Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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Figure 7.8 Normalized Vickers indentation fracture toughness values vs nanotube content for Al 2O 3/CNT nanocomposites sintering at different temperatures. 7.3 Oxide-Based Nanocompositesj195 conditions, the fracture toughness of dense Al2O3/10 vol% SWNT nanocomposite is nearly three times that of pure alumina. The normalized fracture toughness values of the nanocomposites as a function of filler content is also plotted in Figure 7.8. The beneficial toughening effect of SWNTs can be readily seen in this figure. Figure 7.9(a)–(c) show SEM fractographs of the Al2O3/ 10 vol% SWNT nanocomposite. Crack bridging, crack deflection and nanotube pull-out can be readily seen in an indentation crack of this nanocomposite. These mechanisms are responsible for the improved fracture toughness of the nanocomposite. The enhanced indentation fracture toughness of Al 2O 3/CNT nanocomposites associated with CNT additions has raised a doubt over the reliability of KIC values obtained from Vickers indentation tests. Recently, there has been a serious discussion and debate on the toughening effect of adding SWNT to alumina determined by Vickers indentation [21–23]. Padture and coworkers re-examined the effect of SWNTaddition on the fracture toughness of the Al2O3/10 vol% SWNT nanocomposite. Both VIF and SEVNB standard test methods were used to evaluate the fracture toughness [21]. They reported that CNT has little effect in improving the fracture toughness of alumina. The Al 2O 3/10 vol% SWNT nanocomposite was prepared by powder mixing in methanol, ball milling and SPS at 1450–1550 C for 2–3 min under a pressure of 40 MPa. For the purpose of comparison, Al2O3/ 10 vol% graphite composite was also fabricated under the same processing conditions. Further, dense and porous alumina specimens with grain sizes of 1–2 mm were prepared by cold isostatic pressing of the green compact and pressureless sintering at 1600 C for 30 min or 1350 C for 60 min. The r.d. of dense alumina, porous alumina, Al2O3/10 vol% SWNT nanocomposite and Al2O3/10 vol% graphite composite is 98.1, 90.6, 95.1 and 97.7%, respectively.
196j 7 Mechanical Properties of Carbon Nanotube–Ceramic Nanocomposites Figure 7.9 (a) Crack bridging; (b) crack deflection; (c) nanotube pull-out, observed in an indentation crack of the alumina/10 vol% SWNT nanocomposite. Reproduced with permission from [Chap. 5, Ref. 70]. Copyright Ó (2007) Elsevier. Figure 7.10(a) shows low and high-magnification SEM micrographs of the dense alumina subjected to VIF test. Radial cracks can be readily seen in these micrographs, and the toughness is determined to be 3.01 MPa m 1/2 using the Antis equation. A similar radial cracking behavior can be observed in the porous alumina subjected to Vickers indentation (Figure 7.8(b)). However, classical radial cracks are absent in the Al2O3/10 vol% SWNT composite (Figure 7.10(c)). Consequently, the Antis equation becomes invalid for fracture toughness determination. It is considered that the highly shear-deformable SWNTs can assist redistribution of the stress field under indentation, endowing the nanocomposite with contact-damage resistance. Thus, it appears that the VIF test cannot be used to characterize the fracture toughness of alumina-CNT nanocomposites, but the test results can provide useful information relating the resistance of such materials to indentation fracture. Direct fracture toughness measurements using the SEVNB specimens under four-point bending yield an average KIC of 3.22 MPa m 1/2 for the dense alumina, 3.32 MPa m 1/2 for the Al2O3/10 vol% SWNT nanocomposite and 3.51 MPa m 1/2 for the Al2O3/10 vol% graphite composite. These results indicate that there is little
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Sie Chin Tjong Carbon Nanotube Rein
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Sie Chin Tjong Carbon Nanotube Rein
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Contents Preface IX List of Abbrevi
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5 Carbon Nanotube-Ceramic Nanocompo
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Preface Carbon nanotubes are nanost
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XII List of Abbreviations HIP hot i
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1 Introduction 1.1 Background Compo
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Figure 1.2 Transmission electron mi
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1.3 Synthesis of Carbon Nanotubes 1
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MWNTs can be as high as 70% of the
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Figure 1.7 In situ TEM images recor
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1.3 Synthesis of Carbon Nanotubesj1
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show TEM images of MWNTs synthesize
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Since then, large efforts have been
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1.3.4 Patent Processes Carbon nanot
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1.4 Purification of Carbon Nanotube
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Table 1.3 Patent processes for the
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Figure 1.13 Schematic illustration
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1.5 Mechanical Properties of Carbon
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Figure 1.14 Carbon nanotubes in hig
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Figure 1.15 In situ tensile deforma
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Table 1.7 Theoretical and experimen
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Nomenclature ~a 1 , ~a 2 Unit vecto
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carbon nanotubes. Physical Review L
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70 Jang, I., Uh, H.S., Cho, H.J., L
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108 Shelimov, K.B., Esenaliev, R.O.
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Bonnamy, S., Beguin, F., Burnham, N
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2 Carbon Nanotube-Metal Nanocomposi
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isostatic pressing. In certain case
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This implies the absence of effecti
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coating material, the plasma gun an
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Table 2.4 Changes in the size and v
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Figure 2.6 (a) Low and (b) high mag
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Figure 2.8 SEM micrographs showing
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Figure 2.11 Bright field TEM microg
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Figure 2.13 TEM image of bulk Al/5
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2.5 Magnesium-Based Nanocomposites
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limited improvement in ultimate ten
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Figure 2.18 SEM micrographs of the
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Figure 2.19 (a) Low and (b) high ma
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Figure 2.22 SEM micrographs of (a)
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Figure 2.25 (a) TEM micrograph show
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2.8 Transition Metal-Based Nanocomp
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Figure 2.27 TEM image of electrodep
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Figure 2.29 SEM micrographs of (a)
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Figure 2.31 Schematic representatio
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einforcement content SiCp/Al compos
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Materials Science Forum, 534-536 (P
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composite. Materials Science and En
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111 Cha, S.I., Kim, K.T., Arshad, S
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90j 3 Physical Properties of Carbon
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100j 3 Physical Properties of Carbo
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102j 3 Physical Properties of Carbo
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104j 4 Mechanical Characteristics o
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132j 5 Carbon Nanotube-Ceramic Nano
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