IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 22 (2011) 295702 (7pp) doi:10.1088/0957-4484/22/29/295702 Time-dependent contact behavior between diamond and a CNT turf A Qiu 1 , S P Fowler 2 , J Jiao 2 , D Kiener 3 and D F Bahr 1 1 School of Mechanical and Materials Engineering, Washington State University, 99164-2920 WA, USA 2 Department of Physics, Portland State University, Portland, 97207-0751 OR, USA 3 Department of Materials Physics, University of Leoben, A-8700 Leoben, Austria E-mail: anqi qiu@wsu.edu and dbahr@wsu.edu Received 16 February 2011, in final form 6 May 2011 Published 14 June 2011 Online at stacks.iop.org/Nano/22/295702 Abstract The elastic and adhesive properties of nominally vertically aligned carbon nanotube (CNT) turfs have been measured using nanoindentation. The perceived stiffness of a CNT turf is dependent on the unloading rate, which decreases at slower unloading rates. Depth-controlled nanoindentation was used to examine adhesion effects. Adhesive loads between the turf and the probe tip increased as the time the tip is in contact with the turf increased. As these effects could be from either more tubes coming into contact with the tip due to relaxation and motion of CNTs relative to one another or each tube in contact increasing its adhesive behavior and sub-contact stiffness due to tube–tube interactions within the turf, electrical resistance measurements during nanoindentation were carried out. When the tip is held at a fixed nominal depth, the current remains constant while the contact load decreases, suggesting the number of tubes in contact with the tip stays constant with time while the relaxation mechanisms in the turf occur at positions lower than the contact surface. These observations, in conjunction with in situ TEM compression test of CNT arrays, are used to describe the relative effects the various length and time scales may have on the perceived properties measured during experiments, including elastic modulus and adhesion for gecko-like dry adhesives. S Online supplementary data available from stacks.iop.org/Nano/22/295702/mmedia (Some figures in this article are in colour only in the electronic version) 1. Introduction Carbon nanotube (CNT) turfs [1], structures which consist of CNTs grown on a substrate in a generally vertically aligned array [2] (sometimes called VACNTs), have possible applications in electronic devices [3] as they have high electri- cal conductivity [4] and relatively high thermal conductivity along the nominal growth direction [5]. Researchers have investigated the turf’s transfer and compression behavior [6, 7] in order to test the functionality of turfs for flip chip applications and thermal interface materials [8]. During thermocompression bonding of CNT turfs the structure (the turf) is subjected to compressive loading, and therefore assessing the mechanical properties is of interest. The overall compression behavior of turfs has been shown to result in buckling after elastic loading [9–12]. These observations have led to models to predict buckling, which requires a knowledge of the effective modulus of the turf. Similarly, the possibility of using CNT turfs as adhesive connectors [13] or as a gecko- type adherent [14] was shown, requiring again assessing the mechanical properties of the turfs. Nanoindentation is a logical choice to evaluate the properties of these turfs in a manner which is relatively rapid and non-destructive, and indentation models for this type of experiment have been described previously [15]. However, the tip–turf interaction is complex due to adhesion between the tip and turf as well as viscoelastic and/or viscoplastic behavior not commonly noted in bulk materials, and to date the material’s time-dependent behavior has not been examined in detail. Instrumented indentation, commonly referred to as nanoindentation, has been widely used to measure the mechanical properties of small volumes of materials. This process, wherein a sharp tip is pressed into a sample while the load and penetration depth are measured, is well established for 0957-4484/11/295702+07$33.00 © 2011 IOP Publishing Ltd Printed in the UK & the USA 1