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Al composite
Applied Surface Science 239 (2004) 87–93
Investigation of the corroded surface of SiCw/Al composite J. Hu*, W.D. Fei School of Materials Science and Engineering, Harbin Institute of Technology, Box 433, Harbin 150001, PR China Received in revised form 13 May 2004; accepted 13 May 2004 Available online 24 June 2004
Abstract The change on the surface morphology of SiCw/Al composite during the early stage of corrosion in 3.5% NaCl solution has been observed by atomic force microscopy. The effect of interface (between whisker and matrix) and matrix on the corrosion process has been studied. The corrosion behavior of the SiCw/Al composite is summarized as: (1) the surface of the composite became rougher during corrosion, the average roughness of surface increased with the corrosion time; (2) pits preferentially formed at the end of whisker and propagated along the interface between whisker and matrix easily; (3) some of smooth peaks appeared on the corroded surface with the progression of corrosion, but they disappeared with increasing the corrosion time and the matrix became remarkably rougher. # 2004 Elsevier B.V. All rights reserved. Keywords: SiCw/Al composite; Corrosion; Atomic force microscopy
1. Introduction In recent years much attention has been paid to the scanning probe techniques, the best known and most widespread representatives of which are the scanning tunneling microscope (STM) and the atomic force microscopy (AFM). Surface manipulation by AFM is unfolding many interesting and promising new fields [1,2]. SiCw/Al composite can be used in the aircraft industry and marine environments because of their excellence properties and low density [3,4]. But it is susceptible to localized corrosion such as pitting and stress corrosion. These kinds of localized corrosion are dangerous to the safety since the crevices resulted *
Corresponding author. Tel.: þ86 45186415894; fax: þ86 45186413922. E-mail address: [email protected] (J. Hu).
from localized corrosion may lead to cracks of a structure. Thus the study on the corrosion properties of the composite is very importance for their practical applications. To understand a corrosion process, it is essential to study the initiation of the corrosion process. For this purpose, observations of surface change during corrosion are useful because they provide a record of the corrosion process. However, it is difficult to observe the initial stages of corrosion by conventional microscopy because of its limited resolution. AFM has the potential to characterize the surface from the atomic lever to the nanometer scale. The purpose of this article is to report the change in the surface morphology of SiCw/Al composite, focus the behavior in the early stages of the corrosion process, and study the influence of the interface between whisker and matrix and the matrix in the composite on the corrosion behavior.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.080
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J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
2. Experimental The composite used in this study is SiC whiskerreinforced pure aluminum composite. The composite
was fabricated by squeeze casting. The size of the SiC whisker ranged from 0.1 to 1 mm in diameter and 30 to 100 mm in length. The volume fraction of the whisker was about 20%. Before testing, the investigated sur-
Fig. 1. Typical AFM images of surface of the SiCw/Al composite.
J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
89
Fig. 1. (Continued ).
faces were first wet ground with grit paper to 2400 grit, and then polished with diamond paste, cleaned mechanically with pure ethanol and acetone and dried naturally in air. The specimens were exposed to 3.5% NaCl solution for various time. Afterwards the specimens were cleaned with ultrasonic and dried in air before the AFM investigation. Prior to the observations by AFM, the corroded surfaces were examined by scanning electron microscopy (SEM) to confirm the corroded surfaces are smooth enough for AFM study.
3. Results and discussion Typical AFM images of the surface of the SiCw/Al composite are shown in Fig. 1. Fig. 1(a) is the AFM image of the surface of the SiCw/Al composite before corrosion, it can be seen that some of whiskers presented on the surface. The interfaces between whisker and matrix in the composite are smooth, and holes are difficult to be found from these interfaces. Fig. 1(b) and (c) are the AFM images of the corroded surface of the composite exposed to 3.5% NaCl solution for 30
and 60 min, respectively. From Fig. 1(a), the average roughness was measured to be 25.9 nm. After 30 min of corrosion in 3.5% NaCl solution, the average roughness was measured to be 40.8 nm. After 60 min of corrosion in 3.5% NaCl solution, the average roughness was measured to be 62.7 nm. The surface roughness increases with increasing the corrosion time, it is suggested that corrosion occurred at the surface of the composite. From Fig. 1(b) and (c), some holes can be seen on the surfaces of the composite, they are at the interface between whisker and matrix, which means that the matrixes near the whiskers were corroded. At the same time, it can be found that pits preferentially formed at the end of whiskers where was a region with concentrated stress because of the presence of thermal mismatch residual stress. As corrosion proceeds, pits propagated along the interface between the whisker and matrix, it suggested that the matrix surrounding the whisker is dissolved. It is attributed to the greater difference of the composition, which led to the great number of active sites present. The pits size increased with the corrosion time because pits grew up during corrosion process, so the
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Fig. 2. Typical AFM three-dimension images of surface of the SiCw/Al composite.
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Fig. 2. (Continued )
pits size is bigger in Fig. 1(c) than that of in Fig. 1(b). Interface corroded is the mainly mechanism for the SiCw/Al composite. Fig. 2 shows the typical AFM three-dimension images of SiCw/Al composite. Fig. 2(a) is the AFM three-dimension image of the surface of SiCw/Al composite before corrosion, its surface is not smoothness due to mechanical polishing, and the fluctuation of the highness can be seen. Fig. 2(b) is the AFM three-dimension image of the corroded surface after 30 min, the height fluctuation on the surface appears to be increased slightly, some of raised regions (such as edges and corners) are found to disappear, and some of smooth peaks appear on the corroded surface (as shown in Fig. 2(b)). Fig. 2(c) is the AFM three-dimension image of the corroded surface after 60 min. Many of sharp sawtooth peaks can be seen on the surface in the image, the fluctuation of the highness also increased. This indicated that the height fluctuation of the surface resulted from mechanical polishing (natural roughness formed on the surface of specimen) gradually
disappeared during corrosion process, that is to say, some raised regions were corroded gradually during corrosion process, and thereby resulting in some of smooth peaks appear on the surface after 30 min of corrosion. With the progress of corrosion, the matrix became rougher due to localized corrosion, sharp sawtooth peaks appeared on the surface after 60 min of corrosion. This indicated that not only the interface was corroded but also the matrix far from the whisker was also corroded during corrosion process. It is well known, the dislocation with high density in the matrix of the composite is introduced by quenching due to the difference of thermal expansion coefficient between whisker and matrix [5,6]. Anodic dissolution should occur at the places where the energy densities and the chemical potential were high, such as the dislocation tangle and the point of emergence of dislocation [7]. It seems that the introduction of residual stress leads to the dissolution rate of the matrix enhance, resulting in localized corrosion in the matrix of composite increase with corrosion time.
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Fig. 3 shows AFM images of section analysis. The AFM section analysis image of mechanically polished composite is shown in Fig. 3(a). From Fig. 3(a), it can be seen that the section line is not smooth before corrosion, and the natural roughness was formed on the surface of specimen. When a polished composite surface was exposed to 3.5% NaCl solution, the
mechanically polished mark gradually disappeared due to dissolution of edges and corners, and the section line becomes smoother as shown in Fig. 3(b). With the increasing of the time of exposure in corrosion solution, the section line becomes rougher again, which means corrosion occurred at localized defects in the matrix, resulting in a rougher surface as shown in
Fig. 3. AFM images of section analysis of the SiCw/Al composite.
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93
Fig. 3. (Continued ).
Fig. 3(c). These phenomena are consistent with the tests results in Fig. 2.
sion time and the matrix becomes remarkably rougher.
4. Conclusions
References
The surface of the composite becomes rougher during corrosion; the average surface roughness increases with the corrosion time. Corrosion occurs preferentially at the interface between whisker and matrix, and pits formed at the end of the whisker and propagated along the interface. The matrix far away the whisker is corroded because of the presence of localized defects. Some of smooth peaks appear on the corroded surface with the progression of corrosion, but they disappear with increasing the corro-
[1] Y. Watanaba, Y. Nakamura, S. Hirayama, N. Taniguchi, J. Vac. Sci. Technol. B 13 (1995) 1702. [2] A.A. Ejov, S.V. Savinov, V. Yaminsky, J. Vac. Sci. Technol. B 12 (1994) 1547. [3] C.P. Dogan, J.A. Hank, Wear 203 (1997) 267. [4] L. Wang, T. Kobayashi, H. Toda, M. Hayakawa, Mater. Sci. Eng. A 280 (2000) 214. [5] M. Taya, Mater. Trans. JIM 32 (1991) 19. [6] D.C. Dunand, A. Mortensen, Scripta Metall. Mater. 25 (1991) 761. [7] J. Hu, R.S. Luo, C.K. Yao, L.C. Zhao, Mater. Chem. Phys. 70 (2001) 163.
Investigation of the corroded surface of SiCw/Al composite J. Hu*, W.D. Fei School of Materials Science and Engineering, Harbin Institute of Technology, Box 433, Harbin 150001, PR China Received in revised form 13 May 2004; accepted 13 May 2004 Available online 24 June 2004
Abstract The change on the surface morphology of SiCw/Al composite during the early stage of corrosion in 3.5% NaCl solution has been observed by atomic force microscopy. The effect of interface (between whisker and matrix) and matrix on the corrosion process has been studied. The corrosion behavior of the SiCw/Al composite is summarized as: (1) the surface of the composite became rougher during corrosion, the average roughness of surface increased with the corrosion time; (2) pits preferentially formed at the end of whisker and propagated along the interface between whisker and matrix easily; (3) some of smooth peaks appeared on the corroded surface with the progression of corrosion, but they disappeared with increasing the corrosion time and the matrix became remarkably rougher. # 2004 Elsevier B.V. All rights reserved. Keywords: SiCw/Al composite; Corrosion; Atomic force microscopy
1. Introduction In recent years much attention has been paid to the scanning probe techniques, the best known and most widespread representatives of which are the scanning tunneling microscope (STM) and the atomic force microscopy (AFM). Surface manipulation by AFM is unfolding many interesting and promising new fields [1,2]. SiCw/Al composite can be used in the aircraft industry and marine environments because of their excellence properties and low density [3,4]. But it is susceptible to localized corrosion such as pitting and stress corrosion. These kinds of localized corrosion are dangerous to the safety since the crevices resulted *
Corresponding author. Tel.: þ86 45186415894; fax: þ86 45186413922. E-mail address: [email protected] (J. Hu).
from localized corrosion may lead to cracks of a structure. Thus the study on the corrosion properties of the composite is very importance for their practical applications. To understand a corrosion process, it is essential to study the initiation of the corrosion process. For this purpose, observations of surface change during corrosion are useful because they provide a record of the corrosion process. However, it is difficult to observe the initial stages of corrosion by conventional microscopy because of its limited resolution. AFM has the potential to characterize the surface from the atomic lever to the nanometer scale. The purpose of this article is to report the change in the surface morphology of SiCw/Al composite, focus the behavior in the early stages of the corrosion process, and study the influence of the interface between whisker and matrix and the matrix in the composite on the corrosion behavior.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.080
88
J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
2. Experimental The composite used in this study is SiC whiskerreinforced pure aluminum composite. The composite
was fabricated by squeeze casting. The size of the SiC whisker ranged from 0.1 to 1 mm in diameter and 30 to 100 mm in length. The volume fraction of the whisker was about 20%. Before testing, the investigated sur-
Fig. 1. Typical AFM images of surface of the SiCw/Al composite.
J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
89
Fig. 1. (Continued ).
faces were first wet ground with grit paper to 2400 grit, and then polished with diamond paste, cleaned mechanically with pure ethanol and acetone and dried naturally in air. The specimens were exposed to 3.5% NaCl solution for various time. Afterwards the specimens were cleaned with ultrasonic and dried in air before the AFM investigation. Prior to the observations by AFM, the corroded surfaces were examined by scanning electron microscopy (SEM) to confirm the corroded surfaces are smooth enough for AFM study.
3. Results and discussion Typical AFM images of the surface of the SiCw/Al composite are shown in Fig. 1. Fig. 1(a) is the AFM image of the surface of the SiCw/Al composite before corrosion, it can be seen that some of whiskers presented on the surface. The interfaces between whisker and matrix in the composite are smooth, and holes are difficult to be found from these interfaces. Fig. 1(b) and (c) are the AFM images of the corroded surface of the composite exposed to 3.5% NaCl solution for 30
and 60 min, respectively. From Fig. 1(a), the average roughness was measured to be 25.9 nm. After 30 min of corrosion in 3.5% NaCl solution, the average roughness was measured to be 40.8 nm. After 60 min of corrosion in 3.5% NaCl solution, the average roughness was measured to be 62.7 nm. The surface roughness increases with increasing the corrosion time, it is suggested that corrosion occurred at the surface of the composite. From Fig. 1(b) and (c), some holes can be seen on the surfaces of the composite, they are at the interface between whisker and matrix, which means that the matrixes near the whiskers were corroded. At the same time, it can be found that pits preferentially formed at the end of whiskers where was a region with concentrated stress because of the presence of thermal mismatch residual stress. As corrosion proceeds, pits propagated along the interface between the whisker and matrix, it suggested that the matrix surrounding the whisker is dissolved. It is attributed to the greater difference of the composition, which led to the great number of active sites present. The pits size increased with the corrosion time because pits grew up during corrosion process, so the
90
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Fig. 2. Typical AFM three-dimension images of surface of the SiCw/Al composite.
J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
91
Fig. 2. (Continued )
pits size is bigger in Fig. 1(c) than that of in Fig. 1(b). Interface corroded is the mainly mechanism for the SiCw/Al composite. Fig. 2 shows the typical AFM three-dimension images of SiCw/Al composite. Fig. 2(a) is the AFM three-dimension image of the surface of SiCw/Al composite before corrosion, its surface is not smoothness due to mechanical polishing, and the fluctuation of the highness can be seen. Fig. 2(b) is the AFM three-dimension image of the corroded surface after 30 min, the height fluctuation on the surface appears to be increased slightly, some of raised regions (such as edges and corners) are found to disappear, and some of smooth peaks appear on the corroded surface (as shown in Fig. 2(b)). Fig. 2(c) is the AFM three-dimension image of the corroded surface after 60 min. Many of sharp sawtooth peaks can be seen on the surface in the image, the fluctuation of the highness also increased. This indicated that the height fluctuation of the surface resulted from mechanical polishing (natural roughness formed on the surface of specimen) gradually
disappeared during corrosion process, that is to say, some raised regions were corroded gradually during corrosion process, and thereby resulting in some of smooth peaks appear on the surface after 30 min of corrosion. With the progress of corrosion, the matrix became rougher due to localized corrosion, sharp sawtooth peaks appeared on the surface after 60 min of corrosion. This indicated that not only the interface was corroded but also the matrix far from the whisker was also corroded during corrosion process. It is well known, the dislocation with high density in the matrix of the composite is introduced by quenching due to the difference of thermal expansion coefficient between whisker and matrix [5,6]. Anodic dissolution should occur at the places where the energy densities and the chemical potential were high, such as the dislocation tangle and the point of emergence of dislocation [7]. It seems that the introduction of residual stress leads to the dissolution rate of the matrix enhance, resulting in localized corrosion in the matrix of composite increase with corrosion time.
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Fig. 3 shows AFM images of section analysis. The AFM section analysis image of mechanically polished composite is shown in Fig. 3(a). From Fig. 3(a), it can be seen that the section line is not smooth before corrosion, and the natural roughness was formed on the surface of specimen. When a polished composite surface was exposed to 3.5% NaCl solution, the
mechanically polished mark gradually disappeared due to dissolution of edges and corners, and the section line becomes smoother as shown in Fig. 3(b). With the increasing of the time of exposure in corrosion solution, the section line becomes rougher again, which means corrosion occurred at localized defects in the matrix, resulting in a rougher surface as shown in
Fig. 3. AFM images of section analysis of the SiCw/Al composite.
J. Hu, W.D. Fei / Applied Surface Science 239 (2004) 87–93
93
Fig. 3. (Continued ).
Fig. 3(c). These phenomena are consistent with the tests results in Fig. 2.
sion time and the matrix becomes remarkably rougher.
4. Conclusions
References
The surface of the composite becomes rougher during corrosion; the average surface roughness increases with the corrosion time. Corrosion occurs preferentially at the interface between whisker and matrix, and pits formed at the end of the whisker and propagated along the interface. The matrix far away the whisker is corroded because of the presence of localized defects. Some of smooth peaks appear on the corroded surface with the progression of corrosion, but they disappear with increasing the corro-
[1] Y. Watanaba, Y. Nakamura, S. Hirayama, N. Taniguchi, J. Vac. Sci. Technol. B 13 (1995) 1702. [2] A.A. Ejov, S.V. Savinov, V. Yaminsky, J. Vac. Sci. Technol. B 12 (1994) 1547. [3] C.P. Dogan, J.A. Hank, Wear 203 (1997) 267. [4] L. Wang, T. Kobayashi, H. Toda, M. Hayakawa, Mater. Sci. Eng. A 280 (2000) 214. [5] M. Taya, Mater. Trans. JIM 32 (1991) 19. [6] D.C. Dunand, A. Mortensen, Scripta Metall. Mater. 25 (1991) 761. [7] J. Hu, R.S. Luo, C.K. Yao, L.C. Zhao, Mater. Chem. Phys. 70 (2001) 163.