18% Ni-Co-Mo-Ti Ferrous base alloys are special materials, widely used in the industry of isotopic enrichment after specific annealing and aging thermal treatment. The desirable high mechanical properties can then be attained by adequate aging heat treatment, answering the structural materials specifications required by defense applications in aerospace and nuclear engineering. For instance, the isotopic enrichment, in rocket engine envelope application, when associated with high temperature and chemical residues like acidic solutions, can induce corrosion and hydrogen embrittlement in martensite structures. To limit these corrosion and hydrogen embrittlement phenomena, an adherent and protective layer of iron oxides can be grown on the material surface by using adequate atmosphere during the aging treatment. Due to its application in strategic areas, the characterization of these oxide layers in maraging steels is of importance as well as the understanding of their growth kinetics. For this purpose, several techniques, such as Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Glow Discharge Optical Emission Spectroscopy (GDOES), Microabrasive wear testing, Hardness, Grazing Incidence X-ray Diffraction (GIXRD) and X-ray Photoelectron Spectroscopy (XPS), have been performed for chemical and structural characterization of the oxide films formed after vapor exposed thermal aging at 510°C . The oxide layer consists mostly in two sub-layers composed by magnetite (Fe3O4) and an external layer of hematite (Fe2O3). A thick interface between the oxide layer and the bulk is enriched in Ti and Mo, whereas the analyses of deep bulk material show an enriched area with Ni and Co.
Research Article
Structural Characterization of Iron Oxide Grown On 18% Ni-Co-Mo-Ti Ferrous Base Alloy Aged Under Superheated Steam Atmosphere
https://doi.org/10.21203/rs.3.rs-626449/v1
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18% Ni-Co-Mo-Ti Ferrous base alloys
Maraging steel
Hydrogen embrittlement
Characterization oxide layers
- Several techniques were applied for Maraging steel layers characterization.
- A new approach by calotest, nanohardness and X-ray quantification were employed.
- Layers characterization was analyzed and discussed in agreement with literature.
- The layers distinction was satisfactorily evaluated.
Maraging steels, basically Fe-Ni-Co-Ti alloys with low carbon content, are often used in the ultracentrifuge envelope,. Thus, their microstructural matrix consists in a relatively soft martensitic structure (BCC- body centered cubic) [1, 2] some research compare this structural with low tetragonality [3]. Nonetheless, these materials exhibit excellent mechanical properties at the aged condition due to precipitation hardening of intermetallic phases such as Ni3(Ti, Mo) and Fe2Mo [2, 4]. Besides, some previous works has shown that the nickel atoms in the Ni3Ti unit cell can be replaced by cobalt and iron atoms, while titanium atoms can be replaced by molybdenum atoms [3]. It is important to note that distortions could have been observed while performing high temperature aging, due to the formation of reverse austenite (FexNi). Therefore, the general formula for these precipitates would be (Ni, Fe, Co)3(Ti, Mo) and their fraction of Mo and Ti would depend on the alloy composition. However, when adding cobalt to these alloys family, it’s also possible to find Laves phase (Fe2Mo), which are responsible for the high hardness peak and softening resistance during thermal aging [5].
As depicted before, the great advantage of this steel family is the excellent formability in the solution annealed condition and the possibility to achieve a combination of good toughness with a high mechanical strength after aging. But despite these good properties, when subjected to hydrogen containing environment, the maraging steels are prone to embrittlement phenomenon, altering its mechanical behaviour and ultimately leading to brittle fracture [6–8]. The general mechanism of hydrogen embrittlement involves the adsorption and subsequent diffusion of atomic hydrogen into the bulk metal, reducing its ductility and fracture resistance [9]. The formation of atomic hydrogen on the surface of the steel when exposed to a gaseous hydrogen environment requires the dissociation of molecular hydrogen, and further details of this mechanisms could be found elsewhere [9, 10].
An oxide layer can be used as a protective barrier against hydrogen diffusion and uptake, and can withstand stress and corrosion phenomena commonly occuring during nuclear fission[9, 10]. Such an oxide layer can be formed during thermal aging, while being exposed to a superheated water vapor atmosphere. So, studying the oxidation kinetics involving the alloying elements and estimating the layer composition is of primary importance. Various studies pointing out the Gibbs free energies of oxide formation [11, 12] lead to the conclusion that the oxide layer formation could be approximated by 3 different kinetic models: linear, parabolic and logarithmic. The model which will better describe the particular phenomenon will depend on the temperature and the alloy chemical composition. In this study, the model which better describes the oxidation process is logarithmic, with a quickinitial phase of the oxide growth followed by a decrease of the growth rate as fast as the thickness increased, indicating the protective effect [5, 13].
Malafaia and Shreir et al.[14] proposes the formation of three overlapping layers: the outermost, composed by hematite or iron oxide II (Fe2O3 hexagonal compact is considered the highest state of iron oxidation, with almost 70 at.% iron in its composition, while oxygen occupies the other 30 at.%) ; an intermediate one corresponds to the magnetite oxide (Fe3O4 - CFC) and the inner one by wustite (FeO - CFC) [14, 15]. These three layers act together as a barrier to the progress of oxidation and also hinders the hydrogen uptake, thus playing a vital role in reducing hydrogen damage [16].
The growth mechanism of the oxide film was studied by Klein et al. [10, 17], which in this case has been described as an outward migration of iron cations during the thermal oxidation of Fe-Ni-Co alloys, occurring in air, water steam and also carbon dioxide environment. Experimental results obtained by Auger electron spectroscopy and depth profile determination, SEM-XES analysis and X-ray diffraction supported these assumptions, but besides the fact that the oxide formation and cation migration were proved measuring their binding energies, they didn’t quantify the oxide layer thicknesses.
On the other hand, Rezek et al. [10] studied the microstructure and mechanical behavior changes when producing an oxide layer in a 250 maraging steel by high temperature heating in a water steam containing environment. The coating consisted in an innermost layer of austenitic phase and a magnetite Fe3O4 external layer. The results showed lower elongation and only a limited reduction in the measured hydrogen diffusion coefficient for the coated maraging when comparing with the bare steel. Despite that fact, the threshold stress in stress corrosion cracking (SCC) evaluation was significantly higher for the coated steel. This benefit is attributed to presence of the austenitic sub-layer.
Another interesting study, promoted by Greyling et al. [18], investigated the oxidation kinetics of a grade 300 maraging steel heated in air at temperatures ranging from 300 to 600°C. The authors used Auger electron spectroscopy in conjunction with argon ion sputtering, and noted that, a thin gold layer sputtered onto the surface before oxidation resulted to a change in the kinetics from limited-thickness to approximately parabolic growth at temperatures below 480°C. The limited‐thickness oxide growth is described in terms of space‐charge effects, as outlined by the theory of Formhold. The insulating properties of a thin mixed spinel oxide present at the metaloxide interface is critical in bringing about the space charge in the oxide.
Recently Florez et al. [19] showed that oxide characterization is important for embrittlement environments because surface treatments lead to inducing a superficial layer that in some cases could be protective. An oxide layer was generated under different atmospheres during the thermal aging treatment of different maraging grades. Afterwards, this layer was microstructural and mechanically characterized by advanced characterization techniques and the results showed that the oxide layer was made up of several compounds like: TiO2, MoO3, α-Fe2O3, and CoFe2O4. Furthermore, a nickel-rich austenitic phase at the interphase was mainly made up cobalt ions (Co2+), instead of iron ions (Fe2+), within the spinel lattice [19].
So, the purpose of the present study is to perform a thorough characterization, of the structure and composition of the iron oxide layer grown on a maraging steel 350 class after thermal aging in a superheated steam media. Thus, the characterization methods employed in this study consisted of OM, SEM coupled with EDS, Mosbauer, GDOES, Microabrasive wear testing, Hardness, GIXRD and XPS. The results obtained in this work are of great relevance, since maraging 350 class are commonly employed in reactors or high energy fusion housing, both environments which are considered highly aggressive to martensitic materials regarding its low resistance to stress corrosion cracking. The formation of an oxide protective layer during thermal aging can enhance the material’s performance in those applications without further increases in the total manufacturing cost and time.
The chemical composition of the maraging 350 steel employed in this study is presented in Table 1.
|
Ni |
Co |
Mo |
Ti |
C |
S |
P |
Al |
Cr |
Mn |
Fe |
|---|---|---|---|---|---|---|---|---|---|---|
|
17,65 |
11,65 |
4,69 |
1,44 |
˂0,1 |
˂0,1 |
˂0,1 |
˂0,1 |
˂0,1 |
˂0,1 |
Balance |
Twenty samples (20 mm x 20 mm x 2 mm) were prepared by precision cutting. All of them were then annealing treated (ST) at 910°C for 1 h, in a vacuum oven with controlled atmosphere with argon. The samples were sand the 200 to 1200 microns. Then, ten samples were aged (ST + A) at 510°C for 6 h in a Lindberg MPH type 12-1416 Steam Homo Pit Furnace with controlled atmosphere, according to the methodology presented by COESTER et al [20].
Cross section samples were prepared by low speed cutting to be characterized by the several techniques described beforehand. In order to perform the optical microcopy (OM) characterization, the ST and STA cross section treated samples were carefully embedded, grinded and then polished by metallographic conventional procedure, followed by chemical attack with both Potassium Metabisulfite and Marble regents. OM was then performed in a model Axio Imager 2 Carl Zeissin, whereas the Scanning Electron Microscopy (SEM) took place in a model Vega 3 Tescan, operated at 20 kV in a secondary electron (SE) mode in both chemical attacks. Microelementar analyses by EDS (dispersive energy spectroscopy) were performed along the sample’s depth, from the the oxide layer to the bulk. Finally, Mössbauer spectroscopy was performed using “converting electron” (CEMS) geometry. Thus, it was possible to study the most superficial oxide layer (~ 1000Å) [21, 22].
GDOES test was performed in a Horiba Jobin-Yvon GD-Profiler®, using a Pure Argon (800 Pa) R.F. plasma with applied power of 20W on 2 mm diameter electrode. Light intensities emitted by desexcitation of sputtered atoms/ions were separated in a polychromator enabling simultaneous analysis of the elements of concern during the analysis duration. Quantification of element concentration was calculated thanks to previous calibrations curves based on references samples. The sputtering rate was determined on each sample by measuring the produced crater, enabling to determine the concentration profile. Each sample was analysed several times, stopping the analysis sputtering at different depths to determine the sputtering rate of each layer.
The ST and STA samples were also characterized by micro-abrasive wear tests, which were performed on a small scale abrasive wear test instrument from Anton Paar. The test was conducted on a oxidized sample with a flat surface, whose dimensions are 25 X 15 mm², using a 25.4 mm diameter steel sphere with a normal force N of 0.2 N with a duration of 5s. The peripheral speed of the sphere was equal to 6.65 mm/s, and the abrasive solution consisted in diamond paste from with an average particle size of ¼ µm. The results obtained were corroborated by Rupetsov and Minchev [23].
The analyzed thicknesses were calculated by \(h=xy/D\) [23–25], where x is the width of the halo; d is the inner diameter of the groove (\(y=d-x\)) and D the diameter of the ball. After the wear testing, a Vickers hardness profile was raised along the layers of the hubcap, from the oxidized layer to the substrate, using nano-hardness measurements. There was used two penetrator, first a Square Base Pyramidal penetrator and second the Berkovich penetrator, with regular triangular base pyramidal geometry. Eighteen measurements in cross section from surface to the bulk were obtained with a 2.5 gf load maintained during 10 seconds, while on the surface it was used 0.01 gf and for 15 seconds. Micro hardness HV test were also operated with load of 25 gf and time of 10 seconds in triplicate in cross section and on surface. The complementar ultrasonic contact impedance test was conducted in triplicate using the MIC-205 31991 − 7747 digital durometer, with a load of 5 kgf and the MIC 10 digital display, in triplicate, on surface.
The technique of GIXRD was used in order to allow the structural characterization of the most superficial oxide layer [16, 18, 20]. The GIXRD technique was performed with the Rigaku equipment, model Ultima IV, equipped with CuKα source (λ = 1.5406 Å). The equipment operated with an angular step of 0.02 °, under voltage of 40 kV and 30 mA current. The critical angle for this case, to avoid the total external reflection of the X-ray beam, was 0.32° [26]. Consequently, the grazing incidences were chosen as follow 0.5°, 1°, 1.5°, 2°, 3° and 5°, in continuous scan mode from 20° to 90°, leading to 0,09 µm until 8,7 µm probed depths [24]. Rigaku PDXL was used for the identification and estimated quantification of volumetric fraction of the present phases [23]. The computational calculation of the mass attenuation coefficient of the mixture were based on NIST Standard (NBSIR 87-3597 [27]) with the relative weights for each compound quantified by de GIXRD.
Another technique used to evaluate the chemical composition on the surface of the oxidized sample was the X-ray Photoelectron Spectroscopy (XPS), using the Kratos model AXIS ULTRA DLD. XPS uses monochromatic Al-Kα radiation and fixed transmission mode analyzer (80 and 40 eV). The spectra were referenced from line C 1s (binding energy, BE = 284.5 eV) of the hydrocarbon. Samples were analysed with 400 \(\mu\)m diameter beams with 300 W. Survey spectra and high-resolution spectra were acquired using pass energies of 300 eV and 50 eV, respectively. Power step size 0.100 eV and number of power steps 401. The acquisition and quantification of the data were performed with the Kratos software, while the spectra were analyzed by peak adjustment followed by Gaussian second order and second derivative, taking into account the baseline according to the Shirley method in the origin software.
The microstructure of class 350 maraging steel obtained by OM analyses is shown in Figs. 1 and 2 for the ST and STA samples respectively, allowing the observation of the surface characteristics in the ST condition, and the presence of lath martensite. The STA condition leads to an oxide microlayer with 2 ± 0,3µm formed at the samples surface.
The SEM-EDS analysis, presented in Fig. 3 and Table 2, indicated the segregation of Ni, Co, Mo and Ti from the base steel to the external sub-layer causing local enrichment near the layer boundary [28]. However, the semi-quantitative analysis by EDS presented in Table 2 indicates that the chemical composition of the bulk material was not affected by the heat treatment. In this sense, Klein et al.[17] stipulated that elemental oxidation in steam oxidized Fe-Ni-Co alloys can be explained by the iron diffusion mechanism. Since Ni and Co can’t be oxidized in the process, they serve as inert markers and their position indicates the interface between base material and oxidized layer.
Table 2: Cross Section and Surface top semi quantitative point chemical analyses of EDS in Wt%
|
Cross Section Analyses |
||||||
|
Point Analyses |
Fe |
O |
Ni |
Co |
Mo |
Ti |
|
1 |
68.0 |
27.0 |
0 |
2.5 |
2.4 |
0.0 |
|
2 |
60.6 |
27.4 |
4 |
3.4 |
3.5 |
1.1 |
|
3 |
50.5 |
25.6 |
7.8 |
6.8 |
7.5 |
1.8 |
|
4 |
49.8 |
23.3 |
8.7 |
8.0 |
8.1 |
2.2 |
|
5 |
35.7 |
6.0 |
31.7 |
19.15 |
5.5 |
1.8 |
|
6 |
32.2 |
7.1 |
32.2 |
20.66 |
6.1 |
1.8 |
|
7 |
48.3 |
3,2 |
26.0 |
15.78 |
5.3 |
1.4 |
|
8 |
58.5 |
1.9 |
19.7 |
13.02 |
5.49 |
1.41 |
|
Surface Analyses |
||||||
|
Point Analyses |
Fe |
O |
Ni |
Co |
Mo |
Ti |
|
1 |
65.6 |
31.7 |
0.9 |
1.1 |
0.5 |
0.1 |
The SEM images also allowed the measurement of the oxidized layer thickness, as observed in Fig. 3. It is important to note the existence of two distinct regions, one external and one internal sub-layer, with a total thickness of approximately 2 µm. The innermost layer presents diffusion behavior of the different alloying elements added to 350-grade maraging steel. These elements were involved in the formation of the protective oxide layer during thermal aging under superheated water vapor atmosphere [16]. In Fig. 5 and Table 2 the bulk material region near the boundary with the oxide layer, it is formed an intermediate layer, rich in Ni and Co in points 3 until 5, and point 6 and 7 showed a leading to austenite stabilization because Ni was increasing [21, 22, 29].
However, in Fig. 5 and Table 2, it is shown that the outermost layer formation follows a semi-qualitative stoichiometry of approximately 66% Wt of Fe to 32% Wt for O with traces of Mo, Ni and Co, indicating that the outermost formation is composed of hematite (Fe2O3) [14, 15]. This result was corroborated by Mössbauer spectroscopy on CEMS that confirms the presence of the sextet referring to the formation of hematite oxide, as presented in Fig. 6.
On the other hand, the Microabrasive wear testing characterization results shown in Fig. 7 presented the formation of two layers formed on the substrate. Following the V RUPETSOV and R MINCHEV method after 5 s of microabrasive wear testing, the outermost layer thickness measures approximately 1,5 ± 0,2 µm and the innermost layer one is about 0,7 ± 0,1 µm [30].
Hardness nano measurements were carried out at the spherical cap, at different depths, allowing to access the hardness profile as indicated by Figs. 7 and 8 with tree diferent layers, in top of surface until 0.8 µm the hardness is 1600HV, the second layer is in 0.8 µm until 1.5 µm the hardness is 2000 HV following with a gradual decrease in 1.5 µm until 1.8 µm with 1500 HV, than in the bulk depleted in iron and rich in nickel from 2.0 µm until 2,6 µm with 900 HV, and 2,6 µm is the bulk with 750 HV, average. The analysis of these regions means hardness values from the surface to bulk, presenting high standard deviation. The growth kinetics of oxides is non-linear, this behaviour is due to the fact that the atomic diffusion is not unidirectional, resulting in a non-planar boundary surface between substratemagnetite and magnetite-hematite, which leads to the high observed deviations. So, the initial measurements on the samples surface can be a hardness peak or a valley that characterizes the growth of the iron oxide layer under study. This high deviations indicates the existence of sub-layer interfaces, corroborating that the material presents a double layer formation, originated from the growth of the protective oxide during the heat treatment in a steam furnace.
Furthermore, the hardness measurements, accessed by nano indentation into the cap region, made it possible to observe average values for each separate layer: the white layer next to the substrate presented 900 ± 200 HV, the intermediate layer 2090 ± 380 HV, the region near the boundary 1640 ± 250 HV and the substrate 785 ± 120 HV. Nano-, micro- and impedance hardness results obtained on the surface and in the bulk aregathered in Table 3. In this sense, the impedance hardness measurements at the bulk region, in the ST condition, resulted in 442 ± 6 HV and 768 ± 6 HV for the STA condition. The oxidized surface presented a value of 830 ± 310 HV. It is clear that the surface measurements with nano and impedance techniques have a high deviation due to the porous characteristic of the outermost layer and these are techniques sensitive to heterogeneous surface formation [1, 5].
|
Conditions |
Nano |
Micro |
Contact ultrasonic impedance |
|
|---|---|---|---|---|
|
ST – bulk |
399 ± 17 |
445 ± 25 |
442 ± 6 |
|
|
STA – bulk |
772 ± 20 |
998 ± 47 |
768 ± 6 |
|
|
STA – top surface |
790 ± 535 |
772 ± 20 |
826,04 ± 309 |
|
The element concentration profiles determined by GDOES are presented in Fig. 9. An oxide layer of about 1 µm is clearly visible, with oxygen content decreasing form 80 at.% to about 60 at.% and iron content increasing between 20 and 40 at.%. Ni, Co and Mo were not observed in this layer before ~ 0.7 µm, likely because of the rough interface between the top iron oxide and the underneath oxide. Below this iron oxide layer, the oxygen gradient is quickly decreasing till depth of about 2 µm whereas iron is still increasing till its bulk value. Within this intermediary layer located between 1 and 2 µm depth, Ni, Co and Mo are increasing till a maximum at 1.5 µm before decreasing back to their bulk value.. These profiles are confirming the growth mechanism of the oxide, based on iron diffusion toward the surface to build the Fe2O3 oxide, some inward diffusion of oxygen and then the relative enrichment in alloying elements (Ni, Co, Mo) in the intermediate oxidized depth due to the lower Fe content. [18, 31].
In order to obtain structural information about the surface layers, grazing incidence X-ray diffraction (GIXRD) was operated with different incidence angles to allow different penetration depths of the radiation. Figure 10 shows the diffractograms generated for incidence angles of 0.5, 1°, 1.5°, 2°, 3° and 5° [5, 32, 33], and Table 4presents the quantification data acquired by Rietveld method.
|
G. A ° |
Fe2O3 |
Fe3O4 |
γ |
α |
|---|---|---|---|---|
|
0,5° |
65,5% |
25,0% |
8,8% |
0,7% |
|
1° |
58,7% |
29,1% |
7,3% |
4,9% |
|
1,5° |
57,6% |
27,3% |
12,5% |
2,6% |
|
2° |
48,8% |
34,8% |
11,7% |
4,6% |
|
3° |
45,8% |
31,8% |
15,2% |
7,2% |
|
5° |
41,0% |
22,6% |
20,5% |
15,9% |
Using incident angles typically lower than 1.5°, the signal from the substrate can be minimized, increasing the intensity of the diffraction peaks associated with the crystalline structures next to the surface. Some researchers [24] argue that it is possible to obtain a more reliable quantification for the oxide layer using the relationship of photon interaction with the substrate thickness at some angles, especially in the case of phases with low volumetric fraction, as for example the intermetallic, binary and inorganic compounds formed at the substrate interface [2].
At the referred layer, the main crystalline phase is hematite, followed by magnetite. For the incident angles between 0.5° and 1.5°, an average hematite / magnetite ratio of 60% per 40% is maintained, which was also reported by other studies [14, 15]. However, for angles higher than 2°, the magnetite fraction increased, and at 3° the results presented also martensite peaks (charac teristic peaks of α’ Fe (110), (200) and (211)). From these results, it could be presumed that there was a formation of an external hematite layer and an internal magnetite sub layer. According to Rezek et al. [10], X-ray diffraction results indicated a higher percentage of hematite, inferring that a third oxide layer could be formed [2].
In an approach to calculate the penetration depth, it was used the method described by Birkholz [34], considering a sample with infinite thickness and the thickness of the upper layer that accounts for (1–1⁄(e) = 63%) of the measured intensity. With the thickness and phase quantification of each GIXRD diffractogram, an estimative of phase quantification for each thickness was done. Considering that only the depth penetration of the incident beam changes with the increase of α, the Eq. (2) was proposed to perform the layer phase quantification.
Where:
Q: Phase quantification measured in all volume by GIXRD
t: Analyzed film thickness
b: The surface area of analysis
q: Layer Quantification
n = 0,5°⟶5°
Figure 11 show the estimated results of the layer thickness on the material based on the Eq. 2. The result with 5° incidence angle shows a penetration depth of 3,5 µm, and the highest quantity of α and γ phases. This is in good correlation with the previously observed 2 µm thick oxide layer. Concerning the oxide layer, a first surface layer mainly composed of hematite is confirmed within the first 1 µm layer. Below, at the interface with the bulk, the magnetite is presenting a peak of proportion.
Graat et al. [35] made the analysis through the Fe 2p spectrum for a sample of pure Fe and obtained the peaks with binding energy of 706.8 eV for Fe0 and 709.8 eV and 711.2 eV for the Fe2+ and Fe3+ cations (Table 5). Satellite peaks present with a higher energy level, less intensity and always to the left of the main peaks with energy of 722.8 eV and 724.3 eV for Fe2+ and Fe3+ cations [32]. The doublets or multiplets that reveal the formation of satellite peaks in the spectrum of the main ions (Fe2+ and Fe3+). These peaks are due to the movement of electrons from the 3d orbital to the empty 4s orbital during the ejection of the electron photon from the 2p nucleus [35]. In analyses performed in Table 5the peaks summarize that binding energies used for XPS technique.
|
Espectrum Compound Fe 2p |
Peaks of Binding Energies (eV) |
|
|---|---|---|
|
Peak 1 (2p3/2) |
Peak 2 (2p1/2) |
|
|
Fe2O3 |
709,8–710,9 |
724,3 |
|
Fe3+ satellite |
~ 719,0 |
~ 733,0 |
|
Fe3O4 |
709,0–710,4 |
722,0 |
|
Fe2+ satellite |
~ 715,0 |
~ 730,0 |
|
FeO |
708,4–709,4 |
N/A |
|
Fe0 |
706,7–707,0 |
719,8 |
|
Fe 2p |
710,9 |
724,5 |
|
FEOOH |
711,8 |
- |
In Figure 12 the XPS spectrum on the surface of the ST sample can be interpreted as the sum of the following iron components (Fe0, Fe2+, Fe3+ and Fe2O3) in the 2p spectrum and their respective satellite peaks. For the STA condition, Fe in hematite was identified at 709 to 710 eV at 2p3/2 and 722 to 724 eV at 2p1/2; Fe in magnetite is found at 728 eV for the 2p1/2 peak and in the range of approximately 713 to 718 eV for the 2p3/2 Peak. It is also observed the formation of satellite peaks to 2p3/2 Fe3+ at 714.5 eV , to 2p1/2 Fe2+ at 725 eV and to 2p3/2 Fe2+ at 716 eV. The 2p3/2Fe- in FeO is also found at 709.5 eV. In Figure 10, it can be seen that the analysis of the zero point (near the bulk-STA) at approximately 715 eV and 730 eV shows low presence of magnetite in the STA condition, as well as the peaks at 706.5 eV and 720 eV show the high presence of metallic Fe for the ST condition according [21, 33].
In Fig. 12 the XPS spectrum on the surface of the ST sample can be interpreted as the sum of the following iron components (Fe0, Fe2+, Fe3+ and Fe2O3) in the 2p spectrum and their respective satellite peaks. For the STA condition, Fe in hematite was identified at 709 to 710 eV at 2p3/2 and 722 to 724 eV at 2p1/2; Fe in magnetite is found at 728 eV for the 2p1/2 peak and in the range of approximately 713 to 718 eV for the 2p3/2 Peak. It is also observed the formation of satellite peaks to 2p3/2 Fe3+ at 714.5 eV, to 2p1/2 Fe2+ at 725 eV and to 2p3/2 Fe2+ at 716 eV. The 2p3/2Fe− in FeO is also found at 709.5 eV. In Fig. 10, it can be seen that the analysis of the zero point (near the bulk-STA) at approximately 715 eV and 730 eV shows low presence of magnetite in the STA condition, as well as the peaks at 706.5 eV and 720 eV show the high presence of metallic Fe for the ST condition according [21, 33].
The ST top in XPS analysis showed 41% of oxide composed of Fe2O3 in 709,96 eV and 723,27eV, 27% Fe metal formation in 706,78 eV e 720,14 eV, 7% FeO in 707,65 eV, and 16% other components. The top of STA formation of oxide composed of 63% Fe2O3 in 709,96 eV and 723,27eV and 37% of mixture of Fe3O4, FeO, FeOOH, Fe2+, Fe3+ and others is observed in 715,21 707,65, 711,58 eV. However, this analysis technique still needs to be improved, in Fig. 10 shows a movement of the peaks with the layer analysis, showing that the ratio of oxides formed changes with the depth, however the hematite and magnetite peaks divide the energy range very close, making the analysis difficult.
According to microabrasive wear and hardness analyzes, the layers have two characteristic thickness of approximately 1.08µm and 0.84µm, resulting in a 1.92µm thick oxide layer. Therefore, since the scanning of the spectra was carried out up to 1.8 µm, it was not possible to analyze the relative increase of magnetite and iron oxide in the deeper layers. However, it was noticed a decrease in the Fe2+ satellites peaks, which accompany the hematite phase, even though no peaks referring to magnetite were observed. It is also important to note that hematite was the main oxide composing all analyzed layers, showing a higher mass fraction at the surface, which decreased towards the center of the analyzed layer. This behavior was corroborated by XRD and GIXRD results and by the diffusion of iron and oxygen observed by GDOS technique. In brief, all presented techniques showed the formation of a triple oxide layer, composed at its surface majorly by hematite (Mossbauer), followed by a mixture of hematite and magnetite. Near the material’s bulk, it is also present a thin line with wustite and cobalt containing oxides, which leads to iron depletion and cobalt and nickel enrichment, favoring austenite stabilization and diffusion hardening at the matrix next the oxide layer.
The following conclusions could be drawn from the present work on the structural characterization of the iron oxide obtained on 18% Ni (350) maraging steel treated in a superheated vapor steam furnace:
The optical microscopy and scanning electron microscopy analyses pointed out the presence of lath martensite at both conditions (ST and STA). Regardless, the STA sample treated in a water steam environment presented a 2,0 ± 0,3µm thick oxide microlayer on its surface.
The energy dispersive spectroscopy (EDS) evidenced distinct surface regions from the bulk to the top surface: a nickel-rich region with iron impoverishment is covered by 2 sub-layers of iron oxides. At the sample surface, the outermost layer presents a semi-qualitative stoichiometry of approximately 66% wt of Fe to 32% wt for O, with only Mo, Ni and Co traces, indicating that the outermost oxide is composed of hematite (Fe2O3). This result was confirmed by Mössbauer spectroscopy with the sextet formation.
Using Microabrasive wear testing characterization it was possible to corroborate the formation of the two layers on the substrate and to measure the thickness of the outermost layer to approximately 1,5 ± 0,2 µm, while the internal layer was 0,7 ± 0,1 µm thick.
The hardness measurements by nano indentation into the cap region produced by Microabrasive wear testing confirms the formation of a double layer, as well as an interface between these layers and the substrate. The mean nanohardness values for each region studied were of 900 ± 200 HV for the white layer next to the substrate, while 2090 ± 380 HV for the intermediate oxide layer, 1640 ± 250 HV for the region near the bulk interface and 785 ± 120 HV for the substrate itself. Conventional hardness measurementresulted in 442 ± 6 HV for the ST matrix, 768 ± 6 HV for the STA matrix, and finally 830+/-310HV at the oxide layer surface.
The grazing incidence on X-ray diffraction analysis confirmed the presence of hematite and magnetite. Angles between 0.5° and 1.5°, corresponding to the lower probed depths, evidenced the hematite formation in the top oxide, while angles higher than 2° led to the magnetite peaks.
In the XPS peak deconvolution analysis it was detected the interaction of Fe with oxygen at the top until the nearest region of the bulk (1,8µm), using sputtering for depth analysis. On the other hand, the outer layers presented the formation of iron oxides composed by 63% Fe2O3 and a mixture of Fe3O4, FeO, FeOOH, Fe2+, Fe3+ and other residuals. When analyzing the surface region, up to 1 µm deep, it is noted the major presence of hematite (between 60 and 70%), however this technique needs to be improved.
All correlated techniques showed that the oxide coating consists of a double layer, formed from the surface of the iron-depleted substrate by reverse diffusion and oxygen reduction. The substrate close to the layer is significantly richer in Nickel, also with a higher Co and Mo content, which leads to the formation of stable austenite. The interlayer has a mixture of oxides that consists mainly in a certain ration of hematite and magnetite. The amount of hematite increases towards the surface, followed by a reduction in the magnetite, which may indicate hematite formation in its outermost layer.
Acknowledgments
- Pró-Defesa IV – “High Mechanical Resistance Materials for Strategic Applications”.
- French Government program “Investissements d'Avenir” (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01 and EUR INTREE, reference ANR-18-EURE-0010).
a. Funding – Not Applicable
b. Conflicts of interest/Competing interests - Not Applicable
c. Availability of data and material - Not Applicable
d. Code availability – Not Applicable
e. Ethics approval - Not Applicable
f. Consent to participate - Not Applicable
g. Consent for publication - The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
h. Authors' contributions – Andréia de Souza Martins Cardoso - Performed the tests and characterizations; Hugo Ribeiro da Igreja - Treated XRD and calo test results; Pedro Soucasaux Pires Garcia - Prepared and corrected the text; Rodrigo Chales - Prepared and corrected the text; Juan Manuel Pardal - Correlate Abstract and Introduction text and Reference; Sérgio Souto Maior Tavares - Correlate results and conclusions; Maria Margareth da Silva - Treated XPS test results; Andrea Paesano - Treated Mössbauer test results; Luc Pichon - Treated GDOS test results
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