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The coating layer structure of commercial chrome plates
Journal of Electron Spectroscopy and Related Phenomena 202 (2015) 1–6
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
The coating layer structure of commercial chrome plates Sheng Chen ∗ Research Institute of Baoshan Iron & Steel Co. Ltd., 655 Fujin Road, Baoshan District, Shanghai 201900, China
a r t i c l e
i n f o
Article history: Received 28 November 2014 Received in revised form 26 January 2015 Accepted 28 January 2015 Available online 7 February 2015 Keywords: TFS Chrome plate Depth profile AES XPS
a b s t r a c t The surface and cross-sectional morphologies of the commercial chrome plate coating layer with the thickness of dozens of nanometers have been observed. To investigate the detailed structure of the coating layer, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) combined with the low energy Ar+ sputtering technique have been employed. Through careful analysis of experimental data, it can be obtained that the coating layer of commercial chrome plates is composed of four layers from top to bottom with different compositions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Tinplate is widely used for packaging foodstuffs, beverages, oils, grease, paints, powdered, polishes, waxes, chemicals and many other products nowadays [1]. With the continuous growth of tin production, tin resource will eventually be faced with depletion [2]. Thus, different kinds of tin free steels (TFS) have been developed to substitute for tinplates. As the most successful TFS today, chrome plated steel sheets attract more and more attention recently with increasing annual output. Cr plating uses electrolysis to bind a thin Cr layer onto a cold rolling steel plate. The concentration of CrO3 solution used for electroplating is about 150 g/L. The electroplating is usually carried out at the temperature of 40 ◦ C. Because chrome plates have a number of advantages over tinplates such as lower coating weight, better organic coating adhesion and better high temperature tolerance, they have been widely used gradually in recent years. Considering the spot welding performance, commercial chrome plates usually have very thin coating layers with Cr coating mass less than 100 mg/m2 . Because the coating layer thickness of the chrome plate is very thin, which is usually no more than 30 nm, it is difficult to obtain normal direct evaluation and characterization of it. The results obtained by electrochemical methods before indicate that the coating layer of the commercial chrome plate may comprise of an outer hydrated Cr oxide layer and an inner metallic Cr layer [3], but the detailed analysis of the chrome plate coating
layer structure is seldom found [4]. As generally known, the structure and composition of the coating layer play an important role in the performance of the chrome plate. For example, the deep drawing property of the chrome plate will be seriously influenced if the oxide layer in the coating is too thick. So it is important to find out a proper method to characterize the structure of the chrome plate coating layer. Analyzing and researching a very thin coating layer by the traditional grinding and polishing method require highly developed skills in preparing a smooth cross-sectional sample with clear characteristics. If the coating layer is easily corrosive or brittle, the traditional method usually leads to the coating layer damage. A focused ion beam (FIB) system makes accelerated ions focused on the specific location of the sample to remove material from it, and can be employed to easily prepare a cross-sectional sample with high quality. So it is a good choice in the research of chrome plate coating layer. Considering the information depth at nanoscale sizes in solids, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) combined with the low energy Ar+ sputtering technique have great advantages in the analysis of very thin coating layers [5,6]. AES is often used to realize surface analysis of micro-regions, while XPS can be used to investigate the chemical states of elements easily. It is suitable to investigate the thin chrome plate coating layer with Cr in different chemical states at different regions by combining these two methods. 2. Experimental
∗ Tel.: +86 21 26641039; fax: +86 21 26649329. E-mail addresses: chen [email protected], [email protected] http://dx.doi.org/10.1016/j.elspec.2015.01.006 0368-2048/© 2015 Elsevier B.V. All rights reserved.
The surface and cross sectional observations on the commercial chrome plates (the substrate grade of MR T-4CA) made by
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S. Chen / Journal of Electron Spectroscopy and Related Phenomena 202 (2015) 1–6
Fig. 1. The optical microscope (left) and SEM (right) images of the chrome plate coating surface.
was 3 kV. The X-ray source used in XPS system was monochromatic Al K␣ (1486.6 eV) radiation. And the instrumental binding energy scales were calibrated using standard samples of pure Cu, Ag and Au according to ISO15472. The Ar+ with the acceleration voltage of 2 kV was used for either AES or XPS depth profiling. The XPS analysis area was about 300 m × 300 m. 3. Results and discussion
Fig. 2. The cross-sectional SEM image of the chrome plate coating.
Baosteel have been obtained by optical microscope and scanning electron microscope (SEM). There is no other element which can be detected by electron spectroscopy in the substrate except Fe. The cross sectional sample was prepared by SMI3050 FIB system made by SEIKO corporation with an ion source of liquid metallic Ga and an acceleration voltage of 30 kV. The AES and XPS depth profiling experiments of the coating layer have been carried out by PHI700 system and Quantera SXMTM system made by ULVAC-PHI corporation, respectively. The acceleration voltage used in AES experiment
Firstly, the surface morphology of the chrome plate coating layer has been observed by optical microscope and SEM, respectively. It can be seen that the surface with large roughness shaped by previous cold rolling process remains its original morphology after electroplating. The coating layer does not change the surface morphology, and seems continuous and intact without any crack, as shown in Fig. 1. Through the qualitative analysis by AES, it can be obtained that there are only C, O and Cr detected on the surface of coating layer. The cross sectional sample of the chrome plate coating prepared by FIB system has been observed by SEM. From the SEM photo revealed in Fig. 2, it can be seen that the surface of chrome plate substrate is very rough, and the coating layer is very thin. Due to the limit of focused electron beam size, it is difficult to get ideal component analysis result from the cross sectional sample by the method of energy disperse spectroscopy (EDS) or AES. As the analysis of the cross sectional sample has not given a clear coating layer structure, the depth profiling method by electron spectroscopy (AES and XPS) with ion sputtering was employed. AES depth profiling on large area (1 mm × 1 mm) indicates that there are only O, Fe and Cr detected in the coating layer except
Fig. 3. The depth profiling results of the chrome plate coating.
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Fig. 5. Fe2p XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
Fig. 4. Micro zone AES depth profiling results of the chrome plate coating. (a) The analysis zones chosen on the surface, (b) the evolution of Cr LMM, O KLL (left) and Fe LMM (right) peaks with the increasing of sputtering time acquired at zone 1, (c) the evolution of Cr LMM, O KLL (left) and Fe LMM (right) peaks with the increasing of sputtering time acquired at zone 2.
for the C contamination on the topmost surface. According to the peak-to-peak values of signals in AES differentiated spectra and the sensitivity factor method, the normalized atomic concentration of O, Fe and Cr at different sputtering time can be calculated, as shown in the left part of Fig. 3. With the increasing of sputtering time, the information of deeper region is revealed. From this figure, it can be seen that O only exits in the upper layer of the coating, and combines with Cr instead of Fe. Fe is found mainly in the substrate. But in the lower layer of the coating, there is still Fe accompanied by Cr. Cr combines with O in the upper layer, and coexists with Fe in the lower layer. Further, the left part of Fig. 3 can be redrawn as an area figure, as revealed in the right part of Fig. 3. In this figure it can be seen obviously that the whole analyzed region can be roughly separated by three black vertical lines into four parts, which are oxide layer (mainly Cr oxide), elementary Cr layer, Cr–Fe layer (both Cr and Fe) and substrate (mainly Fe) from left to right
(from surface to substrate), respectively. The lines are located at the sputtering time corresponding to 10% or 90% of the differences between the highest and lowest atomic concentrations of O, Cr and Fe. It is well known that the effects of ion sputtering may alter the attained information [7,8], and the uneven thickness of the coating layer may lead to the false result of the elements coexistence when large area depth profiling is carried out. Thus whether Cr is alloyed with Fe in the Cr–Fe layer is still suspicious yet. Next, these two respects are researched. Considering that the sputtering effects are dependent on the ion acceleration voltage, different ion acceleration voltages have been used to estimate the influence of sputtering effects. In the experiment, the acceleration voltages of 2 kV and 4 kV are employed for large area depth AES profiling with other experiment conditions keeping the same. The result concentrationsputtering time curves obtained from the experiments with these two acceleration voltages are remarkably consistent, which indicates the contribution of sputtering effect is not significant in terms of the chrome plate coating sample. To investigate whether the coating layer has a uniform thickness, micro zone (the diameter of the analysis area less than 100 nm) AES depth profiling has been carried out. The result shows the coexistence of the regions with only Cr signal and the regions with only Fe signal at the same sputtering time during sputtering process. This implies the phenomenon that Cr and Fe signals coexist in the same sputtering time observed in large area analysis partially comes from the uneven thickness of the coating layer at least. But it is still difficult to judge whether Cr is alloyed with Fe in the coating layer. To investigate the existence of Cr–Fe alloy, two regions with different heights (as shown in Fig. 4(a), region 1 is higher) have been chosen for AES depth profiling. Using the sputtering time and the kinetic energy of Auger electrons as ordinate and abscissa, the Cr LMM, O KLL and Fe LMM curves obtained at different sputtering time of region 1 and 2 can be summarized as Fig. 4(b) and (c), respectively. The grayscales of these two maps represent the counts of the AES curves. White means high counts, and black means low counts. From these figures, the trends of Cr LMM, O KLL and Fe LMM peaks with the increasing of sputtering time can be seen obviously. The sputtering time when the O signal vanishes in Fig. 4(b) is almost as same as the one in Fig. 4(c). The Fe signal of region 1 appears later than region 2. Moreover, the Cr signal of region 2 fades away with the increasing of sputtering time, but the Cr signal of region 1 can be
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Fig. 6. Cr2p3/2 XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
Fig. 7. O1s XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
observed throughout the whole experiment time. This experiment has been repeated in a lot of different regions, and the vanishing time of O signal obtained in each region reveals less than 1 min, and remains almost the same. This indicates there is an oxide layer with a small and uniform thickness on the top of the coating layer. The difference of Fe signal appearing time corresponding to various regions implies the thickness of the elementary Cr layer is uneven. After the appearance of Fe signal, the vanishing time of Cr signal is early in some regions, but late in others. It can even be found the regions in which the Cr signals remain notable during the whole sputtering experiments, as illustrated in Fig. 4(b). From this it can be deduced that there indeed exists a layer between the elementary Cr layer and the substrate where both Cr and Fe atoms can be found in small region. This is a Cr–Fe alloy layer the thickness of which is not uniform. The Cr–Fe alloy layer is thicker in the region where the substrate is raised, as the situation of region 1 in Fig. 4(a). This very thin alloy layer may be formed by the counterdiffusion of the active Fe atoms in the substrate and the Cr atoms just deposited in the initial period of the electroplating. XPS depth profiling has been carried out to investigate the chemical environments of elements in the chrome plate coating. The C signal is found only on the topmost surface, and removed totally by a very light sputtering. This comes from the adventitious hydrocarbon on the surface. There is no Fe signal detected until the sputtering time of 3 min. According to the positions and shapes of Fe2p peaks obtained, Fe reveals metallic in the whole testing depth (the Fe2p3/2 peak corresponding to oxides locates above 709 eV [9]), as shown in Fig. 5.
This supports the result got from AES depth profiling experiment that O in the coating does not combine with Fe. The evolution of Cr2p3/2 peak with the increasing of sputtering time is displayed in Fig. 6. From this figure it is easy to see that the shape and position of the Cr2p3/2 peak change significantly near the surface (when the sputtering time is short). In the deeper depth the peak only reveals change of intensity. In fact, according to the peak shape and position, it can be confirmed that Cr keeps metallic in the sample after the sputtering time of 1.5 min. This is also in accordance with the AES depth profiling experiment. Fig. 7 shows the change of O1s peak with the increasing of sputtering time. It is obvious that the peak according to the topmost surface is easy to distinguish from the peaks obtained after sputtering. This peak located at 531.9 eV is symmetric and difficult to be fitted to the sum of two or more peaks. Considering the results of Cr and Fe analysis, this implies that there is only one kind of compound containing Cr and O on the topmost surface which is not the same as the oxide in the deeper depth of the coating. O1s peaks obtained at the sputtering time from 0.5 min to 1.5 min do not show obvious differences in the aspects of shape and position. This indicates there is only one kind of Cr oxide in the upper layer of the coating. Because Fe and Cr both reveal metallic after sputtering for 1.5 min as mentioned before, O signals detected after sputtering for 1.5 min come from the adsorbed oxygen, which induced by relatively long testing time of XPS instrument. Based on the analysis above, peak fitting of the Cr2p3/2 peaks obtained at the sputtering time of 0, 0.5, 1 and 1.5 min can be carried out, as shown in Fig. 8. The Cr2p3/2 peak obtained on
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Fig. 8. Analysis of Cr2p3/2 peaks acquired at the sputtering time of 0, 0.5, 1 and 1.5 min.
the topmost surface can be resolved into a peak with the binding energy of 574.4 eV and a peak with the binding energy of 577.7 eV. The former can be attributed to metallic Cr [10]. And the later may be corresponding to some kind of hydrated Cr oxide. The metallic Cr signal comes from metallic Cr below the oxide layer, which is too thin to stop the transmission of photoelectrons generated by metallic Cr below it. Based on this, it can be estimated that the thickness of the oxide layer is about several nanometers. So there is only Cr hydroxide on the surface of the coating. The Cr2p3/2 peaks corresponding to 0.5 min and 1 min can be resolved to be two components. One is attributed to metallic Cr, the other is an asymmetric and broad peak, which comes from trivalent Cr combined with O. The former also comes from the metallic Cr below. The later reveals asymmetric and broad, because it is an envelope of five individual peaks caused by multiplet splitting effect. Due to the limit of the instrument resolution, these five peaks reveal only one broad peak shape [11]. To make it easier, these five peaks are treated as an entire component instead of being handled individually. It can be seen from Fig. 8(b) and (c) that the peak intensity corresponding to Cr oxide drops down, and the peak intensity corresponding to metallic Cr rises up with the increasing of sputtering time, which implicates the decrease of oxide layer thickness. When the sputtering time increases to be 1.5 min, there is only metallic Cr component in the Cr2p3/2 peak obtained, which indicates that there exists only metallic Cr in the depth below. Considering the results got from AES experiment, it is clear that the metallic Cr below contains elementary Cr and alloyed Cr, the XPS peak positions of which cannot be distinguished easily.
Fig. 9. The structure of the coating layer of commercial chrome plate.
To sum up, through detailed analysis, it is deduced that the coating layer of commercial chrome plate consists of surface layer (only the topmost surface, consisting of Cr hydroxide), oxide layer (consisting of Cr2 O3 ), elementary Cr layer and Cr–Fe alloy layer from top to bottom, as shown in Fig. 9. 4. Conclusions Through the morphology observation and electron spectroscopy analysis of the commercial chrome plate coating layer, it can be confirmed that the coating layer is very thin, and consists of surface layer, oxide layer, elementary Cr layer and Cr–Fe alloy layer from top to bottom. The surface layer is the topmost surface made up of Cr hydroxide. The oxide layer consisting of Cr2 O3 has a thin and uniform thickness which is about several nanometers. By contrast, the thicknesses of the elementary Cr layer and the Cr–Fe alloy layer are uneven. Perhaps this is induced by the large roughness of the substrate surface. In the region where the substrate is raised, these two layers are thicker.
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References [1] ITRI Ltd., Guide to Tinplate, ITRA Ltd., Uxbridge, 2000, pp. 41–61. [2] C.F. Izard, D.B. Müller, Tracking the devil’s metal: historical global and contemporary U.S. tin cycles, Resour. Conserv. Recycl. 54 (2010) 1436–1441. [3] Japanese Standards Association, JIS G3315:2008, Tin Free Steel, Japanese Standards Association, Japan, 2008, pp. p2. [4] L. Lu, T. Liu, X. Li, Composition analysis of the plating on electrolytically treated steel sheets in chromic acid solution, Surf. Coat. Technol. 202 (2008) 1401–1404. [5] D.J. O’Connor, B.A. Sexton, R.S.C. Smart, Surface Analysis Methods in Materials Science, second ed., Springer, Berlin, 2003, pp. 107–125. [6] S. Oswald, S. Baunack, Comparison of depth profiling techniques using ion sputtering from the practical point of view, Thin Solid Films 425 (2003) 9–19.
[7] V.S. Smentkowski, Trends in sputtering, Prog. Surf. Sci. 64 (2000) 1–58. [8] E. Lewin, M. Gorgoi, F. Schäfers, S. Svensson, U. Jansson, Influence of sputter damage on the XPS analysis of metastable nanocomposite coatings, Surf. Coat. Technol. 204 (2009) 455–462. [9] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., Chigasaki, 1995, pp. 81. [10] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., Chigasaki, 1995, pp. 76. [11] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W.M. Lau, A.R. Gerson, R.S.C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni, Appl. Surf. Sci. 257 (2011) 2717–2730.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
The coating layer structure of commercial chrome plates Sheng Chen ∗ Research Institute of Baoshan Iron & Steel Co. Ltd., 655 Fujin Road, Baoshan District, Shanghai 201900, China
a r t i c l e
i n f o
Article history: Received 28 November 2014 Received in revised form 26 January 2015 Accepted 28 January 2015 Available online 7 February 2015 Keywords: TFS Chrome plate Depth profile AES XPS
a b s t r a c t The surface and cross-sectional morphologies of the commercial chrome plate coating layer with the thickness of dozens of nanometers have been observed. To investigate the detailed structure of the coating layer, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) combined with the low energy Ar+ sputtering technique have been employed. Through careful analysis of experimental data, it can be obtained that the coating layer of commercial chrome plates is composed of four layers from top to bottom with different compositions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Tinplate is widely used for packaging foodstuffs, beverages, oils, grease, paints, powdered, polishes, waxes, chemicals and many other products nowadays [1]. With the continuous growth of tin production, tin resource will eventually be faced with depletion [2]. Thus, different kinds of tin free steels (TFS) have been developed to substitute for tinplates. As the most successful TFS today, chrome plated steel sheets attract more and more attention recently with increasing annual output. Cr plating uses electrolysis to bind a thin Cr layer onto a cold rolling steel plate. The concentration of CrO3 solution used for electroplating is about 150 g/L. The electroplating is usually carried out at the temperature of 40 ◦ C. Because chrome plates have a number of advantages over tinplates such as lower coating weight, better organic coating adhesion and better high temperature tolerance, they have been widely used gradually in recent years. Considering the spot welding performance, commercial chrome plates usually have very thin coating layers with Cr coating mass less than 100 mg/m2 . Because the coating layer thickness of the chrome plate is very thin, which is usually no more than 30 nm, it is difficult to obtain normal direct evaluation and characterization of it. The results obtained by electrochemical methods before indicate that the coating layer of the commercial chrome plate may comprise of an outer hydrated Cr oxide layer and an inner metallic Cr layer [3], but the detailed analysis of the chrome plate coating
layer structure is seldom found [4]. As generally known, the structure and composition of the coating layer play an important role in the performance of the chrome plate. For example, the deep drawing property of the chrome plate will be seriously influenced if the oxide layer in the coating is too thick. So it is important to find out a proper method to characterize the structure of the chrome plate coating layer. Analyzing and researching a very thin coating layer by the traditional grinding and polishing method require highly developed skills in preparing a smooth cross-sectional sample with clear characteristics. If the coating layer is easily corrosive or brittle, the traditional method usually leads to the coating layer damage. A focused ion beam (FIB) system makes accelerated ions focused on the specific location of the sample to remove material from it, and can be employed to easily prepare a cross-sectional sample with high quality. So it is a good choice in the research of chrome plate coating layer. Considering the information depth at nanoscale sizes in solids, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) combined with the low energy Ar+ sputtering technique have great advantages in the analysis of very thin coating layers [5,6]. AES is often used to realize surface analysis of micro-regions, while XPS can be used to investigate the chemical states of elements easily. It is suitable to investigate the thin chrome plate coating layer with Cr in different chemical states at different regions by combining these two methods. 2. Experimental
∗ Tel.: +86 21 26641039; fax: +86 21 26649329. E-mail addresses: chen [email protected], [email protected] http://dx.doi.org/10.1016/j.elspec.2015.01.006 0368-2048/© 2015 Elsevier B.V. All rights reserved.
The surface and cross sectional observations on the commercial chrome plates (the substrate grade of MR T-4CA) made by
2
S. Chen / Journal of Electron Spectroscopy and Related Phenomena 202 (2015) 1–6
Fig. 1. The optical microscope (left) and SEM (right) images of the chrome plate coating surface.
was 3 kV. The X-ray source used in XPS system was monochromatic Al K␣ (1486.6 eV) radiation. And the instrumental binding energy scales were calibrated using standard samples of pure Cu, Ag and Au according to ISO15472. The Ar+ with the acceleration voltage of 2 kV was used for either AES or XPS depth profiling. The XPS analysis area was about 300 m × 300 m. 3. Results and discussion
Fig. 2. The cross-sectional SEM image of the chrome plate coating.
Baosteel have been obtained by optical microscope and scanning electron microscope (SEM). There is no other element which can be detected by electron spectroscopy in the substrate except Fe. The cross sectional sample was prepared by SMI3050 FIB system made by SEIKO corporation with an ion source of liquid metallic Ga and an acceleration voltage of 30 kV. The AES and XPS depth profiling experiments of the coating layer have been carried out by PHI700 system and Quantera SXMTM system made by ULVAC-PHI corporation, respectively. The acceleration voltage used in AES experiment
Firstly, the surface morphology of the chrome plate coating layer has been observed by optical microscope and SEM, respectively. It can be seen that the surface with large roughness shaped by previous cold rolling process remains its original morphology after electroplating. The coating layer does not change the surface morphology, and seems continuous and intact without any crack, as shown in Fig. 1. Through the qualitative analysis by AES, it can be obtained that there are only C, O and Cr detected on the surface of coating layer. The cross sectional sample of the chrome plate coating prepared by FIB system has been observed by SEM. From the SEM photo revealed in Fig. 2, it can be seen that the surface of chrome plate substrate is very rough, and the coating layer is very thin. Due to the limit of focused electron beam size, it is difficult to get ideal component analysis result from the cross sectional sample by the method of energy disperse spectroscopy (EDS) or AES. As the analysis of the cross sectional sample has not given a clear coating layer structure, the depth profiling method by electron spectroscopy (AES and XPS) with ion sputtering was employed. AES depth profiling on large area (1 mm × 1 mm) indicates that there are only O, Fe and Cr detected in the coating layer except
Fig. 3. The depth profiling results of the chrome plate coating.
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Fig. 5. Fe2p XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
Fig. 4. Micro zone AES depth profiling results of the chrome plate coating. (a) The analysis zones chosen on the surface, (b) the evolution of Cr LMM, O KLL (left) and Fe LMM (right) peaks with the increasing of sputtering time acquired at zone 1, (c) the evolution of Cr LMM, O KLL (left) and Fe LMM (right) peaks with the increasing of sputtering time acquired at zone 2.
for the C contamination on the topmost surface. According to the peak-to-peak values of signals in AES differentiated spectra and the sensitivity factor method, the normalized atomic concentration of O, Fe and Cr at different sputtering time can be calculated, as shown in the left part of Fig. 3. With the increasing of sputtering time, the information of deeper region is revealed. From this figure, it can be seen that O only exits in the upper layer of the coating, and combines with Cr instead of Fe. Fe is found mainly in the substrate. But in the lower layer of the coating, there is still Fe accompanied by Cr. Cr combines with O in the upper layer, and coexists with Fe in the lower layer. Further, the left part of Fig. 3 can be redrawn as an area figure, as revealed in the right part of Fig. 3. In this figure it can be seen obviously that the whole analyzed region can be roughly separated by three black vertical lines into four parts, which are oxide layer (mainly Cr oxide), elementary Cr layer, Cr–Fe layer (both Cr and Fe) and substrate (mainly Fe) from left to right
(from surface to substrate), respectively. The lines are located at the sputtering time corresponding to 10% or 90% of the differences between the highest and lowest atomic concentrations of O, Cr and Fe. It is well known that the effects of ion sputtering may alter the attained information [7,8], and the uneven thickness of the coating layer may lead to the false result of the elements coexistence when large area depth profiling is carried out. Thus whether Cr is alloyed with Fe in the Cr–Fe layer is still suspicious yet. Next, these two respects are researched. Considering that the sputtering effects are dependent on the ion acceleration voltage, different ion acceleration voltages have been used to estimate the influence of sputtering effects. In the experiment, the acceleration voltages of 2 kV and 4 kV are employed for large area depth AES profiling with other experiment conditions keeping the same. The result concentrationsputtering time curves obtained from the experiments with these two acceleration voltages are remarkably consistent, which indicates the contribution of sputtering effect is not significant in terms of the chrome plate coating sample. To investigate whether the coating layer has a uniform thickness, micro zone (the diameter of the analysis area less than 100 nm) AES depth profiling has been carried out. The result shows the coexistence of the regions with only Cr signal and the regions with only Fe signal at the same sputtering time during sputtering process. This implies the phenomenon that Cr and Fe signals coexist in the same sputtering time observed in large area analysis partially comes from the uneven thickness of the coating layer at least. But it is still difficult to judge whether Cr is alloyed with Fe in the coating layer. To investigate the existence of Cr–Fe alloy, two regions with different heights (as shown in Fig. 4(a), region 1 is higher) have been chosen for AES depth profiling. Using the sputtering time and the kinetic energy of Auger electrons as ordinate and abscissa, the Cr LMM, O KLL and Fe LMM curves obtained at different sputtering time of region 1 and 2 can be summarized as Fig. 4(b) and (c), respectively. The grayscales of these two maps represent the counts of the AES curves. White means high counts, and black means low counts. From these figures, the trends of Cr LMM, O KLL and Fe LMM peaks with the increasing of sputtering time can be seen obviously. The sputtering time when the O signal vanishes in Fig. 4(b) is almost as same as the one in Fig. 4(c). The Fe signal of region 1 appears later than region 2. Moreover, the Cr signal of region 2 fades away with the increasing of sputtering time, but the Cr signal of region 1 can be
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Fig. 6. Cr2p3/2 XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
Fig. 7. O1s XPS spectra of the chrome plate coating acquired at different Ar+ sputtering time.
observed throughout the whole experiment time. This experiment has been repeated in a lot of different regions, and the vanishing time of O signal obtained in each region reveals less than 1 min, and remains almost the same. This indicates there is an oxide layer with a small and uniform thickness on the top of the coating layer. The difference of Fe signal appearing time corresponding to various regions implies the thickness of the elementary Cr layer is uneven. After the appearance of Fe signal, the vanishing time of Cr signal is early in some regions, but late in others. It can even be found the regions in which the Cr signals remain notable during the whole sputtering experiments, as illustrated in Fig. 4(b). From this it can be deduced that there indeed exists a layer between the elementary Cr layer and the substrate where both Cr and Fe atoms can be found in small region. This is a Cr–Fe alloy layer the thickness of which is not uniform. The Cr–Fe alloy layer is thicker in the region where the substrate is raised, as the situation of region 1 in Fig. 4(a). This very thin alloy layer may be formed by the counterdiffusion of the active Fe atoms in the substrate and the Cr atoms just deposited in the initial period of the electroplating. XPS depth profiling has been carried out to investigate the chemical environments of elements in the chrome plate coating. The C signal is found only on the topmost surface, and removed totally by a very light sputtering. This comes from the adventitious hydrocarbon on the surface. There is no Fe signal detected until the sputtering time of 3 min. According to the positions and shapes of Fe2p peaks obtained, Fe reveals metallic in the whole testing depth (the Fe2p3/2 peak corresponding to oxides locates above 709 eV [9]), as shown in Fig. 5.
This supports the result got from AES depth profiling experiment that O in the coating does not combine with Fe. The evolution of Cr2p3/2 peak with the increasing of sputtering time is displayed in Fig. 6. From this figure it is easy to see that the shape and position of the Cr2p3/2 peak change significantly near the surface (when the sputtering time is short). In the deeper depth the peak only reveals change of intensity. In fact, according to the peak shape and position, it can be confirmed that Cr keeps metallic in the sample after the sputtering time of 1.5 min. This is also in accordance with the AES depth profiling experiment. Fig. 7 shows the change of O1s peak with the increasing of sputtering time. It is obvious that the peak according to the topmost surface is easy to distinguish from the peaks obtained after sputtering. This peak located at 531.9 eV is symmetric and difficult to be fitted to the sum of two or more peaks. Considering the results of Cr and Fe analysis, this implies that there is only one kind of compound containing Cr and O on the topmost surface which is not the same as the oxide in the deeper depth of the coating. O1s peaks obtained at the sputtering time from 0.5 min to 1.5 min do not show obvious differences in the aspects of shape and position. This indicates there is only one kind of Cr oxide in the upper layer of the coating. Because Fe and Cr both reveal metallic after sputtering for 1.5 min as mentioned before, O signals detected after sputtering for 1.5 min come from the adsorbed oxygen, which induced by relatively long testing time of XPS instrument. Based on the analysis above, peak fitting of the Cr2p3/2 peaks obtained at the sputtering time of 0, 0.5, 1 and 1.5 min can be carried out, as shown in Fig. 8. The Cr2p3/2 peak obtained on
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Fig. 8. Analysis of Cr2p3/2 peaks acquired at the sputtering time of 0, 0.5, 1 and 1.5 min.
the topmost surface can be resolved into a peak with the binding energy of 574.4 eV and a peak with the binding energy of 577.7 eV. The former can be attributed to metallic Cr [10]. And the later may be corresponding to some kind of hydrated Cr oxide. The metallic Cr signal comes from metallic Cr below the oxide layer, which is too thin to stop the transmission of photoelectrons generated by metallic Cr below it. Based on this, it can be estimated that the thickness of the oxide layer is about several nanometers. So there is only Cr hydroxide on the surface of the coating. The Cr2p3/2 peaks corresponding to 0.5 min and 1 min can be resolved to be two components. One is attributed to metallic Cr, the other is an asymmetric and broad peak, which comes from trivalent Cr combined with O. The former also comes from the metallic Cr below. The later reveals asymmetric and broad, because it is an envelope of five individual peaks caused by multiplet splitting effect. Due to the limit of the instrument resolution, these five peaks reveal only one broad peak shape [11]. To make it easier, these five peaks are treated as an entire component instead of being handled individually. It can be seen from Fig. 8(b) and (c) that the peak intensity corresponding to Cr oxide drops down, and the peak intensity corresponding to metallic Cr rises up with the increasing of sputtering time, which implicates the decrease of oxide layer thickness. When the sputtering time increases to be 1.5 min, there is only metallic Cr component in the Cr2p3/2 peak obtained, which indicates that there exists only metallic Cr in the depth below. Considering the results got from AES experiment, it is clear that the metallic Cr below contains elementary Cr and alloyed Cr, the XPS peak positions of which cannot be distinguished easily.
Fig. 9. The structure of the coating layer of commercial chrome plate.
To sum up, through detailed analysis, it is deduced that the coating layer of commercial chrome plate consists of surface layer (only the topmost surface, consisting of Cr hydroxide), oxide layer (consisting of Cr2 O3 ), elementary Cr layer and Cr–Fe alloy layer from top to bottom, as shown in Fig. 9. 4. Conclusions Through the morphology observation and electron spectroscopy analysis of the commercial chrome plate coating layer, it can be confirmed that the coating layer is very thin, and consists of surface layer, oxide layer, elementary Cr layer and Cr–Fe alloy layer from top to bottom. The surface layer is the topmost surface made up of Cr hydroxide. The oxide layer consisting of Cr2 O3 has a thin and uniform thickness which is about several nanometers. By contrast, the thicknesses of the elementary Cr layer and the Cr–Fe alloy layer are uneven. Perhaps this is induced by the large roughness of the substrate surface. In the region where the substrate is raised, these two layers are thicker.
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