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XPS characterization of oxide layers on FeCo-based nanocrystalline alloys
Materials Science and Engineering A 428 (2006) 290–294
XPS characterization of oxide layers on FeCo-based nanocrystalline alloys J.E. May ∗ , S.E. Kuri, P.A.P. Nascente Federal University of S˜ao Carlos, Materials Engineering Department, 13565-905 S˜ao Carlos, SP, Brazil Received 30 March 2006; received in revised form 2 May 2006; accepted 10 May 2006
Abstract The main reason for the study of FeSi-based (FINEMET) and FeZr-based (NANOPERM) alloys was their magnetic properties. However, one of the limiting factors in commercial uses of these alloys is the oxidation, which occurs at high service temperature such as 800 ◦ C. Due to this high service temperature, a non-magnetic oxide layer is formed on the surface and the soft magnetic properties remarkably diminish. In this work, it was carried out mass gain measurements in order to identify the oxidation resistance of four different FeCobased nanocrystalline alloys: Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 . XPS analysis was used in order to identify and quantify the oxides formed during the exposure to an oxidant environment at high temperature. SEM also observed these oxide layers. The main conclusion is that the addition of Si improved the oxidation resistance of the nanocrystalline alloys. This is attributed to the increase of SiO2 amount and decrease of Fe2 O3 content on the surface layer, enhancing the surface oxide characteristics. © 2006 Elsevier B.V. All rights reserved. Keywords: XPS; Corrosion; Oxidation; FeCo-based alloys
1. Introduction The magnetic properties are the main interest to the study of FeSi-based (FINEMET), FeZr-based (NANOPERM), and FeCo-based (HITPERM) alloys. Almost all the efforts in understanding these alloys are dedicated to the structure and magnetic properties. However, during the normal service as a magnetic component, due to the natural potential to the metals oxidation, the alloys suffer an oxidation process. Recent study dealing with oxidation kinetics at temperatures around 300–400 ◦ C under controlled atmosphere demonstrated, by extrapolation to room temperatures, that the thickness growing rate of the surface oxide layer can be as high as 40 nm per year [1]. The same work emphasizes that for a magnetic head the working distance to optimize the efficiency of the component is 10 nm. Thus, if the working distance was increased due to an oxidation process, the efficiency of the component would probably be reduced, because of the exponential reduction in the magnetization with the increase of the working distance. The study of the oxidation process and
∗
Corresponding author. Fax: +55 1935225609. E-mail address: jemay [email protected] (J.E. May).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.012
the improvement of the oxidation resistance of the FeCo-based alloys have not only technological interest, but also scientific interest. Our previous work showed that the corrosion resistance of amorphous and crystalline alloys with Fe80 Nb3.5 Zr3.5 Cu1 B12 (NANOPERM) is much lower than that of Fe73 Nb3 Cu1 Si15.5 B7.5 (FINEMET) [2]. The main conclusion was that the addition of silicon improved the corrosion resistance of the protective film formed during immersion in an acid medium and that the composition of the oxide layer formed was rich in SiO2 . It was also demonstrated that the corrosion process negatively affected the magnetic properties. The magnetic saturation flux density dropped from 1.52–1.25 T for NANOPERM alloy, i.e., a reduction of about 20% of magnetic saturation flux density. This behavior was attributed to the formation of a nonmagnetic oxide layer that changes the magnetic lines around the ribbon. FINEMET alloy lost only 2% of its magnetic saturation. It was showed that the addition of Si and Nb to FeCoZrCuB alloys improved their corrosion resistance and diminished the magnetic losses caused by the corrosion process [3]. The Co-rich amorphous alloys (∼Co74 Fe5 Si2 B19 ) displayed B and Si enrichment in a surface oxide layer promoted during annealing at 380 ◦ C/90 min [4]. In that study, it was concluded
J.E. May et al. / Materials Science and Engineering A 428 (2006) 290–294
Fig. 1. DSC curves for amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , Fe33.5 Co33.5 Nb7 Si15 B11 , and Fe38.5 Co38.5 Nb7 Cu10 B15 alloys (15 ◦ C/min, Ar).
that the surface of the formed SiO2 ·B2 O3 layer decomposed spinodally into two regions: Si-rich and B-rich phases, suggesting that these oxides induced nanocrystallization under the surface. However, the possibility that a spinodally decomposed structure in the metallic bulk led to a partitioned surface oxide layer was not mentioned. According to recent results, there is a possibility of spinodal decomposition prior to the crystallization, which will have impact on the surface oxide layer formed during a temperature exposure under oxidation environment [5]. Some evidences of spinodal decomposition for FeCo(Nb,Zr)SiB alloys have already been shown in previous work [6]. In this work, we present the oxidation resistance of Fe36 Co36 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe36 Co36 Nb7 Si15 B11 alloys analyzed by mass gain measurements, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), and discuss the influence of the structure on the surface oxide layer composition and on the oxidation resistance.
291
Differential scanning calorimetry (DSC) scans were conducted by continuous heating from room temperature to 1000 ◦ C at a heating rate of 15 ◦ C/min. Crystallization treatments at 800 ◦ C were performed in argon atmosphere during 90 min for Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys using heating and cooling rates of 25 and 60 ◦ C/min, respectively. Crystallization was confirmed by XRD. Oxidation resistance measurements were carried out by mass gain from 25 to 1100 ◦ C. Mass gain oxidation testing was performed with a NETZSCH STA-409 with a mixing atmosphere of O2 and Ar (20 and 80 ml/min, respectively) and a heating rate of 15 ◦ C/min. SEM was used to observe the oxide surface layer after heat treatment in oxidation environment (atmosphere) during 90 min at 800 ◦ C. A Philips TMP microscope was employed, with a voltage of 25 kV and working distance of 12 mm. XPS was used to characterize the surface with a Kratos XSAM HS spectrometer having a non-monochromatic Mg K␣ (hν = 1253.6 eV) X-ray source, with power given by the emission of 10 mA at a voltage of 13 kV. The high-resolution spectra were obtained with analyzer pass energy of 20 eV. The accuracy of the electron analyzer is 0.1 eV. Gaussian line shapes were used to fit the curves for C 1s and O 1s, and a mixed Gaussian/Lorentzian fuction was employed for the other peaks. The Shirley background and a least-square routine were used for peak fitting. 3. Results Fig. 1 shows the DSC curves for amorphous Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. The Fe36 Co36 Nb7 Si10 B11 and Fe33.5 Co33.5 Nb7 Si15 B11 alloys presented three crystallization peaks, while the
2. Experimental procedure Amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , Fe33.5 Co33.5 Nb7 Si15 B11 , and Fe73 Nb3 Si13.5 B9.5 Cu1 alloy ribbons were produced using a single roll melt spinner with a copper wheel at a speed of 55 ms−1 in an argon atmosphere. X-ray diffraction (XRD) measurements were performed with a D5000 Siemens diffractometer, using a Co K␣ source (λ = 0.17889 nm), and a 2θ step of 2 min−1 . Table 1 Crystalline fraction related with the (FeCo)76 Nb6 B18 phase Sample
Crystalline fraction (%)
Fe36 Co36 Nb6 Cu1 Si10 B11 Fe36 Co36 Nb7 Si10 B11
73 55
Fig. 2. Mass gain of amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys (15 ◦ C/min, Ar = 80 ml/min and O2 = 20 ml/min).
292
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Fig. 3. SEM micrographs for crystalline: (a) Fe38.5 Co38.5 Nb7 Cu1 B15 , (b) Fe36 Co36 Nb6 Cu1 Si10 B11 , (c) Fe36 Co36 Nb7 Si10 B11 , and (d) Fe33.5 Co33.5 Nb7 Si15 B11 alloys (after oxidation at room atmosphere during 90 min at 800 ◦ C).
J.E. May et al. / Materials Science and Engineering A 428 (2006) 290–294
Fe36 Co36 Nb6 Cu1 Si10 B11 alloy displayed only two peaks. The Fe33.5 Co33.5 Nb7 Si15 B11 showed just one single crystallization peak. It is interesting to notice that above 800 ◦ C there is no crystallization process occurring. Thus the choosen heat treatment for the annealing resulted in a fully crystalline structure for all samples. The XRD, SEM, and XPS measurements were done in the fully crystalline samples. Table 1 shows the crystalline fraction related with the first crystallization peak, which corresponds to the (FeCo)76 Nb6 B18 phase. Fig. 2 shows the dynamic mass gain as a function of increasing temperature in all samples. The oxidation rates did not differ significantly at temperatures up to 500 ◦ C, but the samples with Si addition showed lower values of mass gain. This difference increased above 500 ◦ C. The traditional FeCobased Fe36 Co36 Nb7 Cu1 B15 showed a major mass gain rate at around 550 ◦ C, although this behavior only became evident for Fe36 Co36 Nb6 Cu1 Si10 B11 and Fe36 Co36 Nb7 Si10 B11 at temperatures exceeding 700 ◦ C. The Fe33.56 Co33.5 Nb7 Si15 B11 alloy showed a better performance than the other alloys, with an increase in mass gain at 950 and 1030 ◦ C, respectively. Fig. 3 shows the SEM micrographs for Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. As it can be observed for the micrographs with lower amplification (∼500×), the oxide surface has different characteristics for each sample. The Fe38.5 Co38.5 Nb7 Cu10 B15 alloy has bigger cracks in the oxide surface than the other alloys. The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys present a more uniform oxide surface as compared to the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. Table 2 shows the composition of the oxide surface layer formed during 90 min of exposure in room atmosphere at 800 ◦ C. There are three main differences among the alloys. (a) The Fe38.5 Co38.5 Nb7 Cu10 B15 alloy presented a higher iron oxide (Fe2 O3 ) content while the other alloys did not display a high amount of iron oxide at the surface. (b) The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys showed a higher content of boron oxide (B2 O3 ) compared to the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. (c) The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys presented silicon oxide (SiO2 ) in the surface oxide layer, and the other alloying elements (CoO, Nb2 O5 , Cu2 O) were not detected in the surface oxide layer, while these alloying elements were detected for the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. Table 2 Oxide layer composition analyzed by XPS after oxidation at room atmosphere during 90 min at 800 ◦ C Sample
Fe38.5 Co38.5 Nb7 Cu1 B15 Fe36 Co36 Nb6 Cu1 Si10 B11 Fe36 Co36 Nb7 Si10 B11 Fe33.5 Co33.5 Nb7 Si15 B11 ND: not detected.
Oxide composition (%) Fe
Co
Nb
Cu
Si
B
16.4 ND ND 1.4
2.7 0.4 0.3 0.5
1.7 ND ND ND
1.4 ND – –
– 4.0 9.2 12.7
8.1 39.4 29.5 20.2
293
Fig. 4. XRD diffractograms of crystallized samples after annealing during 90 min at 800 ◦ C (K␣ = Co). Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys (␣-(FeCo)—cubic BCC; (FeCo)76 Nb6 B11 [7]).
Fig. 4 shows the XRD diffractograms of the nanocrystalline Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. All these alloys show a ␣ -FeCo and a (FeCo)76 Nb6 B18 phases in their structure after heat treatment during 90 min at 800 ◦ C. It is interesting to notice that the Fe38.5 Co38.5 Nb7 Cu10 B15 and Fe36 Co36 Nb7 Si10 B11 alloys show a third unidentified phase. 4. Discussion As can be observed in the DSC curves, above 800 ◦ C there is no crystallization process occurring. Thus, the heat treatment conducted in this work led to fully crystallized structure. This is important because there was no amorphous residual matrix and the oxidation process in the surface was processed in a completely crystallized structure. The oxidation measurements were conducted from an amorphous structure at 25–1000 ◦ C. The heating rate was 15 ◦ C/min, which means that all the crystallization process can occur during the experiment. The main conclusion is that the oxidation resistance increases in the order: Fe38.5 Co38.5 Nb7 Cu10 B15 < Fe36 Co36 Nb6 Cu1 Si10 B11 < Fe36 Co36 Nb7 Si10 B11 < Fe33.5 Co33.5 Nb7 Si15 B11 . This behavior can be related with the morphology, the oxide compositions, and the crystalline structure characterized by SEM, XPS, and XRD, respectively. The SEM micrographs indicated that the surface oxide layer of the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy had bigger cracks than the surface oxide layer of the other alloys. The XPS measurements showed a higher iron oxide content in the surface oxide layer of the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy compared to the other alloys. Actually, the main oxides presented on the surface oxide layers for the Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and
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Fe33.5 Co33.5 Nb7 Si15 B11 alloys were silicon and boron oxides, as encountered in Co-rich amorphous alloys (∼Co74 Fe5 Si2 B19 ) [4]. Iron oxide was not detected or its amount was very low. The Si addition to the FeCo-based alloys improved the oxidation resistance and this improvement was related with the silicon oxide formed on the surface oxide layer. This silicon oxide and the boron oxide produced a more packed oxide layer. The affinity of the Si for oxygen is higher than the affinity of the oxygen with the other elements (Co, Nb, Cu). Thus, in the alloys that have Si addition this element was preferentially oxidized. When the Si amount in the alloy was increased, the silicon oxide also increased. However, in spite of the Si content in the Fe36 Co36 Nb7 Si10 B11 alloy do not differ a lot compared with the Fe36 Co36 Nb6 Cu1 Si10 B11 alloy, the surface amount of silicon oxide is strongly different. This could be attributed to the crystalline phases formed during the annealing and the crystalline fraction at the end of the crystallization. As observed in the Table 1, the crystalline fraction for the Fe36 Co36 Nb6 Cu1 Si10 B11 alloys is 73%, associated with the (FeCo)76 Nb6 B18 phase, but only 55% for the Fe36 Co36 Nb7 Si10 B11 alloy. 5. Conclusion The Si addition improves the oxidation resistance of the FeCo-based alloys and this improvement is related with the
silicon and boron oxides encountered in the surface oxide layer. Acknowledgements This work has been supported by the Brazilian agencies FAPESP and CNPq. The authors would like to thank Silvia S. Maluf for her assistance in some of the XPS measurements. References [1] C. Dubarry, A. Galerie, G. Poupon, Mater. Sci. Forum. 369–372 (2001) 247–254. [2] C.A.C. Souza, J.E. May, I.A. Carlos, M.F. de Oliveira, S.E. Kuri, C.S. Kiminami, J. Non-Cryst. Solids 304 (2002) 210. [3] J.E. May, C.A.C. Souza, C.L. Morelli, S.E. Kuri, J. Alloys Compd. 390 (2005) 106–111. [4] C.K. Kim, C.S. Yoon, T.Y. Byun, K.S. Hong, Oxidation Met. 55 (3/4) (2001) 177–187. [5] A. Serebryakov, A. Gurov, N. Novokhatskaya, J. Non-Cryst. Solids 260 (1999) 59–64. [6] J.E. May, M.F. de Oliveira, S.E. Kuri, J. Non-Cryst. Solids 348 (2004) 250–257. [7] A. Inoue, H. Koshiba, T. Zhang, A. Makino, J. Appl. Phys. 83 (4) (1998) 1967.
XPS characterization of oxide layers on FeCo-based nanocrystalline alloys J.E. May ∗ , S.E. Kuri, P.A.P. Nascente Federal University of S˜ao Carlos, Materials Engineering Department, 13565-905 S˜ao Carlos, SP, Brazil Received 30 March 2006; received in revised form 2 May 2006; accepted 10 May 2006
Abstract The main reason for the study of FeSi-based (FINEMET) and FeZr-based (NANOPERM) alloys was their magnetic properties. However, one of the limiting factors in commercial uses of these alloys is the oxidation, which occurs at high service temperature such as 800 ◦ C. Due to this high service temperature, a non-magnetic oxide layer is formed on the surface and the soft magnetic properties remarkably diminish. In this work, it was carried out mass gain measurements in order to identify the oxidation resistance of four different FeCobased nanocrystalline alloys: Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 . XPS analysis was used in order to identify and quantify the oxides formed during the exposure to an oxidant environment at high temperature. SEM also observed these oxide layers. The main conclusion is that the addition of Si improved the oxidation resistance of the nanocrystalline alloys. This is attributed to the increase of SiO2 amount and decrease of Fe2 O3 content on the surface layer, enhancing the surface oxide characteristics. © 2006 Elsevier B.V. All rights reserved. Keywords: XPS; Corrosion; Oxidation; FeCo-based alloys
1. Introduction The magnetic properties are the main interest to the study of FeSi-based (FINEMET), FeZr-based (NANOPERM), and FeCo-based (HITPERM) alloys. Almost all the efforts in understanding these alloys are dedicated to the structure and magnetic properties. However, during the normal service as a magnetic component, due to the natural potential to the metals oxidation, the alloys suffer an oxidation process. Recent study dealing with oxidation kinetics at temperatures around 300–400 ◦ C under controlled atmosphere demonstrated, by extrapolation to room temperatures, that the thickness growing rate of the surface oxide layer can be as high as 40 nm per year [1]. The same work emphasizes that for a magnetic head the working distance to optimize the efficiency of the component is 10 nm. Thus, if the working distance was increased due to an oxidation process, the efficiency of the component would probably be reduced, because of the exponential reduction in the magnetization with the increase of the working distance. The study of the oxidation process and
∗
Corresponding author. Fax: +55 1935225609. E-mail address: jemay [email protected] (J.E. May).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.012
the improvement of the oxidation resistance of the FeCo-based alloys have not only technological interest, but also scientific interest. Our previous work showed that the corrosion resistance of amorphous and crystalline alloys with Fe80 Nb3.5 Zr3.5 Cu1 B12 (NANOPERM) is much lower than that of Fe73 Nb3 Cu1 Si15.5 B7.5 (FINEMET) [2]. The main conclusion was that the addition of silicon improved the corrosion resistance of the protective film formed during immersion in an acid medium and that the composition of the oxide layer formed was rich in SiO2 . It was also demonstrated that the corrosion process negatively affected the magnetic properties. The magnetic saturation flux density dropped from 1.52–1.25 T for NANOPERM alloy, i.e., a reduction of about 20% of magnetic saturation flux density. This behavior was attributed to the formation of a nonmagnetic oxide layer that changes the magnetic lines around the ribbon. FINEMET alloy lost only 2% of its magnetic saturation. It was showed that the addition of Si and Nb to FeCoZrCuB alloys improved their corrosion resistance and diminished the magnetic losses caused by the corrosion process [3]. The Co-rich amorphous alloys (∼Co74 Fe5 Si2 B19 ) displayed B and Si enrichment in a surface oxide layer promoted during annealing at 380 ◦ C/90 min [4]. In that study, it was concluded
J.E. May et al. / Materials Science and Engineering A 428 (2006) 290–294
Fig. 1. DSC curves for amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , Fe33.5 Co33.5 Nb7 Si15 B11 , and Fe38.5 Co38.5 Nb7 Cu10 B15 alloys (15 ◦ C/min, Ar).
that the surface of the formed SiO2 ·B2 O3 layer decomposed spinodally into two regions: Si-rich and B-rich phases, suggesting that these oxides induced nanocrystallization under the surface. However, the possibility that a spinodally decomposed structure in the metallic bulk led to a partitioned surface oxide layer was not mentioned. According to recent results, there is a possibility of spinodal decomposition prior to the crystallization, which will have impact on the surface oxide layer formed during a temperature exposure under oxidation environment [5]. Some evidences of spinodal decomposition for FeCo(Nb,Zr)SiB alloys have already been shown in previous work [6]. In this work, we present the oxidation resistance of Fe36 Co36 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe36 Co36 Nb7 Si15 B11 alloys analyzed by mass gain measurements, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), and discuss the influence of the structure on the surface oxide layer composition and on the oxidation resistance.
291
Differential scanning calorimetry (DSC) scans were conducted by continuous heating from room temperature to 1000 ◦ C at a heating rate of 15 ◦ C/min. Crystallization treatments at 800 ◦ C were performed in argon atmosphere during 90 min for Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys using heating and cooling rates of 25 and 60 ◦ C/min, respectively. Crystallization was confirmed by XRD. Oxidation resistance measurements were carried out by mass gain from 25 to 1100 ◦ C. Mass gain oxidation testing was performed with a NETZSCH STA-409 with a mixing atmosphere of O2 and Ar (20 and 80 ml/min, respectively) and a heating rate of 15 ◦ C/min. SEM was used to observe the oxide surface layer after heat treatment in oxidation environment (atmosphere) during 90 min at 800 ◦ C. A Philips TMP microscope was employed, with a voltage of 25 kV and working distance of 12 mm. XPS was used to characterize the surface with a Kratos XSAM HS spectrometer having a non-monochromatic Mg K␣ (hν = 1253.6 eV) X-ray source, with power given by the emission of 10 mA at a voltage of 13 kV. The high-resolution spectra were obtained with analyzer pass energy of 20 eV. The accuracy of the electron analyzer is 0.1 eV. Gaussian line shapes were used to fit the curves for C 1s and O 1s, and a mixed Gaussian/Lorentzian fuction was employed for the other peaks. The Shirley background and a least-square routine were used for peak fitting. 3. Results Fig. 1 shows the DSC curves for amorphous Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. The Fe36 Co36 Nb7 Si10 B11 and Fe33.5 Co33.5 Nb7 Si15 B11 alloys presented three crystallization peaks, while the
2. Experimental procedure Amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , Fe33.5 Co33.5 Nb7 Si15 B11 , and Fe73 Nb3 Si13.5 B9.5 Cu1 alloy ribbons were produced using a single roll melt spinner with a copper wheel at a speed of 55 ms−1 in an argon atmosphere. X-ray diffraction (XRD) measurements were performed with a D5000 Siemens diffractometer, using a Co K␣ source (λ = 0.17889 nm), and a 2θ step of 2 min−1 . Table 1 Crystalline fraction related with the (FeCo)76 Nb6 B18 phase Sample
Crystalline fraction (%)
Fe36 Co36 Nb6 Cu1 Si10 B11 Fe36 Co36 Nb7 Si10 B11
73 55
Fig. 2. Mass gain of amorphous Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys (15 ◦ C/min, Ar = 80 ml/min and O2 = 20 ml/min).
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Fig. 3. SEM micrographs for crystalline: (a) Fe38.5 Co38.5 Nb7 Cu1 B15 , (b) Fe36 Co36 Nb6 Cu1 Si10 B11 , (c) Fe36 Co36 Nb7 Si10 B11 , and (d) Fe33.5 Co33.5 Nb7 Si15 B11 alloys (after oxidation at room atmosphere during 90 min at 800 ◦ C).
J.E. May et al. / Materials Science and Engineering A 428 (2006) 290–294
Fe36 Co36 Nb6 Cu1 Si10 B11 alloy displayed only two peaks. The Fe33.5 Co33.5 Nb7 Si15 B11 showed just one single crystallization peak. It is interesting to notice that above 800 ◦ C there is no crystallization process occurring. Thus the choosen heat treatment for the annealing resulted in a fully crystalline structure for all samples. The XRD, SEM, and XPS measurements were done in the fully crystalline samples. Table 1 shows the crystalline fraction related with the first crystallization peak, which corresponds to the (FeCo)76 Nb6 B18 phase. Fig. 2 shows the dynamic mass gain as a function of increasing temperature in all samples. The oxidation rates did not differ significantly at temperatures up to 500 ◦ C, but the samples with Si addition showed lower values of mass gain. This difference increased above 500 ◦ C. The traditional FeCobased Fe36 Co36 Nb7 Cu1 B15 showed a major mass gain rate at around 550 ◦ C, although this behavior only became evident for Fe36 Co36 Nb6 Cu1 Si10 B11 and Fe36 Co36 Nb7 Si10 B11 at temperatures exceeding 700 ◦ C. The Fe33.56 Co33.5 Nb7 Si15 B11 alloy showed a better performance than the other alloys, with an increase in mass gain at 950 and 1030 ◦ C, respectively. Fig. 3 shows the SEM micrographs for Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. As it can be observed for the micrographs with lower amplification (∼500×), the oxide surface has different characteristics for each sample. The Fe38.5 Co38.5 Nb7 Cu10 B15 alloy has bigger cracks in the oxide surface than the other alloys. The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys present a more uniform oxide surface as compared to the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. Table 2 shows the composition of the oxide surface layer formed during 90 min of exposure in room atmosphere at 800 ◦ C. There are three main differences among the alloys. (a) The Fe38.5 Co38.5 Nb7 Cu10 B15 alloy presented a higher iron oxide (Fe2 O3 ) content while the other alloys did not display a high amount of iron oxide at the surface. (b) The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys showed a higher content of boron oxide (B2 O3 ) compared to the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. (c) The Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys presented silicon oxide (SiO2 ) in the surface oxide layer, and the other alloying elements (CoO, Nb2 O5 , Cu2 O) were not detected in the surface oxide layer, while these alloying elements were detected for the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy. Table 2 Oxide layer composition analyzed by XPS after oxidation at room atmosphere during 90 min at 800 ◦ C Sample
Fe38.5 Co38.5 Nb7 Cu1 B15 Fe36 Co36 Nb6 Cu1 Si10 B11 Fe36 Co36 Nb7 Si10 B11 Fe33.5 Co33.5 Nb7 Si15 B11 ND: not detected.
Oxide composition (%) Fe
Co
Nb
Cu
Si
B
16.4 ND ND 1.4
2.7 0.4 0.3 0.5
1.7 ND ND ND
1.4 ND – –
– 4.0 9.2 12.7
8.1 39.4 29.5 20.2
293
Fig. 4. XRD diffractograms of crystallized samples after annealing during 90 min at 800 ◦ C (K␣ = Co). Fe38.5 Co38.5 Nb7 Cu1 B15 , Fe36 Co36 Nb6 Cu1 Si10 B11 , Fe36 Co36 Nb7 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys (␣-(FeCo)—cubic BCC; (FeCo)76 Nb6 B11 [7]).
Fig. 4 shows the XRD diffractograms of the nanocrystalline Fe38.5 Co38.5 Nb7 Cu10 B15 , Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and Fe33.5 Co33.5 Nb7 Si15 B11 alloys. All these alloys show a ␣ -FeCo and a (FeCo)76 Nb6 B18 phases in their structure after heat treatment during 90 min at 800 ◦ C. It is interesting to notice that the Fe38.5 Co38.5 Nb7 Cu10 B15 and Fe36 Co36 Nb7 Si10 B11 alloys show a third unidentified phase. 4. Discussion As can be observed in the DSC curves, above 800 ◦ C there is no crystallization process occurring. Thus, the heat treatment conducted in this work led to fully crystallized structure. This is important because there was no amorphous residual matrix and the oxidation process in the surface was processed in a completely crystallized structure. The oxidation measurements were conducted from an amorphous structure at 25–1000 ◦ C. The heating rate was 15 ◦ C/min, which means that all the crystallization process can occur during the experiment. The main conclusion is that the oxidation resistance increases in the order: Fe38.5 Co38.5 Nb7 Cu10 B15 < Fe36 Co36 Nb6 Cu1 Si10 B11 < Fe36 Co36 Nb7 Si10 B11 < Fe33.5 Co33.5 Nb7 Si15 B11 . This behavior can be related with the morphology, the oxide compositions, and the crystalline structure characterized by SEM, XPS, and XRD, respectively. The SEM micrographs indicated that the surface oxide layer of the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy had bigger cracks than the surface oxide layer of the other alloys. The XPS measurements showed a higher iron oxide content in the surface oxide layer of the Fe38.5 Co38.5 Nb7 Cu10 B15 alloy compared to the other alloys. Actually, the main oxides presented on the surface oxide layers for the Fe36 Co36 Nb7 Si10 B11 , Fe36 Co36 Nb6 Cu1 Si10 B11 , and
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Fe33.5 Co33.5 Nb7 Si15 B11 alloys were silicon and boron oxides, as encountered in Co-rich amorphous alloys (∼Co74 Fe5 Si2 B19 ) [4]. Iron oxide was not detected or its amount was very low. The Si addition to the FeCo-based alloys improved the oxidation resistance and this improvement was related with the silicon oxide formed on the surface oxide layer. This silicon oxide and the boron oxide produced a more packed oxide layer. The affinity of the Si for oxygen is higher than the affinity of the oxygen with the other elements (Co, Nb, Cu). Thus, in the alloys that have Si addition this element was preferentially oxidized. When the Si amount in the alloy was increased, the silicon oxide also increased. However, in spite of the Si content in the Fe36 Co36 Nb7 Si10 B11 alloy do not differ a lot compared with the Fe36 Co36 Nb6 Cu1 Si10 B11 alloy, the surface amount of silicon oxide is strongly different. This could be attributed to the crystalline phases formed during the annealing and the crystalline fraction at the end of the crystallization. As observed in the Table 1, the crystalline fraction for the Fe36 Co36 Nb6 Cu1 Si10 B11 alloys is 73%, associated with the (FeCo)76 Nb6 B18 phase, but only 55% for the Fe36 Co36 Nb7 Si10 B11 alloy. 5. Conclusion The Si addition improves the oxidation resistance of the FeCo-based alloys and this improvement is related with the
silicon and boron oxides encountered in the surface oxide layer. Acknowledgements This work has been supported by the Brazilian agencies FAPESP and CNPq. The authors would like to thank Silvia S. Maluf for her assistance in some of the XPS measurements. References [1] C. Dubarry, A. Galerie, G. Poupon, Mater. Sci. Forum. 369–372 (2001) 247–254. [2] C.A.C. Souza, J.E. May, I.A. Carlos, M.F. de Oliveira, S.E. Kuri, C.S. Kiminami, J. Non-Cryst. Solids 304 (2002) 210. [3] J.E. May, C.A.C. Souza, C.L. Morelli, S.E. Kuri, J. Alloys Compd. 390 (2005) 106–111. [4] C.K. Kim, C.S. Yoon, T.Y. Byun, K.S. Hong, Oxidation Met. 55 (3/4) (2001) 177–187. [5] A. Serebryakov, A. Gurov, N. Novokhatskaya, J. Non-Cryst. Solids 260 (1999) 59–64. [6] J.E. May, M.F. de Oliveira, S.E. Kuri, J. Non-Cryst. Solids 348 (2004) 250–257. [7] A. Inoue, H. Koshiba, T. Zhang, A. Makino, J. Appl. Phys. 83 (4) (1998) 1967.