Creep-resistant composites of alumina and single-wall carbon nanotubes

Creep-resistant composites of alumina and single-wall carbon nanotubes Eugenio Zapata-Solvas, Rosalía Poyato, Diego Gómez-García, Arturo Domínguez-Rodríguez, Velimir Radmilovic, and Nitin P. Padture Citation: Applied Physics Letters 92, 111912 (2008); doi: 10.1063/1.2899945 View online: http://dx.doi.org/10.1063/1.2899945 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Fractal model for estimating fracture toughness of carbon nanotube reinforced aluminum oxide J. Appl. Phys. 107, 123532 (2010); 10.1063/1.3445869 Strong and ductile nanostructured Cu-carbon nanotube composite Appl. Phys. Lett. 95, 071907 (2009); 10.1063/1.3211921 Tribological and Strength Properties of Alumina/Multi‐Walled Carbon Nanotube Composites AIP Conf. Proc. 898, 154 (2007); 10.1063/1.2721269 Toughening and reinforcing alumina matrix composite with single-wall carbon nanotubes Appl. Phys. Lett. 89, 121910 (2006); 10.1063/1.2336623 Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes Appl. Phys. Lett. 83, 1228 (2003); 10.1063/1.1600511 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.92 On: Mon, 24 Aug 2015 09:39:30 Creep-resistant composites of alumina and single-wall carbon nanotubes Eugenio Zapata-Solvas,1Rosalía Poyato,1Diego Gómez-García,1 Arturo Domínguez-Rodríguez,1,a兲Velimir Radmilovic,2and Nitin P. Padture3,b兲 1Departamento Física de la Materia Condensada, Universidad de Sevilla, 41080 Sevilla, Spain 2National Center for Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA 3Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA 共Received 17 December 2007; accepted 29 February 2008; published online 20 March 2008兲 Composites of alumina 共Al2O3兲ceramic and single-wall carbon nanotubes 共SWNTs兲have been tested in uniaxial compression at 1300 and 1350 °C 共Ar atmosphere兲, and they have been found to be about two orders of magnitude more creep-resistant compared to a pure alumina of about the same grain size 共0.5 ␮ m兲. This is attributed to partial blocking of grain-boundary sliding by SWNTs in the composites. Since the grain boundaries in the ceramic/SWNTs composites are amenable to being engineered, this constitutes an attractive approach to the design of creep-resistant ceramic composites. © 2008 American Institute of Physics.关DOI: 10.1063/1.2899945兴 Over the past decade or so there has been growing interest in composites of ceramics and carbon nanotubes 共CNTs兲.1–17 This interest is primarily being driven by the idea that by combining CNTs with ceramics, one can impart some of the attractive mechanical properties of the CNTs 共Ref. 18兲to the resulting composites. There have been limited studies on single-wall CNTs 共SWNTs兲reinforced ceramics.6,9,13,15,19–25 An interesting aspect of alumina/ SWNTs composites is the unique grain-boundary structures they possess. In these composites, bundles of SWNTs have been found to segregate at the alumina grain boundaries.6,9,13 This grain-boundary structure is hierarchical in nature, and it comprises a three-dimensional network of two-dimensional mats made up of random one-dimensional SWNTs, which can be engineered.15 While grain boundaries have a profound influence on the high-temperature creep of polycrystalline materials,26 there have been no investigations on the influence of this unusual grain-boundary structure on the creep of alumina/SWNTs composites. Thus, the objective of this work is to determine if the unprecedented grain-boundary structures result in unusual creep behavior in alumina/SWNTs composites. To that end, alumina/10 vol % SWNTs composites were fabricated as part of an earlier study.13,15 The densities of the as-fabricated composites were ⬎97%. These composites were cut into 4⫻2⫻2mm 3rectangular parallelepipeds samples for creep tests. Uniaxial compression creep testing was performed using a method and equipment described elsewhere,27 at temperatures 1300 and 1350 °C, and in the stress range of 47–256 MPa. The temperatures used are high enough for diffusion processes to occur, but are sufficiently low to avoid grain growth. Argon gas atmosphere was used in order to avoid oxidation of the SWNTs. Transmission electron microscopy 共TEM兲specimens from the as-prepared and the creep-deformed composites were prepared using conventional methods involving successive steps of grinding, polishing, dimpling, and ion-beam thinning 共Fischione Instruments, Export, PA兲. The specimens were studied using a conventional 200 kV TEM 共CM200, Philips Electron Optics, Eindhoven, The Netherlands兲,a high-resolution 300 kV TEM 共CM300兲, and a subangstrom resolution 300 kV TEM 共Titan, FEI, Hillsboro, OR兲. Figure 1共a兲shows a bright-field TEM image of the asfabricated alumina/SWNTs composite. The segregation of what appear to be SWNTs at the alumina grain boundaries is evident 共see Fig. 2for higher magnification images兲.Itwas noted in an earlier study that not all SWNTs survive the spark-plasma sintering treatment used to consolidate these composites.13 Therefore, in addition to SWNTs, other carbonaceous matter 共amorphous carbon, graphite, multiwall CNTs兲are likely to be present at the grain boundaries in these composites. Also evident in Fig. 1共a兲is a lack of significant dislocation contrast within the alumina grains, and the average alumina grain size was determined to be ⬃0.5 ␮ m.13 Figure 1共b兲shows a TEM micrograph of the alumina/ SWNTs composite deformed by ⬃30% in compression at 1350 °C. The presence of dislocation debris in the form of a兲Electronic mail: [email protected]. b兲Electronic mail: [email protected]. FIG. 1. Bright-field TEM micrographs of alumina/SWNTs composites: 共a兲 before and 共b兲after creep deformation 共⬃30% strain兲. APPLIED PHYSICS LETTERS 92, 111912 共2008兲 0003-6951/2008/92共11兲/111912/3/$23.00 © 2008 American Institute of Physics92, 111912-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.92 On: Mon, 24 Aug 2015 09:39:30 subgrain cells within alumina grains is clearly evident in this micrograph. The grain size of the alumina was found to be unchanged after creep deformation. Figures 2共a兲and 2共b兲are high-resolution TEM 共HRTEM兲images of the as-prepared alumina/SWNTs composite, showing what appear to be SWNTs bundles of different orientations at alumina grain boundaries. These images are similar to what were reported in earlier studies.13,15 Once again, the presence of some other carbonaceous matter at grain boundaries in these composites cannot be ruled out. Figure 3共a兲is a HRTEM image of the creep-deformed 共⬃30% strain兲alumina/SWNTs composite, clearly showing the presence of entangled SWNTs. The average diameter of these SWNTs appears to be around 0.33 nm, which is much smaller than their typical size range between 0.5 and 2 nm.28 Although the SWNTs smaller than 0.4 nm are energetically less favorable, it has been shown that they could be mechanically stable at temperature above 1000 °C.29 Figure 3共b兲 shows that, besides SWNTs, graphitic material is also present at alumina grain boundaries after deformation. The composites, before and after creep testing, were characterized using a Raman confocal microspectrometer 共LabRAM, Jobin Yvon, Edison, NJ兲, with 532 nm wavelength laser. Figure 4shows Raman spectra in the Gband 共1550–1605 cm−1兲,Dband near 1350 cm−1, and the radial breathing mode 共RBM兲共100–325 cm−1兲共Refs. 30 and 31兲 ranges, from the alumina/SWNTs composite before and after creep deformation. The Raman spectra remain unchanged, confirming the preservation of the carbon structure after creep deformation. Figure 5shows plots of steady-state creep rate 共␧ ˙兲as a function of applied stress 共 ␴ 兲for the alumina/SWNTs composite at two different temperatures: 1300 and 1350 °C. For comparison, uniaxial compression creep data from the literature32 for high-purity alumina of about the same grain size 共0.5 ␮ m兲and density 共⬎98%兲, collected in the same temperature and applied-stress ranges, are included in Fig. 5. The high-temperature steady-state creep of materials can be analyzed using the following equation:26 ␧ ˙=A 冉 ␴ G 冊 n D0 eff exp 冋 −Q kT 册 ,共1兲 where Ais a constant that includes the grain size dependence, Gis the shear modulus, nis the stress exponent, D0 eff is the preexponential term of an effective diffusion coefficient, and Qis the activation energy for the diffusion coefficient that controls the thermally activated transport of the atomic/ionic species. Analysis of the data in Fig. 5using Eq. 共1兲shows that for the pure alumina n⬃1.7 and Q ⬃460 kJ/mol,32 while for the alumina/SWNTs composite n⬃2.6 and Q⬃660 kJ/mol. More importantly, the composite is found to be about two orders of magnitude more creep resistant as compared to the pure alumina. FIG. 2. HRTEM images of the as-prepared alumina/SWNTs composite showing what appear to be SWNTs bundles of different orientations at alumina grain boundaries. FIG. 3. HRTEM images of the creep-deformed alumina/SWNTs composite 共⬃30% strain兲showing 共a兲SWNTs and 共b兲graphitic material, at alumina grain boundaries. FIG. 4. 共Color online兲Raman spectra from alumina/SWNTs composites before and after creep deformation 共⬃30% strain兲in the G-band and D-band ranges. Inset shows corresponding spectra in the RBM range. FIG. 5. 共Color online兲Plot of steady-state strain rate 共␧ ˙兲vs applied compressive stress 共 ␴ 兲data from the alumina/SWNTs composite at 1300 and 1350 °C. Solid lines are linear fits. For comparison, compression creep data at 1300 and 1350 °C for pure alumina of ⬃0.5 ␮ m grain size from the literature 共Ref. 32兲are included as dashed lines. 111912-2 Zapata-Solvas et al. Appl. Phys. Lett. 92, 111912 共2008兲 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.92 On: Mon, 24 Aug 2015 09:39:30 First consider pure, fine-grained alumina, where hightemperature creep deformation generally occurs by coupled atomic/ionic diffusion and grain-boundary sliding.26 This mechanism results in a low stress exponent nin the range of 1–2.33 The preferred diffusion path is grain boundaries, and the associated activation energy is in the range of 400–500 kJ/mol.33,34 Furthermore, dislocation debris is generally not observed in creep-deformed fine-grained alumina at low and moderate applied-stress levels. Thus, a stress exponent of n⬃1.7 and an activation energy of Q ⬃460 kJ/mol for creep of pure alumina in Fig. 5typify diffusional-transport creep coupled with grain-boundary sliding.32 Creep deformation can also occur by coupled dislocation slip 共power law creep兲and grain-boundary sliding, which results in higher stress exponents 共n艌2兲.33,35,36 A stress exponent of n⬃2.6 for the alumina/SWNTs composite points to power law creep in that material. The presence of a subgrain dislocation network in the creep-deformed alumina/ SWNTs composite 关Fig. 1共b兲兴supports this hypothesis.35 Thus, the presence of SWNTs at grain boundaries in fine-grained alumina appears to be responsible for 共i兲a possible shift in the dominant creep mechanism from diffusional creep to power law creep and 共ii兲enhancement of creep resistance by about two orders of magnitude. The nature of grain-boundary sliding, which is associated with both diffusional creep and power law creep and which is influenced profoundly by the grain-boundary structure holds the key to understanding this effect. In the case of pure, fine-grained alumina the grain boundaries are assumed to be “clean,” where sliding is expected to occur unimpeded.35,36 Even if alumina grain boundaries have a thin film of glass, which is typically the case in aluminas of nominal purity,37 sliding is still expected to be unimpeded. This is due to the flow of the softened glassy phase, which is expected to be isotropic.35 In the case of the alumina/SWNTs composite, however, there is a tangled mat of random SWNT bundles at the grain boundaries.15 Sliding between SWNTs within the bundles 关see, e.g., Fig. 2共a兲兴is likely to occur readily, as the secondary bonding between individual SWNTs is relatively weak. However, due to the random nature of the SWNTs it is highly likely that some SWNTs will be in orientations where they will subjected to tension 关see e.g., Fig. 2共b兲, and HRTEM images in Ref. 15兴. This is particularly true at triplegrain junctions. Since the anisotropy in the mechanical properties of SWNTs bundles is extreme, where SWNTs in uniaxial tension have a Young’s modulus of ⬃0.64 TPa and a strength of ⬃37 GPa,18 it would take a small fraction of SWNTs that are oriented in the tension direction to impede significantly grain-boundary sliding in the composite. With the grain-boundary sliding partially blocked, higher stresses are needed to maintain a given steady-state strain rate, promoting dislocation slip. In summary, we have shown that SWNTs at the alumina grain boundaries in alumina/SWNTs composites exert a significant influence on the creep properties of these composites. The two orders of magnitude improvement in the creep resistance of alumina/SWNTs composites over pure alumina of the same grain size could be highly beneficial in the design of creep-resistant ceramic composites. However, these composites would have to be protected from oxidation at high temperatures. The authors thank Dr. H. Tanaka and Dr. T. Nishimura for performing spark-plasma sintering of the samples used in the study; Dr. A. L. Vasiliev and Dr. Z. Lee for performing some of the electron microscopy characterization; and Professor G. S. Daehn and Professor M. J. Mills for fruitful discussions. Funding for this work was provided by the Spanish CICYT 共Research Project No. MAT2006-10249C02-02 and MAT2003-04199-C02-02兲, and by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, U.S. Department of Energy under Contract No. DE-AC02-05CH11231. 1R. Z. Ma, J. Wu, B. Q. Wei, J. Liang, and D. H. Wu, J. Mater. Sci. 33, 5243 共1998兲. 2C. Laurent, A. Peigney, O. Dumortier, and A. Rousset, J. Eur. Ceram. Soc. 18, 2005 共1998兲. 3A. Peigney, C. H. Laurent, E. Flahaut, and A. Rousset, Ceram. Int. 26, 677 共2000兲. 4R. W. Siegel, S. K. Chang, B. J. Ash, J. Stone, P. M. Ajayan, R. W. Doremus, and L. S. Schadler, Scr. Mater. 44, 2061 共2001兲. 5J. Sun, L. Gao, and W. Li, Chem. Mater. 14, 5169 共2002兲. 6G.-D. Zhan, J. D. Kuntz, J. Wan, and A. K. Mukherjee, Nat. Mater. 2,38 共2003兲. 7W. A. Curtin and B. W. Sheldon, Mater. Today 7,44共2004兲. 8L. An, S. Rajagopalan, C. Wang, H. Wang, Y. Fan, L. Zhang, D. Jiang, J. Kapat, L. Chow, B. Guo, J. Liang, and R. Vaidyanathan, Adv. Mater. 共Weinheim, Ger.兲16, 2036 共2004兲. 9X. Wang, N. P. Padture, and H. Tanaka, Nat. Mater. 3,539共2004兲. 10Z. Xia, L. Riester, W. A. Curtin, H. Li, B. W. Sheldon, J. Liang, B. Chang, and J. M. Xu, Acta Mater. 52, 931 共2004兲. 11C. Balazsi, F. Weber, Z. Kover, Z. Konya, I. Kiricsi, L. P. Biro, and P. Arato, Key Eng. Mater. 290, 135 共2005兲. 12M. Daraktchiev, B. V. D. Moortele, R. Schaller, E. Couteau, and L. Forro, Adv. Mater. 共Weinheim, Ger.兲17,88共2005兲. 13R. Poyato, A. L. Vasiliev, N. P. Padture, H. Tanaka, and T. Nishimura, Nat. Nanotechnol. 17,1770共2006兲. 14N. Grobert, Mater. Today 10,28共2007兲. 15A. L. Vasiliev, R. Poyato, and N. P. Padture, Scr. Mater. 56,461共2007兲. 16A. A. White, S. M. Best, and I. A. Kinloch, Int. J. Appl. Ceram. Technol. 4,1共2007兲. 17D. Jiang, K. Thomson, J. D. Kuntz, J. W. Ager, and A. K. Mukherjee, Scr. Mater. 56, 959 共2007兲. 18R. H. Baughman, A. A. Zakhidov, and W. A. deHeer, Science 297, 787 共2002兲. 19S. Rul, F. Lefevre-Schlick, E. Capria, C. Laurent, and A. Peigney, Acta Mater. 52, 1061 共2004兲. 20B. W. Sheldon and W. A. Curtin, Nat. Mater. 3,505共2004兲. 21Z. Burghard, D. Schon, J. Bill, and F. Aldinger, Int. J. Mater. Res. 97, 1667 共2006兲. 22J. P. Fan, D. M. Zhuang, D. Q. Zhao, G. Zhang, M. S. Wu, F. Wei, and Z. J. Fan, Appl. Phys. Lett. 89, 121910 共2006兲. 23T. Wang, Z. Iqbal, and S. Mitra, Carbon 44, 2804 共2006兲. 24G. Yamamoto, K. Yokomizo, M. Omori, Y. Sato, B. Jeyadevan, K. Motomiya, T. Hashida, T. Takahashi, A. Okubo, and T. Tohji, Nanotechnology 18, 145614 共2007兲. 25N. P. Padture and W. A. Curtin, Scr. Mater. 共to be published兲. 26J. Poirier, Creep of Crystals 共Cambridge University Press, Cambridge, UK, 1985兲. 27H. Gervais, B. Pellisier, and J. Castaing, Rev. Int. Hautes Temp. Refract. 15,43共1978兲. 28P. Kondratyuk and J. T. Yates, Acc. Chem. Res. 40, 995 共2007兲. 29L. M. Peng, Z. L. Zhang, Z. Q. Xue, Q. D. Wu, Z. N. Gu, and D. G. Pettifor, Phys. Rev. Lett. 85, 3249 共2000兲. 30M. S. Dresselhaus and P. C. Eklund, Adv. Phys. 49,705共2000兲. 31M. S. Dresselhaus, G. Dresselhaus, A. Joriob, A. G. S. Filho, and R. Saito, Carbon 40, 2043 共2002兲. 32L. A. Xue and I. W. Chen, J. Am. Ceram. Soc. 73, 3518 共1990兲. 33W. R. Cannon and T. G. Langdon, J. Mater. Sci. 23,1共1988兲. 34R. M. Cannon, W. H. Rhodes, and A. H. Heuer, J. Am. Ceram. Soc. 63, 46 共1980兲. 35A. H. Chokshi and T. G. Langdon, Mater. Sci. Technol. 7,577共1991兲. 36O. A. Ruano, J. Wadsworth, and O. D. Sherby, Acta Mater. 51, 3617 共2003兲. 37D. R. Clarke, Annu. Rev. Mater. Sci. 17,57共1987兲. 111912-3 Zapata-Solvas et al. Appl. Phys. Lett. 92, 111912 共2008兲 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.92 On: Mon, 24 Aug 2015 09:39:30