Copyright © 2008 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 8, 5561–5565, 2008
The Fabrication of Carbon Nanotubes Reinforced Copper Coating by a Kinetic Spray Process Yuming Xiong, Kicheol Kang, Sanghoon Yoon, and Changhee Lee∗ Kinetic Spray Coating Lab. (NRL), Division of Materials Science and Engineering, Hanyang University, 17 Haendang-dong, Seongdong-Ku, Seoul 133-791, Korea
RESEARCH ARTICLE
In this paper, multiwalled carbon nanotubes (MWCNTs) reinforced copper coating was deposited on copper sheet through kinetic spraying process. Effect of heat treatment on microstructure, conductivity, and hardness of the coating was investigated. The incompact MWCNTs reinforced copper coating exhibits a comparable hardness, but higher electrical resistivity than pure copper coating. After heat treatment at 600 C for 2 h, the hardness of copper coatings significantly decreased due to the substantial grain growth. MWCNTs reinforced copper coating showed stable hardness and electrical conductivity against heat treatment owing to the inhibition of CNTs to grain growth and the intimate contact between CNTs and copper matrix.
Keywords: Multi-Walled Carbon Nanotubes, Kinetic Sprayed Coating, Hardness, Electrical Resistivity.
1. INTRODUCTION Copper, due to its excellent electrical and thermal conductivities, has been widely used in the electronics industry, such as electrical contact materials (brushes in electric motors),1 but failure of those electronic components always occurs due to the softness and low oxidation/ wear resistance of copper under service environments. In general, improving strength of metals via various traditional approaches, including alloying, cold-working, and grain refinement, usually leads to a pronounced decrease in conductivity.2 Carbon nanotubes (CNTs) are increasingly attracting scientific and technological interest by virtue of their properties and potential applications.3 The high strength, elastic modulus, flexibility, and unique conductivity along with other properties have led to the applications of CNTs as novel fiber reinforcement for a variety of composite materials. It has been found that carbon nanotubes (CNTs) reinforced metals have prominent enhancement on mechanical properties.4–7 Recent studies have indicated that the CNTs reinforced copper composite materials exhibit excellent mechanical performance and remarkable enhancement of microhardness in comparison with copper.8 9 However, it is also reported that the electrical conductivity of CNTs reinforced copper (CNTs-Cu in abbreviation) materials is decreased due to the electron ∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 10
scattering at boundaries.8 10 A solution by adding nickel coated CNTs has been reported to significantly improve the interfacial bonding between CNTs and matrix.6 In the present paper, a new emergent method is proposed which is Kinetic Spraying (KS), to fabricate dense and thick CNTs-Cu composite coating. In this process, the deposition of coatings mainly depends on high strain rate (∼109 sec−1 ) deformation of materials delivered by a high speed (300–1200 m/sec) impact of solid state powder (around 50 m in particle size) at room temperature. Thus, the coatings could retain the properties of initial feedstock against microstructure change and oxidation which always happen to the melted or semi-melted incident powder in the conventional thermal spray processes.11–14 High bond and cohesive strength implies an intimate contact between particles in kinetic sprayed coatings, which may decrease the boundary effect on electron scattering. In order to declare the CNTs enhancement effect, pure Cu coating was also evaluated for comparing the microstructures and resultant properties (hardness and electrical conductivity) influenced by heat treatment.
2. EXPERIMENTAL DETAILS 2.1. Kinetic Spraying Process A commercially available CGT kinetic spraying system (KINETIC3000, Germany) was used in this study. The detailed experimental set-up and coating process
1533-4880/2008/8/5561/005
doi:10.1166/jnn.2008.1346
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The Fabrication of Carbon Nanotubes Reinforced Copper Coating by a Kinetic Spray Process
Xiong et al.
were described elsewhere.15 A de-Laval type convergingdiverging MOC nozzle with round exit was used. Diameter of the nozzle exit and the diameter ratio of exit to throat are 6.34 mm and 2.34 respectively. Helium and nitrogen gases were used as process and carrier gases respectively. Temperature and pressure of the process gas were kept constants as 450 C and 2.5 MPa respectively. The carrier gas was set as 8% of process gas. High powder feed rate (20 g/min) and low nozzle transverse travel speed (80 mm/sec) were used for coating. The substrate of pure Cu sheet was polished (Ra < 15) and fixed at 3 cm in front of the nozzle exit.
RESEARCH ARTICLE
2.2. Characteristics of Feedstock The commercial pure Cu (Sulzer Metco) and CNTs (3 Vol.%) reinforced Cu composite (ACN, Korea) powders were used as feed stocks. The morphologies of the powders are shown in Figure 1. The Cu powder is spherical in shape with particle size ranging from 40 m to 65 m. The multiwalled carbon nanotubes (MWCNTs) used in this work was fabricated by thermal chemical vapor deposition. MWCNTs were treated with H2 at 900 C for 24 h, refluxed with 6 M HCl for 24 h, and then washed with distilled water to remove impurities. As shown in Figure 1(c) of a TEM image, the outer diameter and length of MWCNTs are ∼30 nm and ∼10 m respectively. The MWCNTs with a purity as high as 95 wt% has higher values of electrical conductivity (>10−2 S/cm), Young’s Modulus (1200 GPa), and Tensile strength (150 GPa). Cu and MWCNTs powders in a volume ratio of 97:3 were mixed by mechanical ball milling for 8 hrs. The diameter and thickness of MWCNTs-copper particle in flake shape are about 50 m and 3 m respectively. The hardness of initial powder was measured by a nanoindentation technique (Nanoindenter XP, MTS) with a diamond Berkovich (three-sided pyramid) indenter mounted in a nanoindenter at a constant strain rate (0.05 sec−1 ). The hardness of MWCNTs-Cu and Cu powders is 1.8 GPa and 0.8 GPa respectively. It is shown that MWCNTs can improve the hardness of Cu markedly. 2.3. Characterization of Coatings The two as-sprayed coatings (Cu and MWCNTs-Cu) were heat treated under 600 C in vacuum (<1 × 10−5 bar) for 2 h under a heating rate of 10 C/sec followed by a furnace cooling. The as-sprayed and as-heat-treated coatings were polished for surface electrical resistivity measurement using Jandel Four-Point-Probe Heads (CMT-SR2000N, Korea) and microhardness test (HMV-2, Shimadzu) using a load of 490.3 mN. The porosity was measured from the cross section of coatings using an image analyser software (Image-Pro Plus 5.02, MediaCybernetics) attached to an Optical Microscopy (OM). All the measured values were 5562
Fig. 1. Morphologies of feedstocks: (a) pure copper, (b) MWCNTscopper, and (c) TEM images of essential MWCNTs (inset of selected area electron diffraction patterns).
obtained from the average of five readings at different regions. Those values were found to be within ±10%. After etching for 20 sec in an aqueous solution containing 10%HCl, 5%FeCl3 , and 100 ml H2 O, the cross sectional microstructures of as-sprayed and as-heattreated coatings were characterized by scanning electron microscopy (SEM, JSM5600, JEOL). The morphology of MWCNTs in the composite coatings was characterized by transmission electron microscopy (TEM, JEM-2000 EX-II, JEOL) at an applied excitation voltage of 200 keV. Selected area electron diffraction (SAED) patterns were also performed to characterize phase structures of coatings. J. Nanosci. Nanotechnol. 8, 5561–5565, 2008
Xiong et al.
The Fabrication of Carbon Nanotubes Reinforced Copper Coating by a Kinetic Spray Process
3. RESULTS AND DISCUSSION
Fig. 2. Cross-sectional SEM microstructures of (a) as-sprayed and (b) as-heat-treated Cu coating after etching.
J. Nanosci. Nanotechnol. 8, 5561–5565, 2008
Properties of coatings before and after heat treatment at 600 C Pure Cu coating
Porosity, area% Microhardness, HV Electrical resistivity, m/square
CNTs-Cu coating
As-sprayed
As-heat treated
As-sprayed
As-heat treated
0079 144 00795
0065 67 0580
0660 137 0158
0361 110 0135
as-sprayed one. Although the detailed reason has not been found, it might be related with the atomic diffusion at the inter-particles boundaries, which leads to structural inhomogeneities of large-sized pore distribution in the as-heat-treated coating.16 As shown in Figure 2(b), the porosity along “vertical” direction was obviously lower than that of “horizontal” one after heat treatment, comparing with the homogeneous distribution of fine pores in as-sprayed coating (as shown in Fig. 2(a)). It is worth noting that the volume electrical resistivity of Cu coating could decrease by atomic diffusion during heat treatment,13 in spite of the increasing surface electrical resistivity due to the anisotropic pore distribution. In as-sprayed MWCNTs-Cu coating, many discontinuous cracks occurred (as shown in Figs. 3(a and c)), which lead to high electrical resistivity and weaker adhesion. In general, a bow shock within the impingement zone in front of substrate always formed to reduce the impact velocity of entrained particles in a kinetic spraying process.17 Although flake MWCNTs-Cu particles could be effectively accelerated in process gas flow, the resistance of bow shock against the flight of particles was identically considerable. Consequently, the impact velocity of flake MWCNTs-Cu particle was less than that of spherically shaped particle. Also, the deformability of relatively hard MWCNTs reinforced Cu particle is lower than that of soft Cu particle. Low deformation degree would lead to the formation of cracks in kinetic sprayed coatings. In terms of finite element analysis (FEA), the maximum plastic deformation of impacted materials is concentrated at the surrounding of the impact interface rather than the centre.18 Thus, the cohesive strength between particles is dominated by atomic bonding within an extremely narrow peripheral impact interface. As shown in Figure 3(c), more than 70% inter-particles boundaries exhibited weak contact. However, lowly work-hardened MWCNTs-Cu coating had a comparable hardness with Cu coating owing to the enhancement of CNTs (Table I). After heat treatment, the MWCNTs-Cu coating became compact through atomic diffusion (Fig. 3(b)). Along the initial inter-particles boundaries, many discontinuous micron-sized open pores (marked by black arrow in Fig. 3(d)) were formed. Interestingly, the growth and coarsening of Cu grains seems not to be serious according to SEM observation. The fact is that introducing 5563
RESEARCH ARTICLE
Consistent with the previous researches,12 the as-sprayed Cu coating showed a compact microstructure through a severe plastic deformation of incident feedstock materials (as shown in Fig. 2(a)). More than 60% particle/ particle boundary was well welded. Also, the metallic jet was welded at the boundaries. At those well bonded boundaries, the grains were elongated with a high orientation, which indicates the deformation directions of particles and substrate upon impact. Preferential etching of internal particle boundaries is limited to a narrow region, in turn implying a tight inter-particles cohesive strength. In addition, many small size grains (less than 1 m) were formed along inter-particle boundaries through recrystallization process upon impact. After heat treatment, substantial grain growth and coarsening could be obviously observed to release internal stress in coating which is induced by high strain plastic deformation upon impact. The size of fine grains at boundaries was increased up to +3–5 microns. Meanwhile, some discontinuous large-sized open pores formed at the boundaries through the accumulation of small pores due to atomic diffusion (Fig. 2(b)). As given in Table I, the hardness of coating decreased markedly due to grain coarsening. Interestingly, the surface electrical resistivity of as-heattreated Cu coating increased as high as 7 times that of
Table I. for 2 h.
RESEARCH ARTICLE
The Fabrication of Carbon Nanotubes Reinforced Copper Coating by a Kinetic Spray Process
Xiong et al.
Fig. 4. TEM morphologies of MWCNTs in as-sprayed (a) and as-heat treated (b) coatings (insets for SAED patterns).
Fig. 3. Optical micrographs and SEM microstructures of as-sprayed (a, c-etched) and as-heat-treated (b, d-etched) MWCNTs composite coatings, respectively.
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MWCNTs could hinder the coarsening of grains during heat treatment. As shown in Figure 4(a), a uniform distribution of CNTs in the as-sprayed composite coating can be observed. MWCNTs showed an intimate contact with the Cu matrix. Due to the higher strength of CNTs than Cu, severe plastic deformation of Cu leads to occurrence of adiabatic shear instability and dynamic recrystallization to tightly enwrap MWCNTs upon kinetic spraying impact. Also, the high strain rate impact of Cu matrix may lead to deformation and structural variation of MWCNTs. According to Figure 4(b), the size of Cu grains was not significantly coarsened due to the inhibition of CNTs at the grain boundaries. The boundaries between MWCNTs and Cu matrix became blurry due to a possible atomic interdiffusion during heat treatment. SAED patterns showed that partial MWCNTs might have experienced an amorphization process during impact process. Although the detailed mechanisms of the amorphization needs further research, the features of high strain rate (generally up to 109 ) and fast cooling (up to 1010 K/sec) upon kinetic spraying impact may play a vital role in this phase transformation.19 Also, the amorphous carbon might be formed during fabrication of MWCNTs through CVD process of carbon. Accordingly, comparing with Cu coating, MWCNTs-Cu coating showed stable properties against heat treatment, such as hardness and electrical conductivity, due to the uniform distribution and inhibition of CNTs against grain coarsening of Cu, as well as their intimate contact with matrix. J. Nanosci. Nanotechnol. 8, 5561–5565, 2008
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The Fabrication of Carbon Nanotubes Reinforced Copper Coating by a Kinetic Spray Process
4. CONCLUSION In this paper, multi-walled carbon nanotubes (3 Vol.%) reinforced copper coating was successfully deposited on copper sheet through kinetic spraying. The coating showed a comparable hardness but higher electrical resistivity than the pure copper coating. After heat treatment, the porosity of the coatings decreased. The hardness of MWCNTsCu coating decreased slightly due to the enhancement of CNTs, other than the significant decrease of pure Cu coating through substantial grain growth. Meanwhile, MWCNTs composite coating exhibited a stable electrical conductivity against heat treatment due to the intimate contact of CNTs with metallic matrix. Acknowledgment: This work was supported by the Korea Science and Engineering Foundation (KOSEF)
grant funded by (No. 2006-02289).
the
Korea
government
(MOST)
References and Notes 1. K. Kuniya, H. Arakawa, T. Sakaub, H. Minorikawa, K. Akeyama, and T. Sakamoto, Japan. J. Inst. Met. 50, 583 (1986). 2. L. Lu, Y. F. Shen, X. H. Chen, L. H. Qian, and K. Lu, Science 304, 422 (2004). 3. F. S. Rober, Science 281, 940 (1998). 4. Z. Bian, R. J. Wang, W. H. Wag, T. Zhang, and A. Iniue, Adv. Func. Mater. 14, 55 (2004). 5. C. L. Xu, B. Q. Wei, R. Z. Ma, J. Liang, X. K. Ma, and D. H. Wu, Carbon 37, 855 (1999). 6. B. Lim, C. Kim, B. Kim, U. Shim, S. Oh, B. Sung, J. Choi, and S. Baik, Nanotech. 17, 5795 (2006). 7. Y. L. Yang, Y. D. Wang, Y. Ren, C. S. He, J. N. Deng, J. Nan, J. G. Chen, and L. Zuo, Mater. Lett. 62, 47 (2008). 8. S. R. Dong, J. P. Tu, and X. B. Zhang, Mater. Sci. Eng. A 313, 83 (2001). 9. K. T. Kim, S. I. Cha, and S. H. Hong, Mater. Sci. Eng. A 449–451, 46 (2007). 10. Y. Feng, H. L. Yuan, and M. Zhang, Mater. Charact. 55, 211 (2005). 11. A. P. Alkhimov, A. N. Papyrin, V. F. Kosarev, N. I. Nesterovich, and M. M. Shushpanov, U.S. Patent No. 5302414 (1994). 12. T. Stoltenhoff, C. Borchers, F. Gartner, and H. Kreye, Surf. Coat. Tech. 200, 4947 (2006). 13. P. S. Phani, V. Vishnukanthan, and G. Sundararajan, Acta Mater. 55, 4741 (2007). 14. W. B. Choi, L. Li, V. Luzin, R. Neiser, T. Gnäupel-Herold, H. J. Prask, S. Sampath, and A. Gouldstone, Acta Mater. 55, 857 (2007). 15. S. Yoon, C. Lee, H. Choi, and H. Jo, Mater. Sci. Eng. A 415, 45 (2006). 16. M. Perez-Ramos, A. Manzano-Ramirez, Y. Vorobiev, and J. Gonzalez-Hernandez, Metall. Mater. Trans. B 34, 129 (2003). 17. J. Pattison, S. Celotto, A. Khan, and W. O’Neill, Surf. Coat. Tech. 202, 1443 (2008). 18. W. Li, H. Liao, C. Li, G. Li, C. Coddet, and X. Wang, Appl. Surf. Sci. 252, 2852 (2006). 19. M. Chen, J. W. McCauley, and K. J. Hemker, Science 299, 1563 (2003). 20. T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H. F. Ghaemi, and T. Thio, Nature 382, 54 (1996). 21. H. J. Dai, E. W. Wong, and C. M. Lieber, Science 72, 523 (1996). 22. G. D. Zhan, J. D. Kuntz, J. E. Garay, and A. K. Mukherjee, Appl. Phys. Lett. 83, 1228 (2003).
Received: 23 January 2008. Accepted: 20 February 2008.
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RESEARCH ARTICLE
However, it is worth noting that MWCNTs in Cu matrix, as scattering centers of electrons, might increase the resistivity of Cu composites although they have the same or even better electrical conductivity (up to 108 S · m−1 ) than pure Cu.20 21 On the other hand, the uniform distribution of MWCNTs along grain boundaries leads to the formation of an interlinked electrical pathway of nanotube network to decrease the resistivity of the composites.22 Thus, the electrical conductivity of the MWCNTs metallic composites is dependent on the fraction, distribution, and interface contact of MWCNTs in metallic matrix. The kinetic sprayed coatings could exhibit satisfying properties due to the improved intimate contact between MWCNTs and matrix through high strain plastic deformation at a high strain rate. However, deformability of the composite powder and deposition efficiency of the coatings would be decreased with increasing CNTs fraction. In addition, flake powder is not recommended to be used in kinetic spray process according to the present results. The detailed impact and bonding mechanisms of CNTs metallic composite particle, as well as the CNTs behavior in the composite coatings, should be figured out through further research.
