, and XRD Analysis of Al2O3 Scales Grown on PM2000 Alloy

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Introduction
Most of the metallic materials functioning at high temperature need to have oxidation resistance. This resistance can be achieved when the chosen materials develop through oxidation, an oxide film which acts as a diffusion barrier while keeping a good adherence. Several studies have shown that the oxide layers as SiO 2 , Cr 2 O 3 , and -Al 2 O 3 provide a satisfactory protective role, a protection based on the formation of a layer of -Al 2 O 3 . Al 2 O 3 is the most powerful principle. In this prospect, -Al 2 O 3 is a very good candidate. Before reaching the most stable Al 2 O 3 [1], alumino-former materials developed transition Al 2 O 3 among which the most common are , , and/or phases. Nowadays, it is not clear whether the growth of transition Al 2 O 3 as a first step improves the protective properties of the further formed -Al 2 O 3 film. Moreover, one difficulty associated with the understanding of the influence of transition Al 2 O 3 on the further oxidation resistance concerns the fact that, as mentioned in [2], the techniques which allow us to detect and characterize transition Al 2 O 3 formed as thin layers (1 to 3 m) are scarce and provide ambiguous answers. Indeed, the most common technique, the XRD, provides patterns for various Al 2 O 3 which are relatively close to each other. Moreover, it seems that, in many cases, several transition phases can be simultaneously present [3]. In previous studies, transmission electron microscopy (TEM) was used to probe the oxidation of either an intermetallic alloy, Fe 3 Al, or an ODS (oxide dispersion strengthening) FeCrAl alloy strengthened by very small Y 2 O 3 particles, PM2000 [4][5][6][7]. The formation of transition Al 2 O 3 for various heat treatment conditions was evidenced and the transition to the -Al 2 O 3 was studied. The formed oxide scales were characterized using analysis techniques such as scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The present study aims at examining whether FTIR and XPS analysis may provide a simple probe to various structural varieties of Al 2 O 3 for applications of high temperature materials.  (iii) cooling it down rapidly to room temperature (air quench).

Characterization of Samples.
All the samples were then characterized using XRD, SEM observations, EDX qualitative analysis, FTIR spectroscopy, and XPS spectroscopy.
(ii) SEM images were taken on a field emission scanning microscope (JEOL 7500-F). The SEM used for the characterization is equipped by Genesis EDX spectroscopy system that was used to measure the composition of the elements constituting the films.
(iii) FTIR spectra were obtained using a Perkin-Elmer spectrometer at a resolution of 8 cm −1 . FTIR technique was used in the transmission mode in the 400-4000 cm −1 range. For each sample, 120 scans were used. After oxidation, ∼100 g of the oxides was scraped. The oxide was then compressed together with 23±2 mg of KBr in a cold 150 MPa isostatic press (CIP) in order to obtain a 200-250 m thick pellet. All IR spectra are reporting absorbance ( = − log( / 0 )) as a function of the incident wavenumbers.
(iv) An XPS spectrometer was used to qualitatively and quantitatively verify the composition of the different powder compounds. The spectra were treated using the Thermo Advantage V5.27 software. The photoelectrons are excited using a monochromatic Al-K radiation as the excitation source, collected at = 0 ∘ with respect to the surface normal and detected with a hemispherical analyzer. The spot size of the XPS source on the sample is 200 m, and the analyzer is operated with a pass energy of 150 eV for the survey spectra and 20 eV for the accumulation spectra of the core levels. The pressure is maintained below 10 −8 Torr during data collection, and the binding energies (Eb) of the obtained peaks are referenced to the C1s signal for C-H, which is set to 285.0 eV. XPS measurements are made with an uncertainty of about 0.1 eV to 0.2 eV.

SEM and EDX Analysis.
For each tested temperature, oxidized specimens, contained in crucibles, were cooled to room temperature. For studies of scale morphology, composition, and crystal structure, EDX analysis was used. Figure 1 shows EDX analysis of the as-received PM2000, and the presence of the elements Fe, Cr, Al, Y, Zr, Si, Mn, and C is detected. Significant differences in the scale morphology were observed by SEM, depending on the oxidation temperature. Both materials developed Al 2 O 3 scales. Figure 2(a) shows nodules form during the early stages of oxidation; they consist of oxides enriched by Al, Fe, Cr, Si, or Mg. Fe/Cr oxides were detected for temperatures between 873 and 1073 K, and cracks and porosities are observed at 1073 K, as shown in Figure 2(b). Our observations are consistent with those in the literature [8][9][10]. In addition to -Al 2 O 3 (detected by XRD), Figure 2(c) shows metastable Al 2 O 3 which is clearly observed at 1173 K, the so-called "platelet-like" oxide, and recognized to propagate by an outward Al anion diffusion in contrary to -Al 2 O 3 scale. The presence of this type of particles was correlated with the XRD analysis and they were assumed to be -Al 2 O 3 . It is well established [11,12] that the growth rate of the metastable Al 2 O 3 is of factor two to four times higher than the stable Al 2 O 3 , leading to faster Al consumption and as a result substantial decreasing of component's lifetime. The external surface of metastable oxide is very different from the equiaxed dense -Al 2 O 3 grains.  Concerning the morphology of the oxide obtained at 1473 K, which can be seen in Figure 2(d), the formation of a bulk porous microstructure is observable, with a quite different microstructure in comparison with the a, b, c specimens. The Al 2 O 3 (equiaxed grains) microstructure colonies develop into vermicular morphology containing larger scale interconnected porosity.
Data are analyzed in order to reveal the calcination effectiveness and to check the stoichiometry of the asprepared oxide films; the results are reported in Table 1. Figure 3(a) shows the EDX analysis samples of PM2000 (as-received and oxidized). The analyzed oxide layers are rich in Fe, Cr, and Al in the interval of temperatures between 873 K and 1073 K. For temperatures above 1173 K, Al 2 O 3 becomes predominant. An example of EDX spectra of oxidized PM2000 at 1473 K is shown in Figure 3(b). As shown in Figure 3(b), the presence of Mg appears more important as the temperature increases in the -Al 2 O 3 layer. Its dissemination to the external interface is favored by higher temperatures [13].

XRD Results
. PM2000 is iron-based (Fe-Cr-Al) with a ferritic matrix ( -Fe) as shown in Figure 4(a) and is mechanically alloyed with Y 2 O 3 and ZrO 2 dispersion material. Before oxidation, at room temperature, native oxide, thin film exists on the alloy surface, with only several nanometers in thickness, and it consists of all the alloying elements as mentioned in literature [14]. This native oxides is Al 2 O 3 and a mixture of oxides as Fe, and Cr. The oxide formation at elevated temperatures can be separated in three steps. First, at relatively low temperatures (873, 973, and 1073 K), a mixed oxide similar to the preexisting native oxide forms, the XRD does not allow the revelation of these oxides (they are revealed by SEM observations and detected by FTIR, EDX, and XPS analysis). Second, at 1173 K, the XRD pattern obtained reveals, in Figure 4 Information on the crystallite size ( ) for the compounds (i.e., -Al 2 O 3 ) was obtained from the full width at half maximum of the diffraction peaks using the Scherrer formula [15]: where , (ℎ ), and (ℎ ) are the X-ray wavelength (0.15418 nm), Bragg diffraction angle, and line width at half maximum, respectively. The values of the (ℎ ) and (ℎ ) parameters from the XRD peak are estimated by Gaussian fitting. This formula is not limited by the preferential orientation and is valid for an ordinary XRD profile. To improve the statistics, the most intense peaks in the profiles were chosen to determine the crystallite size. The results are reported in Table 2.

FTIR Characterizations of the Transition Al 2 O 3 on
Oxidized PM2000 Alloys. The FTIR spectral signatures of both -Al 2 O 3 and metastable forms have been thoroughly addressed in the literature using both experimental and theoretical simulations [16][17][18][19]. It is now possible to detect the presence of transition Al 2 O 3 and perhaps their nature on oxide scales formed by oxidation of alumino-former alloys. For this purpose, PM2000 samples (PM2000, ODS alloy), oxidized at different temperatures (from 873 K to 1473 K in air for 7 hours) were studied by IR spectroscopy. FTIR spectra are reported in Figure 5. The spectrum (Figure 6(a)) at 873 K in the range 400-1000 cm −1 represents a poorly crystallized structure characterized by a broadband with no apparent thin peak. This signature is that of the -Al 2 O 3 (the broad Intensity (cps) Intensity (a.u.)  [20,21]. Peak observed at ∼873 cm −1 is due to out-ofplane bending vibration ( 4-CO 3 2− ) of carbonate.

XPS Results.
XPS is sensitive to the chemical composition and the local environment of atoms in the crystal structure, which is reflected by the changes in the binding energy and the occurrence of multiple bands associated with different chemical environments. In the literature, the XPS analyses of Al, Fe, Cr, Mg, and Si oxides were performed and interpreted for the O1s, Al2p, Fe2p, Cr2p, Mg1s, Si1s, and C1s. XPS bands data were compared with the values reported in the literature [23][24][25][26][27][28][29]. XPS spectra of native oxides Fe-Cr oxides, transition Al 2 O 3 , and -Al 2 O 3 have been studied by spectral characterization of each sample (the as-received PM2000, and after oxidation from 873 to 1473 K). Figure 7 shows the XPS survey spectra and the spectrum of each  (Figure 8(b)). The results obtained for the valence Al2p for these alloys show that the variations in binding energies are within an interval of about 1 eV; these are in good agreement with previous observations [23,26,27]. The evolution of -Al 2 O 3 Al2p peaks is represented in Figures 9(a) and 9(b), for temperature increasing from 873 to 1473 K. At 1373 K, the -Al 2 O 3 peak is symmetrical and the BE position is given at 74.45 eV (FWHM 1.97 eV). The XPS decomposition peaks (Figure 9(b)) related to the energies Al2p bands are used to estimate the evolution percentage of the thermal -Al 2 O 3 phase between 873 and 1473 K (Figure 9(c)). This trend can be divided into three parts. First, at temperatures between 873 and 1073 K, the percentage is low and does not exceed 15%; this part corresponds to the -Al 2 O 3 and the (Fe, Cr) oxides formation. The second portion is between 1073 and 1173/1273 K; the percentage of Al is growing rapidly and has a high slope, which means a significant growth of thermal -Al 2 O 3 and -Al 2 O 3 in this field. Finally, in a step between 1173/1273 and 1473 K, -Al 2 O 3 takes place.  The O1s peaks are less symmetrical (relative to Al2p peaks), more complex, and more sensitive to the different states of the minerals. In fact, the O1s band is very important due to its intensity, which allows it to be more sensitive and hence more exploitable, according to the literature [23][24][25][26]. This band can be decomposed into multiple parts: the O1s band corresponding to (Fe, Cr) oxides is located at ∼529 eV, the O1s band corresponding to Al 2 O 3 oxides is located at ∼531 eV, the O1s band corresponding to the OH groups is located at ∼532 eV, and the O1s band indicating the presence of amorphous mixtures containing H 2 O is located at ∼533.5 eV. Figure 10(a) represents O1s peaks of as-received and treated PM2000. We note that these peaks exhibit significant differences depending on the compound state. In the asreceived PM2000, the energy of O1s is shifted to 531.84 eV    Figure 10: XPS spectra in the O1s region for as-received and treated PM2000 at various temperatures (a), decomposed O1s peaks of oxidized PM2000 at 873 K (b), decomposed O1s peaks of oxidized PM2000 at 1073 K (c), and decomposed O1s peaks of oxidized PM2000 at 1373 K (d).
in Figure 10(d), the wide peak at 531.14 (FWHM 2.12 eV) indicates the presence of the -Al 2 O 3 with the percentage of ∼96%, and a small peak at 532.11 eV (1.45 eV) due to the presence of the OH groups.
The Fe oxidation state of the oxides formed at various temperatures oxidations can be derived from the Fe2p spectra in Figure 11. Fe2p 3/2 and Fe2p 1/2 main line peak positions are 710.5 and 724.0 eV, respectively, which is in excellent agreement with the literature values for the mixed Fe 2 O 3 -(maghemite-) Fe 3 O 4 (magnetite) surface [30][31][32]. In addition, the occurrence and intensity of the so-called Fe2p 3/2 charge-transfer satellites, which appear additionally to the Fe2p 3/2 main line, indicate the oxidation state of different Fe oxides. In the case of Fe 2 O 3 , the Fe 3+ charge-transfer satellite should occur at 719 eV, while for divalent FeO, the Fe 2+ satellite appears at 715.5 eV. For the mixed valence state of Fe 3 O 4 (Fe 3+ : Fe 2+ = 2 : 1), both satellites add up in such a way that the spectral region between the 2p 3/2 and 2p 1/2 main lines becomes smooth and less structured [28][29][30][31][32][33]. In all samples, Fe2p peaks have asymmetric shape. In the spectrum of the as-received PM2000, the peaks corresponding to metallic Fe are specified at 706.93 eV and 707.89 eV, satellite is given at 720.16 eV, and the Fe2p 3/2 peak of native oxides is located at 711.33 eV. When oxidized at 873 K and 973 K, the spectra corresponding to the Fe2p 3/2 and Fe2p 1/2 bands are rather wider; peaks shifted at ∼711 eV and 724 eV, respectively. The satellite at 719.22 eV is well defined. At 1200 ∘ C, the peaks Fe2p 3/2 and Fe2p 1/2 are given to the binding energies near ∼714 eV and ∼725 eV. The decomposition of the spectrum between 716 and 725 eV can give satellites at 720 eV; these peaks indicate the presence of -Fe 2 O 3 .
The Cr oxidation state of the oxides formed at various temperatures oxidations can be derived from the Cr2p spectra in Figure 12, and the peaks observed at 577 eV and 586.5 eV indicate Cr2p 3/2 electrons and Cr 2 O 3 presence in the formed oxide. A satellite of the Cr2p 3/2 peak overlaps the Cr2p 1/2 component in Cr 2 O 3 . The peaks at 577.6 eV and 586.38 suggest Cr2p 3/2 and Cr2p 1/2 core electrons, respectively. The Cr2p 3/2 and 2p 1/2 main line peak positions are in the range 775-582 eV and 582-590 eV, respectively, being in excellent agreement with literature values [34,35]. In all samples, Cr2p peaks have asymmetric shape. In the spectrum of the as-received PM2000, the peaks corresponding to metallic Cr at 574.39 eV and 575.02 eV are identified. A Cr2p 3/2 peak of native oxides is located in the range 576-579 eV. When oxidizing at 873 K and 973 K, the spectra corresponding to the Cr2p 3/2 and Cr2p 1/2 bands are significant and located in the range 576-580 eV and 584-590 eV, respectively ( Figures  12(a) and 12(b)). The existence of Cr 2 O 3 was IR from a peak at 579.60 eV. For the oxidations at temperatures above 1073 K, these bands become negligible.
The SiO 2 and SiC are found by XPS peak (Si2p bands) positions 99.4 eV, 103.5 eV, and 100.3 eV, respectively. Peaks obtained are 100.87 eV and 97.75 eV (Figure 13), which reveals the presence of SiC and eventually Si. This compound is quite stable, since the alloy receiver (native oxide) and the amount do not change during oxidation. The addition of small quantities of colloidal SiO 2 to a commercial Al 2 O 3 powder has a significant effect on its densification and microstructure evolution [36,37]. SiO 2 has a detrimental effect on the Al 2 O 3 densification behavior particularly during the intermediate stage of sintering (from 1473 to 1673 K).
As mentioned above, the presence of Mg appears more important as the temperature increases. Its dissemination to the external interface is favored by higher temperatures as shown in Figure 14. Mg doping -Al 2 O 3 improved densification and elimination of residual porosity [38]. Works demonstrate that Mg additions uniformly distributed over a nanometer-sized Al 2 O 3 powder have no effect on the to phase transition, raise the densification rate in the rapidsintering stage, and increase the net shrinkage [39].

Adventitious C (Carbon).
The decomposition of the C1s signal in the domain (oxidation PM2000 at 873-1473 K) results in too bands (Figures 15(a) and 15(b)). The peak at ∼285.0 eV is associated with the binding energy of the C atoms in aromatic C-C/C-H [40], and the peak at 288.55 eV can be attributed to the binding energy of the carboxylic group (O-C=O), which is in agreement with the literature results [41].

Conclusion
The aim of this work was to determine whether IR spectroscopy and XPS could allow us to easily distinguish the different structural varieties of Al 2 O 3 and therefore be used as a rapid diagnostics to evidence the phases present in the protective layers of high temperature materials. It was thus possible to determine the FTIR spectra of Al 2 O 3 phases and the XPS analysis at different temperatures of oxidation and to evidence a continuous evolution leading to the simultaneous presence of several Al 2 O 3 phases. These results have allowed us to determine some characteristic IR and XPS peaks, that is, signatures, for the various transition Al 2 O 3 phases and -Al 2 O 3 . Using these IR and XPS signatures, it is possible to detect the presence of transition Al 2 O 3 naturally grown on Al 2 O 3 -former alloys. A detailed example is presented for the oxidation of PM2000 ODS alloy. Indeed, many high temperature metallic materials develop Al 2 O 3 scales that can act as protective layer against an aggressive environment. In the first stage, mixed Fe, Cr oxides, and transition Al 2 O 3 appear; afterwards Al 2 O 3 oxides become gradually the majority as the oxidation temperature increases before transformation into the most stable -Al 2 O 3 structure. Although the physical properties of the transition Al 2 O 3 differ, their identification is not straightforward.     Figure 15: XPS spectra in the C1s region for treated and as-received PM2000 at various temperatures (a) and decomposed C1s peak of oxidized PM2000 at 1073 K (b).