Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations
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risa Pour Shahid Saeed Abadi George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Samuel Graham and Baratunde A. Cola George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332; and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Julia R. Greer Materials Science, California Institute of Technology (Caltech), Pasadena, California 91125 (Received 31 March 2012; accepted 28 September 2012)
We report the mechanical behavior of vertically aligned carbon nanotube films, grown on Si substrates using atmospheric pressure chemical vapor deposition, subjected to in situ large displacement (up to 70 lm) flat-punch indentations. We observed three distinct regimes in their indentation stress–strain curves: (i) a short elastic regime, followed by (ii) a sudden instability, which resulted in a substantial rapid displacement burst manifested by an instantaneous vertical shearing of the material directly underneath the indenter tip by as much as 30 lm, and (iii) a positively sloped plateau for displacements between 10 and 70 lm. In situ nanomechanical indentation experiments revealed that the shear strain was accommodated by an array of coiled carbon nanotube “microrollers,” providing a low-friction path for the vertical displacement. Mechanical response and concurrent deformation morphologies are discussed in the foam-like deformation framework with a particular emphasis on boundary conditions.
I. INTRODUCTION
Considerable efforts have been dedicated to explore the deformation mechanisms of vertically aligned carbon nanotube (VACNT) forests, in part motivated by their wide range of potential applications in areas such as energy dissipation devices, electrical interconnects, thermal interface materials, microelectromechanical systems, and microelectronics.1–6 To date, instrumented indentation has been a common method for testing the mechanical behavior of VACNT films on substrates. However, due to the limitations of the displacement actuators, most of the existing studies are limited to shallow indentation depths (and strains), varying from a few hundred nanometers to several micrometers. Additionally, while VACNT forest films may have millimeter-sized lateral and vertical dimensions, they are composed of individual nanotubes with diameters in the nanometer range, which drives their mechanical response to be distinct from monolithic materials. Hence, the accurate estimation of the contact area between the VACNT film and the commonly used parabolic7,8 and pyramidal7 indenter tip geometries, a)
Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/jmr.2012.366 984
J. Mater. Res., Vol. 28, No. 7, Apr 14, 2013
http://journals.cambridge.org
Downloaded: 02 Apr 2015
necessary for indentation data analysis, poses a significant challenge. In this work, we utilize flat-punch diamond indenter tips, resulting in a constant contact
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