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Characterisation of oxide and hydroxide layers on technical aluminum materials using XPS
Vacuum 86 (2012) 1216e1219
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterisation of oxide and hydroxide layers on technical aluminum materials using XPSq J. Zähr a, *, S. Oswald b, M. Türpe c, H.J. Ullrich a, U. Füssel a a
TU Dresden, Institute of Surface and Manufacturing Technology, Joining Technology and Assembly, D-01062 Dresden, Germany IFW Dresden, P.O.-Box 270116, D-01171 Dresden, Germany c Behr GmbH & Co. KG, Siemensstraße 164, D-70469 Stuttgart, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2010 Received in revised form 2 February 2011 Accepted 5 February 2011
Aluminum alloys are widely used as technical lightweight materials. For industrial applications, these materials often have to be bonded, e. g. welded or brazed. To get a metallic connection by brazing, the natural oxide layer on Al-materials, which is dense and has a high melting point, has to be eliminated. This layer can be influenced by the temperature and humidity in the surrounding atmosphere. Due to the nm-thickness of the layer, the analysis of its thickness and composition is challenging. An applicable method is X-ray photoelectron spectroscopy (XPS). The present investigation shows the applicability of XPS to estimate the oxide layer thickness as well as to distinguish between Al-hydroxide and Al-oxide phases. Finally, reasons for the differences in brazeability depending on the previous storage conditions are studied. It is shown, that a storage under condensation for 9 days causes an increase of the oxide layer thickness and an aggregation of water inside the pores of the hydroxide layer. This lowers significantly the brazeability of the material. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Al-oxide Brazing Climate tests XPS
1. Introduction Aluminum materials are widely used materials for technical applications, e.g. in automotive production or for heat exchangers, due to their excellent combination of mean density, high strength and good corrosion resistance. The high corrosion resistance is based on the dense natural aluminum oxide layer [9]. This layer is formed in air within seconds [8]. In technical applications the natural layer on aluminum materials is always assigned as aluminum oxide, albeit this term is not correct. The amorphous Al2O3 - -layer, which is located directly at the metallic interface, is the so-called barrier layer [13]. At the transition of the barrier layer to the environment a porous and hydrous layer is formed which can consist of gelatinous boehmite or bayerite [14] [15], see Fig. 1. The oxide layer has to be removed for brazing or welding [16]. Because of the importance for joining technology, its composition and thickness have to be known [17] [18]. The analysis of the natural layer is demanding due to its small thickness of 2e3 nm. One analysis technique for such thin films is
X-ray photoelectron spectroscopy (XPS) because the signals arise from a depth of only some nm. Additionally, the analysis of both oxide and metallic Al is possible using chemical peak shifts [19] [20]. In the present investigation, we distinguish between the Al2p-signal from the metallic substrate and the oxygen signal from the oxide and hydroxide layer by peak fit procedures. The results from this investigation are used for: - Estimation the layer thicknesses and - Determination the oxide/hydroxide content Systematic climate tests in well-defined atmospheres were done to simulate the alteration of the surfaces during transport and storage. Afterward, brazing tests were performed using the stored materials. Hence, a correlation between the oxide/hydroxide layer thickness and the brazeability of the material is possible. 2. Experimental 2.1. Technical aluminum alloy for storage and brazing tests
q A corresponding talk was presented at the 5th Symposium on Vacuum based Science and Technology SVST5, Kaiserslautern (Germany), 28e30 September 2010. * Corresponding author. Tel.: þ49 351 46334346; fax: þ49 351 463 37249. E-mail address: [email protected] (J. Zähr). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.04.004
The material investigated is a both side cladded Al-alloy, which is usually used for heat exchangers [21]. The core material is an AleMn-alloy (EN AW-Al Mn1Cu, EN AW-3003). The clad material
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
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Fig. 2. Scheme of material investigated (not true to scale): Technical aluminum.
Fig. 1. Natural surface layer on aluminum materials [8].
consists of an AleSi-alloy (EN AW-Al Si10, EN AW-4045). A scheme of a cross section of the material is shown in Fig. 2. The exact composition is given in Table 1. Such material is often used for house wares and packaging, whereby the clad material is a typical brazing alloy for Al-based materials [1]. The Si-additions cause a decrease of the melting temperature. Therefore, the core alloy can be joined by brazing without melting at a temperature of about 605 C. 2.2. Climate tests The aim of the investigations is the analysis of the influence of different atmosphere conditions on the surface of technical aluminum alloys. Therefore, climate conditions had to be chosen which are typical for the storage conditions in practice and the transport of Al-materials. Due to these requirements, the following environmental conditions were used: Normal climate (NC) 23 C/50% RH [2] Humid atmosphere (HA) 40 C/92% RH [3] Condensation atmosphere (CA) 23 C, humidity 100% These atmospheres were simulated in both a climate chamber and a condensation chamber under control of temperature and relative humidity (RH). The samples had a size of 100 50 mm2 with a thickness of 0.4 mm, see Fig. 2. 2.3. XPS The XPS measurements were carried out with a PHI 5600 CI spectrometer (Physical Electronics) which is equipped with a hemispherical analyzer operated with a typical pass energy of 29 eV and 800 mm diameter analysis area. Monochromatic AleKa excitation (1486.6 eV, 350 W) was used. Sputter cleaning was done with Arþ ions of 1.5 keV at a scan size of 3 mm 3 mm, and with an ion current leading a sputter rate of approximately 1 nm/min. Depth profile measurements were performed with the same ion
gun at 3.5 keV, a scan size of 2 mm 2 mm and the same sputter rate. Standard single element sensitivity factors were used to estimate the element concentrations, whereby a homogeneous mixture of the elements in the surface region was assumed. If significant charging effects were occurring, a low energy electron flood gun was used with a typical current of 1 mA. Nevertheless, some residual peak shifts resulting from different charging were observed. These were indicated by varying differences of the binding energy between the aluminum metal and aluminum oxide peak. Such changes were considered in the peak fit procedure. The spectra were usually calibrated by the C1s peak. Additionally, all peak intensities were normalized for a better comparison of the different binding states, see Fig. 3. The thickness of the Al-oxide layer was estimated from the ratio between the Al signal from the oxide overlayer (“Al-oxide”) and the Al-metal signal from the bulk material. This method is reliable also for different C-contamination thicknesses, because both signals have the same intensity attenuation through the contamination [4]. This is a very commonly used method as discussed also by PAYNTER [4] and KOZLOWSKA [22]. For the calculations the EXCEL-sheet based on the work of KOZLOWSKA et al. [5] and the attenuation lengths obtained from the CUMPSON-SEAH-2 (CS2) algorithm [6] were used. The peak areas were calculated from peak fit procedures using the PHI-MULTIPAK program [7]. 2.4. Brazing tests Brazing tests were done in a laboratory glass tube furnace to analyze the correlation between the oxide and hydroxide layer thickness as well as the surface layer constitution with the brazeability. All tests were done with a brazing temperature of 605 C and under a nitrogen atmosphere. The clad material, EN AW-Al Si10, see Fig. 2, served as brazing solder. The brazed length was measured to evaluate the brazeability. A natural oxide layer with a thickness of 2e3 nm has a brazing rate of 75e95% in this laboratory scale procedure. This corresponding sample called “Techn. Alloy” was not treated before brazing. 3. Results and discussion 3.1. Differentiation between Al(OH)3 and Al2O3 Two natural layers are existing on aluminum alloys: an amorphous Al2O3-layer and a hydroxide layer [8,9]. The thickness of the Al2O3-layer depends on the environmental temperature and on the
Table 1 Composition of technical aluminum alloy used for storage and brazing tests. Aluminum alloy
Concentration [weight %] [23] Si
EN AW-3003 mod. (EN AW-Al Mn1Cu mod.) EN AW-4045 mod. (EN AW-Al Si10 mod.)
0.071 10.05
Fe
0.205 0.174
Cu
0.464 0.08
Mn
0.957 0.013
Mg
0.242 0.008
Cr
0.006 0
Zn
0.043 0
Ti
0.086 0.012
Additions
Al
Single
All
0.05 0.05
0.15 0.15
Rest Rest
1218
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
a
b
Fig. 4. Atomic ratio of Al-oxide/O. Distinguishing of Al-hydroxide and Al-oxide is possible via this ratio. For technical Al-materials just the Al-oxide signal fraction of the Al2p-signal can be considered.
Fig. 3. XPS-spectra of Al2p- (a) and O1s-signal (b) of different Al-oxides/Al-hydroxides. Distinguishing of Al-oxide and Al-hydroxide is just possible via the O1s-signal. Alhydroxide is existing even at the surface of technical Al.
material composition [9]. The humidity in the environmental atmosphere as well as the temperature influence the thickness and the composition of Al(OH)3-layer [9], which is formed at room temperature at the surface [10]. One aim of the present investigation was the analysis of aluminum surfaces with different storage conditions. Therefore, first tests were performed to get a routine for distinguishing between oxide and hydroxide layers on the technical aluminum alloy. The following surface conditions were tested (see Fig. 4):
condensation influences the shifting more than the storage in water. Furthermore, it has to be noted that the O1s-hydrogen peak of the not-stored initial state of the Al-alloy shows a similar behavior as that of the water - stored sample at higher binding energies. At the low energy tail the behavior corresponds to that of the Al2O3-sample. According to the previously mentioned work [11], it can thus be assumed, that the layer consists of oxide as well as hydroxide phases. As another possibility to distinguish between the different phases the molar ratio of the concentrations of Al and O can be evaluated [11,12]. In the studies of KLOPROGGE et. al. [11]. and WITTBERG et al. [12]. mineral samples of Al-oxide and hydroxide were used. In our study technical Al-materials with a very thin natural oxide layer at the surface are analyzed. Only the oxide-bound Al2p-peak and the total O1s (oxide and hydroxide) signal are used for the calculation of the AleO-ratio. Fig. 4 displays the ratios of the atomic concentration of oxide e bound Al (“Al-oxide”) to O for the initial state sample as well as for the stored samples. By help of this ratio, it can be proven again, that bayerite (Al(OH)3) is existing at the surface of the technical
pure Al2O3 technical Al-alloy without storage (initial state) technical Al-alloy which was stored for1 day in H2O with 20 C technical Al-alloy which was stored for10 days under condensation at 23 C
According to KLOPROGGE et al. [11] a differentiation of the different AleO-phases can be obtained from the O1s- and the Al2p-peak of the XPS-spectra. In Fig. 3 the XPS-peaks of Al2p and O1s are presented. It is obvious, that the Al2p-oxide signal shows, besides a small broadening presumably originating from different charging, no significant tendencies. The Al-oxide peak in the Al2p spectra refers both to Al2O3 and Al-hydroxide. However, the O1s-peak displays differences between the four samples. The Al2O3-sample has the maximum of the peak at the lowest binding energy. The storage under water causes a small shifting of the O1s-peak to higher binding energies as well as a broadening of the signal. It has to be emphasized that the storage under
Fig. 5. Oxide thickness of technical Al-materials in initial state (IS) and after storage. A storage in condensation atmosphere (CA) results in an increase of oxide thickness. Surprisingly, subsequent storage in normal climate (NC) leads to an oxide layer growth.
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
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The influence of the oxide layer properties on the brazing rate is shown in Fig. 7. All surface conditions have astonishingly a satisfying brazing rate. Only the sample with the 9-day storage under condensation atmosphere prohibits brazing. Therefore, we assume that for longer exposure H2O inside the pores of the hydroxide layer influences the brazing behavior significantly. Following storage under normal climate raises the brazing rate again to approximately 75%. The general increase of the total Al-oxide thickness (see Fig. 5) has no major influence.
4. Conclusions Fig. 6. Ratio of atomic concentration of Al-oxide to O. Bayerite is present on technical surfaces at the transition to atmosphere. This is not influenced by storage under normal climate, humid or condensation atmosphere.
non-stored material as well as on Al-materials which were stored under water influence. In contrast to the materials with a hydroxide structure at the surface, the Al2O3-sample has a higher Al-oxide to O-ratio. However, the measured ratio is distinctly lower than the theoretical value for Al2O3. A reason for this difference may be the reaction between the Al2O3-surface with the humidity in the air causing partly the formation of hydroxide structures at the surface of this sample, too.
3.2. Influence of storage conditions on the surface of technical Alalloys In this part of the study the XPS-method is used to analyze the surface composition of technical Al-alloys after storage under normal climate (NC), humid atmosphere (HA) and condensation atmosphere (CA) for different duration. The analyses show that storage under normal climate as well as under humid atmosphere does not influence the oxide thickness, see Fig. 5. Under condensation conditions the oxide layer is growing due to the reaction with H2O. It has to be emphasized, that the oxide thickness is increasing even during a subsequent storage under normal climate. Additionally, H2O may be stored inside the pores of the Al-hydroxide layer. All samples are found to have a ratio of Al-oxide to O of around 0.32 to 0.38, see Fig. 6. Thus it can be assumed, that bayerite is present on all samples.
Fig. 7. Brazing rate of technical Al-materials after different storage conditions. Only a long storage under condensation atmosphere prohibits brazing. This means that the adsorbed water in pores or at the surface influences the brazing process significantly.
XPS was successfully applied to investigate the behavior of Almaterial surfaces during climate tests for simulating material changes during storage and transport. Despite using monochromatic X-ray excitation to obtain good energy resolution for the expected small peak shifts, the identification of the peak positions at our technical surfaces was complicated by charging processes of the dielectric Al2O3. To distinguish between oxide and hydroxide AleO-compositions, the ratio between Al bound in oxide form and the amount of oxygen was considered. With the help of this ratio we could distinguish between Al2O3 and aluminum hydroxide in the surface layers. Additionally, the thickness of the layers was calculated from the oxide- and metal e Al peak ratios. This method was applied for a series of Al-material stored under different climate conditions. From brazing tests it appears that not the increasing oxide thickness, but water inside the pores of the oxide layer may reduce the brazeability.
References [1] Datta J. Aluminium-Werkstoff-Datenblätter. 4. Aufl. Düsseldorf: AluminiumVerlag, ISBN 3-87017-281-9; 2003. [2] DIN 50014. Normalklimate e Klimate und ihre technische Anwendung. Ausg. 7.1985. Zurückgezogen 9.2008. Berlin: Beuth Verlag; 1985. [3] DIN 50015. Konstante Prüfklimate e Klimate und ihre technische Anwendung. Ausg. 08.1975. Zurückgezogen 09.2008. Berlin: Beuth Verlag; 1975. [4] Paynter RW. Surf Interface Anal 1999;27:103. [5] Kozlowska M, Reiche R, Oswald S, Vinzelberg H, Hübner R, Wetzig K. Surf Interface Anal 2004;36:1600. [6] Cumpson PJ, Seah MP. Surf Interface Anal 1997;25:430. [7] Software PHI multipak, V 6.1 A. Eden Prairie: Physical Electronics, Inc; 1994e1999. [8] Altenpohl D. Aluminium und Aluminiumlegierungen. Berlin: Springer Verlag; 1965. [9] Kammer C. überarb. Ausgabe. Aluminium-Taschenbuch. Aufl. 1, 15. Düsseldorf: Aluminium-Verlag, ISBN 3-87017-241-X; 1998. [10] Draley JE, Mori S, Loess RE. J Electrochem Soc 1967;114:353. [11] Klopprogge JT, Duong LV, Wood BJ, Frost RL. J Colloid Interface Sci 2006;296: 572. [12] Wittberg TN, Douglas Wolf J, Wang Pu Sen. J Mater Sci 1988;23:1745. [13] Hunter MS, Fowle P. J Electrochem Soc 1956;103:482. [14] Wefers K, Misra C. Oxides and hydroxides of aluminum. Pennsylvania, USA: ALCOA Laboratories; 1987. [15] Wittenberg TN, Douglas WJ, Wang PS. J Mater Sci 1988;23:1745. [16] Matthes KJ, Riedel F. Fügetechnik e Überblick e Löten e Kleben e Fügen durch Umformen. Leipzig: Fachbuchverlag Leipzig im Carl Hanser Verlag; 2003. [17] Swidersky HW. Aluminum brazing with non-corrosive fluxes e state of the art and trends in NOCOLOKÒ flux technology. In: DVS-Berichte Band, 212; 2001. p. 164e9. [18] Creber DK, Ball J, Field DJ. A mechanistic study of aluminum vacuum brazing. SAE Trans 1987e1988;96:648. [19] Blumenauer H. Werkstoffprüfung. 6. überarbeitete Auflage. Stuttgart: Deutscher Verlag für Grundstoffindustrie; 1994. [20] Frangini S, Giorgi R, Lascovich J, Mignone A. Surf Interface Anal 1994;21:435. [21] Datta J. Aluminium-Werkstoff-Datenblätter. 4. Aufl. Düsseldorf: AluminiumVerlag; 2003. [22] Kozlowska M. (2005). Thesis: Winkelaufgelöste XPS: Optimierung der mathematischen Modellierung und deren experimentelle Überprüfung. TU, Dresden. [23] Behr GmbH & Co. KG. Internal materials analyses; 2009.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterisation of oxide and hydroxide layers on technical aluminum materials using XPSq J. Zähr a, *, S. Oswald b, M. Türpe c, H.J. Ullrich a, U. Füssel a a
TU Dresden, Institute of Surface and Manufacturing Technology, Joining Technology and Assembly, D-01062 Dresden, Germany IFW Dresden, P.O.-Box 270116, D-01171 Dresden, Germany c Behr GmbH & Co. KG, Siemensstraße 164, D-70469 Stuttgart, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2010 Received in revised form 2 February 2011 Accepted 5 February 2011
Aluminum alloys are widely used as technical lightweight materials. For industrial applications, these materials often have to be bonded, e. g. welded or brazed. To get a metallic connection by brazing, the natural oxide layer on Al-materials, which is dense and has a high melting point, has to be eliminated. This layer can be influenced by the temperature and humidity in the surrounding atmosphere. Due to the nm-thickness of the layer, the analysis of its thickness and composition is challenging. An applicable method is X-ray photoelectron spectroscopy (XPS). The present investigation shows the applicability of XPS to estimate the oxide layer thickness as well as to distinguish between Al-hydroxide and Al-oxide phases. Finally, reasons for the differences in brazeability depending on the previous storage conditions are studied. It is shown, that a storage under condensation for 9 days causes an increase of the oxide layer thickness and an aggregation of water inside the pores of the hydroxide layer. This lowers significantly the brazeability of the material. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Al-oxide Brazing Climate tests XPS
1. Introduction Aluminum materials are widely used materials for technical applications, e.g. in automotive production or for heat exchangers, due to their excellent combination of mean density, high strength and good corrosion resistance. The high corrosion resistance is based on the dense natural aluminum oxide layer [9]. This layer is formed in air within seconds [8]. In technical applications the natural layer on aluminum materials is always assigned as aluminum oxide, albeit this term is not correct. The amorphous Al2O3 - -layer, which is located directly at the metallic interface, is the so-called barrier layer [13]. At the transition of the barrier layer to the environment a porous and hydrous layer is formed which can consist of gelatinous boehmite or bayerite [14] [15], see Fig. 1. The oxide layer has to be removed for brazing or welding [16]. Because of the importance for joining technology, its composition and thickness have to be known [17] [18]. The analysis of the natural layer is demanding due to its small thickness of 2e3 nm. One analysis technique for such thin films is
X-ray photoelectron spectroscopy (XPS) because the signals arise from a depth of only some nm. Additionally, the analysis of both oxide and metallic Al is possible using chemical peak shifts [19] [20]. In the present investigation, we distinguish between the Al2p-signal from the metallic substrate and the oxygen signal from the oxide and hydroxide layer by peak fit procedures. The results from this investigation are used for: - Estimation the layer thicknesses and - Determination the oxide/hydroxide content Systematic climate tests in well-defined atmospheres were done to simulate the alteration of the surfaces during transport and storage. Afterward, brazing tests were performed using the stored materials. Hence, a correlation between the oxide/hydroxide layer thickness and the brazeability of the material is possible. 2. Experimental 2.1. Technical aluminum alloy for storage and brazing tests
q A corresponding talk was presented at the 5th Symposium on Vacuum based Science and Technology SVST5, Kaiserslautern (Germany), 28e30 September 2010. * Corresponding author. Tel.: þ49 351 46334346; fax: þ49 351 463 37249. E-mail address: [email protected] (J. Zähr). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.04.004
The material investigated is a both side cladded Al-alloy, which is usually used for heat exchangers [21]. The core material is an AleMn-alloy (EN AW-Al Mn1Cu, EN AW-3003). The clad material
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
1217
Fig. 2. Scheme of material investigated (not true to scale): Technical aluminum.
Fig. 1. Natural surface layer on aluminum materials [8].
consists of an AleSi-alloy (EN AW-Al Si10, EN AW-4045). A scheme of a cross section of the material is shown in Fig. 2. The exact composition is given in Table 1. Such material is often used for house wares and packaging, whereby the clad material is a typical brazing alloy for Al-based materials [1]. The Si-additions cause a decrease of the melting temperature. Therefore, the core alloy can be joined by brazing without melting at a temperature of about 605 C. 2.2. Climate tests The aim of the investigations is the analysis of the influence of different atmosphere conditions on the surface of technical aluminum alloys. Therefore, climate conditions had to be chosen which are typical for the storage conditions in practice and the transport of Al-materials. Due to these requirements, the following environmental conditions were used: Normal climate (NC) 23 C/50% RH [2] Humid atmosphere (HA) 40 C/92% RH [3] Condensation atmosphere (CA) 23 C, humidity 100% These atmospheres were simulated in both a climate chamber and a condensation chamber under control of temperature and relative humidity (RH). The samples had a size of 100 50 mm2 with a thickness of 0.4 mm, see Fig. 2. 2.3. XPS The XPS measurements were carried out with a PHI 5600 CI spectrometer (Physical Electronics) which is equipped with a hemispherical analyzer operated with a typical pass energy of 29 eV and 800 mm diameter analysis area. Monochromatic AleKa excitation (1486.6 eV, 350 W) was used. Sputter cleaning was done with Arþ ions of 1.5 keV at a scan size of 3 mm 3 mm, and with an ion current leading a sputter rate of approximately 1 nm/min. Depth profile measurements were performed with the same ion
gun at 3.5 keV, a scan size of 2 mm 2 mm and the same sputter rate. Standard single element sensitivity factors were used to estimate the element concentrations, whereby a homogeneous mixture of the elements in the surface region was assumed. If significant charging effects were occurring, a low energy electron flood gun was used with a typical current of 1 mA. Nevertheless, some residual peak shifts resulting from different charging were observed. These were indicated by varying differences of the binding energy between the aluminum metal and aluminum oxide peak. Such changes were considered in the peak fit procedure. The spectra were usually calibrated by the C1s peak. Additionally, all peak intensities were normalized for a better comparison of the different binding states, see Fig. 3. The thickness of the Al-oxide layer was estimated from the ratio between the Al signal from the oxide overlayer (“Al-oxide”) and the Al-metal signal from the bulk material. This method is reliable also for different C-contamination thicknesses, because both signals have the same intensity attenuation through the contamination [4]. This is a very commonly used method as discussed also by PAYNTER [4] and KOZLOWSKA [22]. For the calculations the EXCEL-sheet based on the work of KOZLOWSKA et al. [5] and the attenuation lengths obtained from the CUMPSON-SEAH-2 (CS2) algorithm [6] were used. The peak areas were calculated from peak fit procedures using the PHI-MULTIPAK program [7]. 2.4. Brazing tests Brazing tests were done in a laboratory glass tube furnace to analyze the correlation between the oxide and hydroxide layer thickness as well as the surface layer constitution with the brazeability. All tests were done with a brazing temperature of 605 C and under a nitrogen atmosphere. The clad material, EN AW-Al Si10, see Fig. 2, served as brazing solder. The brazed length was measured to evaluate the brazeability. A natural oxide layer with a thickness of 2e3 nm has a brazing rate of 75e95% in this laboratory scale procedure. This corresponding sample called “Techn. Alloy” was not treated before brazing. 3. Results and discussion 3.1. Differentiation between Al(OH)3 and Al2O3 Two natural layers are existing on aluminum alloys: an amorphous Al2O3-layer and a hydroxide layer [8,9]. The thickness of the Al2O3-layer depends on the environmental temperature and on the
Table 1 Composition of technical aluminum alloy used for storage and brazing tests. Aluminum alloy
Concentration [weight %] [23] Si
EN AW-3003 mod. (EN AW-Al Mn1Cu mod.) EN AW-4045 mod. (EN AW-Al Si10 mod.)
0.071 10.05
Fe
0.205 0.174
Cu
0.464 0.08
Mn
0.957 0.013
Mg
0.242 0.008
Cr
0.006 0
Zn
0.043 0
Ti
0.086 0.012
Additions
Al
Single
All
0.05 0.05
0.15 0.15
Rest Rest
1218
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
a
b
Fig. 4. Atomic ratio of Al-oxide/O. Distinguishing of Al-hydroxide and Al-oxide is possible via this ratio. For technical Al-materials just the Al-oxide signal fraction of the Al2p-signal can be considered.
Fig. 3. XPS-spectra of Al2p- (a) and O1s-signal (b) of different Al-oxides/Al-hydroxides. Distinguishing of Al-oxide and Al-hydroxide is just possible via the O1s-signal. Alhydroxide is existing even at the surface of technical Al.
material composition [9]. The humidity in the environmental atmosphere as well as the temperature influence the thickness and the composition of Al(OH)3-layer [9], which is formed at room temperature at the surface [10]. One aim of the present investigation was the analysis of aluminum surfaces with different storage conditions. Therefore, first tests were performed to get a routine for distinguishing between oxide and hydroxide layers on the technical aluminum alloy. The following surface conditions were tested (see Fig. 4):
condensation influences the shifting more than the storage in water. Furthermore, it has to be noted that the O1s-hydrogen peak of the not-stored initial state of the Al-alloy shows a similar behavior as that of the water - stored sample at higher binding energies. At the low energy tail the behavior corresponds to that of the Al2O3-sample. According to the previously mentioned work [11], it can thus be assumed, that the layer consists of oxide as well as hydroxide phases. As another possibility to distinguish between the different phases the molar ratio of the concentrations of Al and O can be evaluated [11,12]. In the studies of KLOPROGGE et. al. [11]. and WITTBERG et al. [12]. mineral samples of Al-oxide and hydroxide were used. In our study technical Al-materials with a very thin natural oxide layer at the surface are analyzed. Only the oxide-bound Al2p-peak and the total O1s (oxide and hydroxide) signal are used for the calculation of the AleO-ratio. Fig. 4 displays the ratios of the atomic concentration of oxide e bound Al (“Al-oxide”) to O for the initial state sample as well as for the stored samples. By help of this ratio, it can be proven again, that bayerite (Al(OH)3) is existing at the surface of the technical
pure Al2O3 technical Al-alloy without storage (initial state) technical Al-alloy which was stored for1 day in H2O with 20 C technical Al-alloy which was stored for10 days under condensation at 23 C
According to KLOPROGGE et al. [11] a differentiation of the different AleO-phases can be obtained from the O1s- and the Al2p-peak of the XPS-spectra. In Fig. 3 the XPS-peaks of Al2p and O1s are presented. It is obvious, that the Al2p-oxide signal shows, besides a small broadening presumably originating from different charging, no significant tendencies. The Al-oxide peak in the Al2p spectra refers both to Al2O3 and Al-hydroxide. However, the O1s-peak displays differences between the four samples. The Al2O3-sample has the maximum of the peak at the lowest binding energy. The storage under water causes a small shifting of the O1s-peak to higher binding energies as well as a broadening of the signal. It has to be emphasized that the storage under
Fig. 5. Oxide thickness of technical Al-materials in initial state (IS) and after storage. A storage in condensation atmosphere (CA) results in an increase of oxide thickness. Surprisingly, subsequent storage in normal climate (NC) leads to an oxide layer growth.
J. Zähr et al. / Vacuum 86 (2012) 1216e1219
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The influence of the oxide layer properties on the brazing rate is shown in Fig. 7. All surface conditions have astonishingly a satisfying brazing rate. Only the sample with the 9-day storage under condensation atmosphere prohibits brazing. Therefore, we assume that for longer exposure H2O inside the pores of the hydroxide layer influences the brazing behavior significantly. Following storage under normal climate raises the brazing rate again to approximately 75%. The general increase of the total Al-oxide thickness (see Fig. 5) has no major influence.
4. Conclusions Fig. 6. Ratio of atomic concentration of Al-oxide to O. Bayerite is present on technical surfaces at the transition to atmosphere. This is not influenced by storage under normal climate, humid or condensation atmosphere.
non-stored material as well as on Al-materials which were stored under water influence. In contrast to the materials with a hydroxide structure at the surface, the Al2O3-sample has a higher Al-oxide to O-ratio. However, the measured ratio is distinctly lower than the theoretical value for Al2O3. A reason for this difference may be the reaction between the Al2O3-surface with the humidity in the air causing partly the formation of hydroxide structures at the surface of this sample, too.
3.2. Influence of storage conditions on the surface of technical Alalloys In this part of the study the XPS-method is used to analyze the surface composition of technical Al-alloys after storage under normal climate (NC), humid atmosphere (HA) and condensation atmosphere (CA) for different duration. The analyses show that storage under normal climate as well as under humid atmosphere does not influence the oxide thickness, see Fig. 5. Under condensation conditions the oxide layer is growing due to the reaction with H2O. It has to be emphasized, that the oxide thickness is increasing even during a subsequent storage under normal climate. Additionally, H2O may be stored inside the pores of the Al-hydroxide layer. All samples are found to have a ratio of Al-oxide to O of around 0.32 to 0.38, see Fig. 6. Thus it can be assumed, that bayerite is present on all samples.
Fig. 7. Brazing rate of technical Al-materials after different storage conditions. Only a long storage under condensation atmosphere prohibits brazing. This means that the adsorbed water in pores or at the surface influences the brazing process significantly.
XPS was successfully applied to investigate the behavior of Almaterial surfaces during climate tests for simulating material changes during storage and transport. Despite using monochromatic X-ray excitation to obtain good energy resolution for the expected small peak shifts, the identification of the peak positions at our technical surfaces was complicated by charging processes of the dielectric Al2O3. To distinguish between oxide and hydroxide AleO-compositions, the ratio between Al bound in oxide form and the amount of oxygen was considered. With the help of this ratio we could distinguish between Al2O3 and aluminum hydroxide in the surface layers. Additionally, the thickness of the layers was calculated from the oxide- and metal e Al peak ratios. This method was applied for a series of Al-material stored under different climate conditions. From brazing tests it appears that not the increasing oxide thickness, but water inside the pores of the oxide layer may reduce the brazeability.
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