Fabrication of Dense ZrO 2 /CNT Composites: Influence of Bead-Milling Treatment
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THE first publications on carbon fibers appeared from 1889 to 1890.[1,2] In 1952, the concept of tubes was introduced by Radushkevich et al.[3] who observed the hollow structure of carbon formed by thermal decomposition. However, it was in 1991 that, through a report in Nature by Iijima,[4] carbon nanotubes (CNTs) were promoted as a new field of study, and since then their fields of application, as well as the number of reports concerning this material, have increased exponentially. In the last decade, CNTs have attracted great interest especially of those researchers working with composite ceramics, and these materials are considered to be promising as reinforcing elements for structural composite materials because of their remarkable physical and mechanical properties.[5–7] The use of CNT-reinforced ceramics has been drawing great interest because of the high mechanical, thermal, and electric properties of these fibers.[7] Owing to the small diameter, large aspect ratio, and surface characteristics of the nanotubes, it is difficult to obtain a good mixture of the two phases before the sintering process, especially while using aqueous suspensions.
GUSTAVO SUA´REZ and ESTEBAN F. AGLIETTI, Researchers, are with the Technological Center of Mineral Resources and Ceramics, CETMIC, Camino Centenario y 506, C.P. 1897 Gonnet, Argentina. Contact e-mail: [email protected] BYUNG-KOOG JANG, Researcher, is with the Surface Kinetics Group, High Temperature Materials Unit, National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan. YOSHIO SAKKA, Director, is with the Advanced Ceramics Group, Materials Processing Unit, Advanced Key Technologies Division, National Institute for Materials Science (NIMS). Manuscript submitted November 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
Some success has been achieved with conventional milling techniques, primarily with the use of lowto-moderate nanotube volume fractions.[8–11] Previous reports on CNT-reinforced ceramics have shown conflicting results. Some of them present results with an increment in fracture toughness due to the incorporation of CNTs.[12–14] Some others found no improvements in their mechanical properties.[6,8,15] Some researchers present the reinforcement mechanisms of the matrix as a result of the bridging and the pull-out effect[15,16] of the nanotubes on the fractures produced by indentation. Sun et al.[17] reported that the CNT agglomerates at grain boundaries, and weak bondings between CNTs and zirconia are responsible for the decrease in the hardness and lack of improvement in other properties. At present, contradictory results are available in the literature regarding mechanical properties. One reason could be the use of different processing methods, and another, the differences in the measurements of the mechanical properties. We present a processing route for homogeneous CNT dispersion in a zirconia matrix and show the full densification of the composite and Vickers hardness and fracture toughness values. Several pr
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