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Interfacial in-situ Al2O3 nanoparticles enhance load transfer in carbon nanotube (CNT)-reinforced aluminum matrix composites
Journal of Alloys and Compounds 789 (2019) 25e29
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interfacial in-situ Al2O3 nanoparticles enhance load transfer in carbon nanotube (CNT)-reinforced aluminum matrix composites B. Chen a, b, *, K. Kondoh b, J. Umeda b, S. Li c, L. Jia c, J. Li a a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China Joining & Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan c School of Materials Science and Engineering, Xi'an University of Technology, Xi'an, 710048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2018 Received in revised form 24 January 2019 Accepted 4 March 2019 Available online 5 March 2019
Dissatisfactory load transfer has been a critical issue in carbon nanotube (CNT)- and graphene-reinforced metal matrix composites (MMCs) mainly because of the intrinsically unpleasant carbon-metal interfaces. Here we show by introducing in-situ Al2O3 nanoparticles at aluminum (Al)-CNTs interface, the load transfer efficiency can be noticeably enhanced in powder metallurgy CNTs/Al composites. From in-situ tensile tests, the nanoparticle-modified Al-CNTs interfaces result in CNT fracture, a sign of high load transfer efficiency; while clean interfaces without nanoparticles lead to CNT pulling-out. The nanoparticle-induced enhancement of interface strength can be explained by the increased sliding resistance of CNTs in MMCs at the wake of cracks under tensile loading. Our study provides a new strategy for designing strong carbon-metal interfaces to fabricate high-performance nanocarbon-reinforced MMCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Metal-matrix composites (MMCs) Carbon nanotubes (CNTs) Nanoparticles Interface Fracture
1. Introduction Carbon nanotubes (CNTs) and graphene have attracted great attention for structural and functional uses in materials science [1,2]. Excellent properties, such as superhigh strength (up to ~ 100 GPa), superhigh Young's modulus (~1 TPa), high aspect ratios (up to ~ 1000) and light weight (~2 g‧cm3), make CNTs and graphene ideal reinforcements for composites materials [3,4]. The precondition of making most use of CNTs and graphene is a high load transfer from matrix to reinforcements, which is determined by two crucial aspects, viz. good reinforcement dispersion and suitable interface bonding [3,5]. In the past decade, homogeneous CNT dispersion has been achieved in aluminum (Al) metal matrix composites (MMCs) reported by many studies using some novel methods [6], such as in-situ grown CNTs [7]. The dispersion of CNTs in Al appears to be no longer a critical problem in laboratory researches [8e10]. However, the dark cloud of insufficient carbonmetal interface still hangs over the area of CNT- and graphenereinforced composites because of the intrinsically weak mechanical bonding between reinforcement and matrix [6], resulting from
* Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.jallcom.2019.03.063 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the poor wettability between carbon and metals [11,12]. Recently, introducing interfacial carbide nanoparticles has been proposed as a potential solution to overcoming the weak bonding in some CNTs/metal nanocomposite systems [13e17]. Nano-sized carbides from the reaction between CNTs and metal matrices or alloy elements produce strong metallurgic bonding between nanocarbon reinforcements and metal matrices. The formation of carbide nanoparticles, such as CrxCy [13], Ni3C [14] and Al4C3 [15,16], was thought responsible for the improved mechanical properties of CNTs/Cu-Cr, CNTs/Ni and CNTs/Al composites, respectively. However, one problem of interfacial carbides is the destruction of graphene walls in CNTs, which degrades the high strength of CNTs. This causes difficulty in balancing the damage of CNT structure and the enhancement of interface to improve the load transfer effect consequently. Therefore, devolving new strong interfacial structures while maintaining the structure of CNTs is urgently required in the area of CNT- and graphene-reinforced MMCs. Herein, we report a progress on designing novel CNTs-Al interfaces by introducing interfacial in-situ Al2O3 nanoparticles for enhancing the load transfer efficiency. The mechanism leading to the breakthrough of load transfer enhancement from CNT pullingout to CNT fracture is discussed based on high resolution microscopy microstructures and in-situ tensile tests. The present findings
26
B. Chen et al. / Journal of Alloys and Compounds 789 (2019) 25e29
may show guidance for fabricating carbon/metal nanocomposites with high strength. 2. Experimental methods In this study, multi-walled CNTs (MWCNTs, ~100 nm in diameter, prepared by a chemical vapor grown process, Showa Denko Group, Japan) and pure Al (99.9% in purity, ~150 mm in diameter, Kojundo Chemical Laboratory CO., Japan) were used as starting materials. CNTs (0.6 wt%) were dispersed in Al powders (100 g) through a rocking ball milling process on a mixing machine (Seiwa Giken, Japan) for 4 h [18]. Al2O3 media balls (ball to powder mass ratio of 1:10) was added as a milling medium during mixing. AlCNTs composite powders were also fabricated by using the same starting materials but dispersed by a wet-coating process [19]. The added CNT content is controlled at 0.6 wt%. The milled powder mixtures were consolidated by sparking plasma sintering (SPS) and following hot-extrusion. SPS was conducted at sintering temperature of 550 C held for 30 min at a pressure of 30 MPa. Before hot extrusion, the as-sintered billets (42 mm in diameter) were preheated at 500 C for 3 min under Ar atmosphere. The billets were then immediately extruded using a 2000 kN hydraulic press machine (SHP-200-450, Shibayama, Japan). The extrusion ratio and the ram speed were 12:1 and 0.5 mm/s, respectively. Details of the fabrication process can be found elsewhere [18]. The CNT-Al interface in as-extruded materials were examined by high resolution transmission electronic microscopy (HR-TEM, JEM-2010, JEOL, Japan). TEM samples with a thickness of ~100 nm were fabricated in a focused ion beam (FIB, HITACHI FB-2000A) system. The in-situ tensile test was performed by operating a tensile stage with a CNT/Al composite specimen inside the field emission scanning electron microscopy (FE-SEM, JEM-6500F, JEOL) chamber. The specimen was machined from the extruded material into a flat dumbbell shape with a length of 10 mm, a width of 2 mm and a thickness of 1 mm. As the tensile loading was manually paused during the tensile test, the displacement was maintained and high-quality SEM figures were captured. The stain rate was 5 104 s1. The fracture morphology of composites after tensile testing was characterized by the FE-SEM. Details of the in-situ tensile test system can be found elsewhere [15,18]. 3. Results and discussion From the previous in-situ tensile test [18], the phenomenon of CNT fracture was clearly observed during composite failure (Fig. 1). At the initial stage of plastic deformation of the composite sample, CNTs are partially exposed in the wake of the micro-cracks and CNTs act as bridges to restrain the growth of cracks (Fig. 1a). As tensile strain is increased, the exposed CNTs fracture into two parts
(Fig. 1b). From the higher-magnification image (Fig. 1c), it is observed that the outer walls of the two CNTs have been ruptured, while the inner part is undamaged in a columnar shape. The breaking of CNT walls has been clarified as a result of effective load transfer from Al matrix to CNTs [18]. However, the nature of the AlCNTs interface leading to the effective load transfer is still unclear. According to the study of Chen et al. [18] based on the shear-lag theory [20], CNTs will be fractured if the interfacial strength (t) is larger than a critical interfacial stress (tc) which is given as [18]:
tc ¼
sf
(1)
2S
where sf is the fracture strength (~4 GPa [21]) and S is the aspect ratio (length to diameter ratio) of CNTs (~100). tc can be calculated as 20 MPa. In the case of CNT fracture, the strengthening efficiency (R) can be expressed as [18]:
R¼
sf s sc sm 1 f 1¼ sm 4St V sm
(2)
where sc and sm are the strength of composite and matrix, respectively, and V is CNT volume content). From Eq. (2), we can get
t¼
V s2f i: h 4S V sf þ ð1 VÞsm sc
(3)
Therefore, from the parameter values in Eq. (2) [18], the interface strength is estimated to be 41 MPa, suggesting a strong interface bonding, which is also in agreement with the condition for CNT fracture, i.e., t > tc. It is well-known that unreacted CNTs usually have weak bonding [22], namely a mechanical bonding in nature, to Al matrix because of the poor wettability between carbon and Al materials [11,12]. Although some recent researchers reported that interfacial nanoscale Al4C3 compounds are beneficial to the formation of strong metallurgic bonding between CNTs and Al [15,16,22], this mechanism does not work in the present composite. This study reveals that the strong CNT-Al interface is resulted from the anchoring effect of interfacial Al2O3 nanoparticles, which will be demonstrated as following. From detailed TEM observations, Al2O3 nanoparticles are observed at the CNT-Al interface in the composite. Fig. 2 shows two typical a-Al2O3 particles, which are identified by a combination of bright-field TEM (Fig. 2a and d), dark-field TEM (Fig. 2b and e) and selected diffraction patterns (Fig. 2c and f). The a-Al2O3 particles have a size of 50e100 nm. A crystal relation between a-Al2O3 and Al is found to be a-Al2O3[02-1]//Al [11-2] (Fig. 2f). One significant phenomenon is that the shape of CNTs in composites becomes tortuous (Fig. 2a, b, d and e), compared with straight morphology of CNTs at starting state [18].
Fig. 1. CNT fracture process in a CNT/Al composite recorded in in-situ tensile tests inside a SEM chamber. (a) A crack encounters a CNT. (b,c) Larger strains lead to CNT fracture phenomena. Reprinted from Ref. [18], with permission from Elsevier.
B. Chen et al. / Journal of Alloys and Compounds 789 (2019) 25e29
27
Fig. 2. Typical interfacial a-Al2O3 particles at two Al-CNT interface positions. (a,d) Bright-field TEM. (b,e) Dark-field TEM. (c,f) Selected area diffraction patterns.
Except for a-Al2O3, g-Al2O3 nanoparticles are also occasionally observed in the CNTs-Al interface. Fig. 3a shows a typical one with size of ~20 nm. The g-Al2O3 phase can be identified by the fast Fourier transformation (FFT) image (inset of Fig. 3b) and the inverse FFT image (Fig. 3b) from the high-resolution TEM image. The Al3þ and O2 ions can be distinguished in the g-Al2O3 crystal because of their large size difference. It is also observed that the g-Al2O3 crystal possesses a less ordered structure with many areas consisted of highly defective lattices. These characteristics agree with the growth process of in-situ Al2O3 phases from amorphous alumina on Al surface [23]: amorphous Al2O3/g-Al2O3/[d]/ [q]/a-Al2O3. The phases shown in brackets are intermediate phases which are unstable and thus may not be produced. The transformation from g-Al2O3 to a-Al2O3 is accompanied with the consummation of structural defects and grain growth [23], leading to the observed size increase of nanoparticles (Figs. 2 and 3). Because of the comparatively low energy input in the rocking ball milling process, undoubtedly the transformation from amorphous film on Al powder surface to the stable and well-crystallized g- and a-Al2O3 particles should take place during the consolidation
process consisted of SPS and hot extrusion process. However, only amorphous Al2O3 phases are observed in a composite fabricated by a very similar consolidation process consisted of SPS þ hot extrusion [24]. A major difference between the two composites' fabrication routes is the dispersion method, i.e., a natural-rubberassisted coating process in Ref. [24] and a rocking ball milling process in the present study. On the other hand, in the Al-CNTs composite powders prepared by high energy ball milling, a widely used method for dispersing CNTs, a comparatively thick amorphous Al2O3 film formed on aluminum powder surface [25]. After sintering and hot extrusion, the film evolved into an amorphous [26] or g-Al2O3 [25] interfacial layer in composites. The results suggest that the rocking ball milling process applied in this study may play a critical role in forming the interfacial Al2O3 nanoparticles. To confirm the role of rocking ball milling, CNTs/Al composites were also fabricated by using the same starting materials but dispersed by a wet-coating process [19]. The interface is found to be clean without any Al4C3 nor Al2O3 nanoparticles (Fig. 4). With the three bulk materials, Al and CNTs/Al fabricated by rocking ball
Fig. 3. High-magnification TEM images of a g-Al2O3 nanoparticle at Al-CNTs interface. (a) High-resolution TEM image. (b) Inverse fast Fourier transformation image.
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B. Chen et al. / Journal of Alloys and Compounds 789 (2019) 25e29
Fig. 4. TEM images of clean Al-CNT interfaces in the composite fabricated by wet coating. Insets of (b) show selected area diffraction patterns of Al and CNTs.
milling, and CNTs/Al by wet coating, the oxygen content was similar at a low level of around 0.15 wt%. It suggests the rocking ball milling process did not increase the oxygen content much. Therefore, the formation of interfacial alumina nanoparticles might be related to the mechanical effect of milling balls on the thin alumina film of Al powder surfaces. Another phenomenon is that CNTs with a clean interface are much straighter (Fig. 4b) than those with interfacial nanoparticles (Fig. 2). Therefore, the formation of the tortuous topography should be due to the increased resistance of CNT extension in Al matrix during the hot-extrusion process induced by the pinning effect of hard Al2O3 nanoparticles. To further confirm the role of interfacial nanoparticles played in enhancing the CNTs-Al interface and consequent load transfer effect, in-situ tensile tests are also applied on the CNTs/Al composite with clean interfaces fabricated from the wet-coating process.
During composite failure, the phenomenon of CNT pulling-out is often observed (Fig. 5a). As the crack expands after the CNT bridging stage, one half part of the CNT is entirely pulled-out from the other part (Fig. 5b), as the scheme suggests in Fig. 6a. However, the CNT fracture mode in the composite with interfacial Al2O3 nanoparticles, which has been confirmed in the in-situ tensile test (Fig. 1), produces comparatively short broken CNTs on the fracture surface (Fig. 5c). Some nanoparticles were occasionally observed on the fractured CNT surface (as an arrow indicated in Fig. 5d). The nanoparticles are probably interfacial Al2O3 nanoparticles because of the similar size and situation observed in TEM (Fig. 2). The scheme of interfacial Al2O3-induced CNT fracture is shown in Fig. 6b. With the addition of interfacial nanoparticles, the straight interface (Fig. 4b) becomes tortuous (Fig. 2a and d). Such morphology is favorable to produce high anchoring effect of
Fig. 5. Failure behaviors of CNTs in Al MMCs during in-situ tensile tests with two kinds of interfaces. (a,b) Clean interface without nanoparticle. (b,c) With in-situ Al2O3 nanoparticles.
B. Chen et al. / Journal of Alloys and Compounds 789 (2019) 25e29
29
Fig. 6. Scheme of CNT failure mechanism of in Al MMCs without (a) and with (b) interfacial in-situ Al2O3 nanoparticles.
nanoparticles that resists the sliding of CNTs under tensile loading (Fig. 6b). In this way, load on the sample can be effectively transferred from matrix to CNTs, leading to the observed CNT fracture phenomena (Figs. 1 and 5c).
[9]
[10]
4. Conclusions [11]
This study reveals a new-type CNTs-Al interface structure modified with in-situ formed Al2O3 nanoparticles. The introduction of nanoparticles causes little damage to the structure completeness of CNTs but produces tortuous morphology of CNTs in as-extruded composites. Such morphology produces strong anchoring effect of nanoparticles to resist the sliding of CNTs at the wake of cracks under tensile loading. Consequently, compared with clean interfaces, the Al2O3-modified interfaces are highly effective for load transfer from Al matrix to CNTs, which leads to CNT fracture during tensile loading. Our study may provide a new insight into designing strong carbon-metal interfaces in CNT- and graphene-reinforced MMCs.
[12]
[13]
[14]
[15]
[16]
Acknowledgements
[17]
B. Chen thanks the fund support from the “Fundamental Research Funds for the Central Universities” under Contract No. G2018KY0301.
[18]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interfacial in-situ Al2O3 nanoparticles enhance load transfer in carbon nanotube (CNT)-reinforced aluminum matrix composites B. Chen a, b, *, K. Kondoh b, J. Umeda b, S. Li c, L. Jia c, J. Li a a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China Joining & Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan c School of Materials Science and Engineering, Xi'an University of Technology, Xi'an, 710048, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2018 Received in revised form 24 January 2019 Accepted 4 March 2019 Available online 5 March 2019
Dissatisfactory load transfer has been a critical issue in carbon nanotube (CNT)- and graphene-reinforced metal matrix composites (MMCs) mainly because of the intrinsically unpleasant carbon-metal interfaces. Here we show by introducing in-situ Al2O3 nanoparticles at aluminum (Al)-CNTs interface, the load transfer efficiency can be noticeably enhanced in powder metallurgy CNTs/Al composites. From in-situ tensile tests, the nanoparticle-modified Al-CNTs interfaces result in CNT fracture, a sign of high load transfer efficiency; while clean interfaces without nanoparticles lead to CNT pulling-out. The nanoparticle-induced enhancement of interface strength can be explained by the increased sliding resistance of CNTs in MMCs at the wake of cracks under tensile loading. Our study provides a new strategy for designing strong carbon-metal interfaces to fabricate high-performance nanocarbon-reinforced MMCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Metal-matrix composites (MMCs) Carbon nanotubes (CNTs) Nanoparticles Interface Fracture
1. Introduction Carbon nanotubes (CNTs) and graphene have attracted great attention for structural and functional uses in materials science [1,2]. Excellent properties, such as superhigh strength (up to ~ 100 GPa), superhigh Young's modulus (~1 TPa), high aspect ratios (up to ~ 1000) and light weight (~2 g‧cm3), make CNTs and graphene ideal reinforcements for composites materials [3,4]. The precondition of making most use of CNTs and graphene is a high load transfer from matrix to reinforcements, which is determined by two crucial aspects, viz. good reinforcement dispersion and suitable interface bonding [3,5]. In the past decade, homogeneous CNT dispersion has been achieved in aluminum (Al) metal matrix composites (MMCs) reported by many studies using some novel methods [6], such as in-situ grown CNTs [7]. The dispersion of CNTs in Al appears to be no longer a critical problem in laboratory researches [8e10]. However, the dark cloud of insufficient carbonmetal interface still hangs over the area of CNT- and graphenereinforced composites because of the intrinsically weak mechanical bonding between reinforcement and matrix [6], resulting from
* Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.jallcom.2019.03.063 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the poor wettability between carbon and metals [11,12]. Recently, introducing interfacial carbide nanoparticles has been proposed as a potential solution to overcoming the weak bonding in some CNTs/metal nanocomposite systems [13e17]. Nano-sized carbides from the reaction between CNTs and metal matrices or alloy elements produce strong metallurgic bonding between nanocarbon reinforcements and metal matrices. The formation of carbide nanoparticles, such as CrxCy [13], Ni3C [14] and Al4C3 [15,16], was thought responsible for the improved mechanical properties of CNTs/Cu-Cr, CNTs/Ni and CNTs/Al composites, respectively. However, one problem of interfacial carbides is the destruction of graphene walls in CNTs, which degrades the high strength of CNTs. This causes difficulty in balancing the damage of CNT structure and the enhancement of interface to improve the load transfer effect consequently. Therefore, devolving new strong interfacial structures while maintaining the structure of CNTs is urgently required in the area of CNT- and graphene-reinforced MMCs. Herein, we report a progress on designing novel CNTs-Al interfaces by introducing interfacial in-situ Al2O3 nanoparticles for enhancing the load transfer efficiency. The mechanism leading to the breakthrough of load transfer enhancement from CNT pullingout to CNT fracture is discussed based on high resolution microscopy microstructures and in-situ tensile tests. The present findings
26
B. Chen et al. / Journal of Alloys and Compounds 789 (2019) 25e29
may show guidance for fabricating carbon/metal nanocomposites with high strength. 2. Experimental methods In this study, multi-walled CNTs (MWCNTs, ~100 nm in diameter, prepared by a chemical vapor grown process, Showa Denko Group, Japan) and pure Al (99.9% in purity, ~150 mm in diameter, Kojundo Chemical Laboratory CO., Japan) were used as starting materials. CNTs (0.6 wt%) were dispersed in Al powders (100 g) through a rocking ball milling process on a mixing machine (Seiwa Giken, Japan) for 4 h [18]. Al2O3 media balls (ball to powder mass ratio of 1:10) was added as a milling medium during mixing. AlCNTs composite powders were also fabricated by using the same starting materials but dispersed by a wet-coating process [19]. The added CNT content is controlled at 0.6 wt%. The milled powder mixtures were consolidated by sparking plasma sintering (SPS) and following hot-extrusion. SPS was conducted at sintering temperature of 550 C held for 30 min at a pressure of 30 MPa. Before hot extrusion, the as-sintered billets (42 mm in diameter) were preheated at 500 C for 3 min under Ar atmosphere. The billets were then immediately extruded using a 2000 kN hydraulic press machine (SHP-200-450, Shibayama, Japan). The extrusion ratio and the ram speed were 12:1 and 0.5 mm/s, respectively. Details of the fabrication process can be found elsewhere [18]. The CNT-Al interface in as-extruded materials were examined by high resolution transmission electronic microscopy (HR-TEM, JEM-2010, JEOL, Japan). TEM samples with a thickness of ~100 nm were fabricated in a focused ion beam (FIB, HITACHI FB-2000A) system. The in-situ tensile test was performed by operating a tensile stage with a CNT/Al composite specimen inside the field emission scanning electron microscopy (FE-SEM, JEM-6500F, JEOL) chamber. The specimen was machined from the extruded material into a flat dumbbell shape with a length of 10 mm, a width of 2 mm and a thickness of 1 mm. As the tensile loading was manually paused during the tensile test, the displacement was maintained and high-quality SEM figures were captured. The stain rate was 5 104 s1. The fracture morphology of composites after tensile testing was characterized by the FE-SEM. Details of the in-situ tensile test system can be found elsewhere [15,18]. 3. Results and discussion From the previous in-situ tensile test [18], the phenomenon of CNT fracture was clearly observed during composite failure (Fig. 1). At the initial stage of plastic deformation of the composite sample, CNTs are partially exposed in the wake of the micro-cracks and CNTs act as bridges to restrain the growth of cracks (Fig. 1a). As tensile strain is increased, the exposed CNTs fracture into two parts
(Fig. 1b). From the higher-magnification image (Fig. 1c), it is observed that the outer walls of the two CNTs have been ruptured, while the inner part is undamaged in a columnar shape. The breaking of CNT walls has been clarified as a result of effective load transfer from Al matrix to CNTs [18]. However, the nature of the AlCNTs interface leading to the effective load transfer is still unclear. According to the study of Chen et al. [18] based on the shear-lag theory [20], CNTs will be fractured if the interfacial strength (t) is larger than a critical interfacial stress (tc) which is given as [18]:
tc ¼
sf
(1)
2S
where sf is the fracture strength (~4 GPa [21]) and S is the aspect ratio (length to diameter ratio) of CNTs (~100). tc can be calculated as 20 MPa. In the case of CNT fracture, the strengthening efficiency (R) can be expressed as [18]:
R¼
sf s sc sm 1 f 1¼ sm 4St V sm
(2)
where sc and sm are the strength of composite and matrix, respectively, and V is CNT volume content). From Eq. (2), we can get
t¼
V s2f i: h 4S V sf þ ð1 VÞsm sc
(3)
Therefore, from the parameter values in Eq. (2) [18], the interface strength is estimated to be 41 MPa, suggesting a strong interface bonding, which is also in agreement with the condition for CNT fracture, i.e., t > tc. It is well-known that unreacted CNTs usually have weak bonding [22], namely a mechanical bonding in nature, to Al matrix because of the poor wettability between carbon and Al materials [11,12]. Although some recent researchers reported that interfacial nanoscale Al4C3 compounds are beneficial to the formation of strong metallurgic bonding between CNTs and Al [15,16,22], this mechanism does not work in the present composite. This study reveals that the strong CNT-Al interface is resulted from the anchoring effect of interfacial Al2O3 nanoparticles, which will be demonstrated as following. From detailed TEM observations, Al2O3 nanoparticles are observed at the CNT-Al interface in the composite. Fig. 2 shows two typical a-Al2O3 particles, which are identified by a combination of bright-field TEM (Fig. 2a and d), dark-field TEM (Fig. 2b and e) and selected diffraction patterns (Fig. 2c and f). The a-Al2O3 particles have a size of 50e100 nm. A crystal relation between a-Al2O3 and Al is found to be a-Al2O3[02-1]//Al [11-2] (Fig. 2f). One significant phenomenon is that the shape of CNTs in composites becomes tortuous (Fig. 2a, b, d and e), compared with straight morphology of CNTs at starting state [18].
Fig. 1. CNT fracture process in a CNT/Al composite recorded in in-situ tensile tests inside a SEM chamber. (a) A crack encounters a CNT. (b,c) Larger strains lead to CNT fracture phenomena. Reprinted from Ref. [18], with permission from Elsevier.
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Fig. 2. Typical interfacial a-Al2O3 particles at two Al-CNT interface positions. (a,d) Bright-field TEM. (b,e) Dark-field TEM. (c,f) Selected area diffraction patterns.
Except for a-Al2O3, g-Al2O3 nanoparticles are also occasionally observed in the CNTs-Al interface. Fig. 3a shows a typical one with size of ~20 nm. The g-Al2O3 phase can be identified by the fast Fourier transformation (FFT) image (inset of Fig. 3b) and the inverse FFT image (Fig. 3b) from the high-resolution TEM image. The Al3þ and O2 ions can be distinguished in the g-Al2O3 crystal because of their large size difference. It is also observed that the g-Al2O3 crystal possesses a less ordered structure with many areas consisted of highly defective lattices. These characteristics agree with the growth process of in-situ Al2O3 phases from amorphous alumina on Al surface [23]: amorphous Al2O3/g-Al2O3/[d]/ [q]/a-Al2O3. The phases shown in brackets are intermediate phases which are unstable and thus may not be produced. The transformation from g-Al2O3 to a-Al2O3 is accompanied with the consummation of structural defects and grain growth [23], leading to the observed size increase of nanoparticles (Figs. 2 and 3). Because of the comparatively low energy input in the rocking ball milling process, undoubtedly the transformation from amorphous film on Al powder surface to the stable and well-crystallized g- and a-Al2O3 particles should take place during the consolidation
process consisted of SPS and hot extrusion process. However, only amorphous Al2O3 phases are observed in a composite fabricated by a very similar consolidation process consisted of SPS þ hot extrusion [24]. A major difference between the two composites' fabrication routes is the dispersion method, i.e., a natural-rubberassisted coating process in Ref. [24] and a rocking ball milling process in the present study. On the other hand, in the Al-CNTs composite powders prepared by high energy ball milling, a widely used method for dispersing CNTs, a comparatively thick amorphous Al2O3 film formed on aluminum powder surface [25]. After sintering and hot extrusion, the film evolved into an amorphous [26] or g-Al2O3 [25] interfacial layer in composites. The results suggest that the rocking ball milling process applied in this study may play a critical role in forming the interfacial Al2O3 nanoparticles. To confirm the role of rocking ball milling, CNTs/Al composites were also fabricated by using the same starting materials but dispersed by a wet-coating process [19]. The interface is found to be clean without any Al4C3 nor Al2O3 nanoparticles (Fig. 4). With the three bulk materials, Al and CNTs/Al fabricated by rocking ball
Fig. 3. High-magnification TEM images of a g-Al2O3 nanoparticle at Al-CNTs interface. (a) High-resolution TEM image. (b) Inverse fast Fourier transformation image.
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Fig. 4. TEM images of clean Al-CNT interfaces in the composite fabricated by wet coating. Insets of (b) show selected area diffraction patterns of Al and CNTs.
milling, and CNTs/Al by wet coating, the oxygen content was similar at a low level of around 0.15 wt%. It suggests the rocking ball milling process did not increase the oxygen content much. Therefore, the formation of interfacial alumina nanoparticles might be related to the mechanical effect of milling balls on the thin alumina film of Al powder surfaces. Another phenomenon is that CNTs with a clean interface are much straighter (Fig. 4b) than those with interfacial nanoparticles (Fig. 2). Therefore, the formation of the tortuous topography should be due to the increased resistance of CNT extension in Al matrix during the hot-extrusion process induced by the pinning effect of hard Al2O3 nanoparticles. To further confirm the role of interfacial nanoparticles played in enhancing the CNTs-Al interface and consequent load transfer effect, in-situ tensile tests are also applied on the CNTs/Al composite with clean interfaces fabricated from the wet-coating process.
During composite failure, the phenomenon of CNT pulling-out is often observed (Fig. 5a). As the crack expands after the CNT bridging stage, one half part of the CNT is entirely pulled-out from the other part (Fig. 5b), as the scheme suggests in Fig. 6a. However, the CNT fracture mode in the composite with interfacial Al2O3 nanoparticles, which has been confirmed in the in-situ tensile test (Fig. 1), produces comparatively short broken CNTs on the fracture surface (Fig. 5c). Some nanoparticles were occasionally observed on the fractured CNT surface (as an arrow indicated in Fig. 5d). The nanoparticles are probably interfacial Al2O3 nanoparticles because of the similar size and situation observed in TEM (Fig. 2). The scheme of interfacial Al2O3-induced CNT fracture is shown in Fig. 6b. With the addition of interfacial nanoparticles, the straight interface (Fig. 4b) becomes tortuous (Fig. 2a and d). Such morphology is favorable to produce high anchoring effect of
Fig. 5. Failure behaviors of CNTs in Al MMCs during in-situ tensile tests with two kinds of interfaces. (a,b) Clean interface without nanoparticle. (b,c) With in-situ Al2O3 nanoparticles.
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Fig. 6. Scheme of CNT failure mechanism of in Al MMCs without (a) and with (b) interfacial in-situ Al2O3 nanoparticles.
nanoparticles that resists the sliding of CNTs under tensile loading (Fig. 6b). In this way, load on the sample can be effectively transferred from matrix to CNTs, leading to the observed CNT fracture phenomena (Figs. 1 and 5c).
[9]
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4. Conclusions [11]
This study reveals a new-type CNTs-Al interface structure modified with in-situ formed Al2O3 nanoparticles. The introduction of nanoparticles causes little damage to the structure completeness of CNTs but produces tortuous morphology of CNTs in as-extruded composites. Such morphology produces strong anchoring effect of nanoparticles to resist the sliding of CNTs at the wake of cracks under tensile loading. Consequently, compared with clean interfaces, the Al2O3-modified interfaces are highly effective for load transfer from Al matrix to CNTs, which leads to CNT fracture during tensile loading. Our study may provide a new insight into designing strong carbon-metal interfaces in CNT- and graphene-reinforced MMCs.
[12]
[13]
[14]
[15]
[16]
Acknowledgements
[17]
B. Chen thanks the fund support from the “Fundamental Research Funds for the Central Universities” under Contract No. G2018KY0301.
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