Length Dependent Foam-Like Mechanical Response of Axially Indented Vertically Oriented Carbon Nanotube Arrays
AIR FORCE RESEARCH LAB WRIGHT-PATTERSON AFB OH MATERIALS AND MANUFACTURING DIRECTORATE
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The axial compressive mechanical response of substrate-supported carbon nanotube CNT arrays with heights from 35 to 1200 lm is evaluated using flat punch nanoindentation with indentation depths to 200 lm. The compressive behavior is consistent with that of an open-cell foam material with array height playing a role similar to that of occupation density for traditional foam. Mechanical yielding of all arrays is initiated between 0.03 and 0.12 strain and arises from localized coordinated plastic buckling. For intermediate CNT array heights between 190 and 650 lm, buckle formation is highly periodic, with characteristic wavelengths between 3 and 6 lm. Buckle formation produced substantial force oscillations in both the compressive and lateral directions. The compressive elastic modulus of the arrays is obtained as a continuous function of penetration depth and attains a value between 10 and 20 MPa for all arrays during mechanical yield. A qualitative model based upon concepts of cellular foam geometry is advanced to explain the observed CNT buckling behavior.
- Refractory Fibers