Bonding strength of a carbon nanofiber array to its substrate
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The bonding strength of a carbon nanofiber array (CNFA) grown on a copper substrate is evaluated based on the measured shearing force-at-failure and the developed analytical stress model that enables one to determine the magnitude and the distribution of the interfacial shearing stress causing the measured (given) shearing force. The experiment is conducted using specially designed test specimens. A table version of the Instron tester is used to measure the applied force and the corresponding displacement in shear. The maximum predicted shear-off stress is about 300 psi (0.211 kgf/m2), and was determined, based on the developed stress model, as a product of the measured 5 kgf/m force at the interface failure and the computed parameter k = 0.0422 m–1 of the interfacial shearing stress.
I. INTRODUCTION 1
Since their discovery in 1991, carbon nanotubes (CNTs) have become a distinct branch of nanotechnology, with numerous attractive applications. One such application is heat transfer. Vertically aligned carbon nanofiber arrays (CNFAs) produced by plasma-enhanced chemical vapor deposition (PECVD) are also attractive because of their good alignment.2 CNTs and CNFs have been demonstrated as effective thermal interface materials.2–5 It goes without saying that satisfactory adhesion of the CNFA to its substrate is critical for making CNF-based technology practical. Accordingly, the analysis that follows is aimed at the development of an easy-to-use and effective experimental method for the evaluation of such adhesion. Note that the adhesion strength of a single CNF to its substrate was addressed qualitatively, apparently for the first time, by Cui et al.6 and by Chen et al.7 This was done in connection with the use of PECVDsynthesized CNFs as probe tips in atomic force microscopy imaging equipment. In the experiments described by these investigators, individual CNFs were directly grown on tipless cantilevers. In the reported observation, the CNF probe was operated on a continuous scan mode for 8 h, and no degradation in image resolution was observed. However, the CNFA in our tests consisted of billions of CNFs. Therefore, the information obtained characterizes the performance of an ensemble of a plurality of CNFs. To translate the experimental data into a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0357 2922
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 11, Nov 2006 Downloaded: 13 Mar 2015
the corresponding shearing stresses, we developed a simple analytical stress model that enables one to calculate the magnitude and the distribution of the interfacial shearing stress from the measured (given) external force. Testing has been conducted using specially designed shear-off test specimens. In the analysis, we determine the maximum effective shear stress-at-failure for a CNFA fabricated on a thick Cu substrate. We use the term “effective” to emphasize that we address a plurality of CNFs and we treat the CNFA “brush” as a sort of continuous bonding layer. This appro
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