Synergistic Strengthening Effect of Ultrafine-Grained Metals Reinforced with Carbon NanotubesThis research was supported by a grant (02K1501-00720) from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea

The recent application of nanotechnology to structural materials is a promising way to produce new strong materials that exceed current limitations. Grain-size refinement is considered as one of the most effective strengthening methods of materials. At the same time, the addition of carbon nanotubes (CNTs) in a material is known to be more effective than conventional reinforcements. Here, CNT/Co nanocomposites were fabricated by reinforcing the CNTs in ultrafine-grained Co matrix via a modified molecular-level mixing process. The CNT/Co nanopowders have a structure resembling a pearl necklace, consisting of Co nanoparticles penetrated by CNTs and were fabricated and then consolidated into CNT/Co nanocomposites using spark plasma sintering. The microstructure of the CNT/Co nanocomposite consisted of a three-dimensional (3D) network of CNTs in ultrafine-grained Co matrix with an average grain size of 300 nm. The CNT/Co nanocomposite showed an outstanding yield strength of 1.5 GPa. This indicates that the synergistic strengthening mechanism of homogeneously dispersed CNTs in an ultrafine-grained-metal matrix could improve the mechanical properties of materials. Since the strength is the most important property for structural metals, many attempts have been made throughout history to fabricate stronger metals. Advances in physics and chemistry have been applied continuously to overcome the limitations on the mechanical performance of metals. The recent emergence of nanotechnology has also contributed to the development of strong structural metals. Grainsize refinement to fabricate ultrafine-grained or nanocrystalline metals and reinforcement with CNTs are known to be one of the most promising methods to strengthen the metals through nanotechnology. Among these, grain-size refinement has characteristic features. The strength of metals increases with decreasing grain size according to the well-known Hall–Petch relationship. When the grain size of metals reaches a submicrometerand nanoscale, very high strengths can be obtained compared to coarse-grained metals with the same chemical composition and phase constitution. There are several ways to fabricate ultrafinegrained or nanocrystalline metals, including the sintering of nanoparticles and the severe deformation of bulk metals. Compared to refinement of metal grain size into ultrafinegrained or nanocrystalline ranges, it is difficult to add CNTs to a metal matrix due to the strong aggregation of CNTs and the weak interfacial strength between the metal matrix and the CNTs. There is therefore some doubt about the strengthening ability of CNTs in a metal matrix. However, with the development of the molecular-level mixing process, in which CNTs are mixed with a metal matrix in the disperse state in a solvent to form CNT-implanted composite powders, it has been observed that the strengthening efficiency of CNTs in a Cu matrix is extraordinarily high. The CNTs have a strengthening efficiency eight times that of SiC particles and three times that of SiC whiskers. However, despite the considerable strengthening efficiency of CNTs in Cu, the resulting mechanical properties are not impressive; the compound has a yield strength of about 500 MPa, which is similar to values for Al-based alloys. This results from the low yield strength of the matrix, in this case the pure Cu, which has a yield strength of around 100 MPa. Our strategy to produce a strong metal was to reinforce the ultrafine-grained Co with CNTs, fabricating a CNT/Co nanocomposite in which the metal matrix did not undergo any changes in chemical composition or constituent phase except for the addition of CNTs as reinforcement. To fabricate CNT/Co nanocomposites with an ultrafine-grained Co matrix, we modified the molecular-level mixing process, which was introduced as a unique method of fabricating CNT/metal nanocomposites, to incorporate the concept of nanocrystal fabrication. First, the CNTs are functionalized with carboxyl and hydroxyl groups by acid treatment to provide an active site for reacting with metal ions. Then, the functionalized CNTs are dispersed within oleylACHTUNGTRENNUNGamine by sonication, and Co acetylacetonate is added to this mixture. It has been reported that the transient metal salts are decomposed in oleylamine during refluxing to form oxide nanocrystals. On refluxing in an Ar atmosphere, CNT/CoO composite powders are fabricated in which cubeshaped CoO nanoparticles about 50 nm in size are threaded by CNTs, as shown in Figure 1. For comparison, CoO nanopowders were fabricated under the same conditions without CNTs. As shown in Figure 1a, and b, the size and shape of the CoO nanocrystals were similar regardless of the addition of CNTs. The X-ray diffraction (XRD) analysis verified the [*] Y. J. Jeong, Dr. S. I. Cha, Dr. K. T. Kim, K. H. Lee, C. B. Mo, Prof. S. H. Hong Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology 373-1 Kusong-Dong Yusong-Gu, Daejeon 305-701 (Korea) Fax: (+82)42-869-3310 E-mail: [email protected] [] Current affiliation: International Center for Young Scientists National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) [] Current affiliation: LCD R&D Center Samsung Electronics Co., Ltd. San 24 Nongseo-dong, Giheung-Gu Yongin, Gyeonggido 446-771 (Korea)