Complex structures consisting of intertwined, nominally vertical carbon nanotubes (CNTs), referred to as turfs, have unique properties that arise from their complex nanogeometry and interactions between individual CNT segments. For applications such as contact switches for electrical or thermal transfer it is necessary to understand the properties that arise from the collective behavior of an assemblage of CNTs rather than the properties of a single tube. In this study, the mechanical response of turfs bonded to substrates under compressive loading is demonstrated experimentally; coordinated alignment and buckling takes place under uniform loads. The mechanical response of turf structures provides some surprising results regarding parameters that control permanent deformation and buckling in assemblages of nanostructures; buckling of the turf structure is controlled by the height and effective modulus of the turf, but not the aspect ratio of the structure. We present and verify a model which describes the coordinated buckling phenomena relevant for applications such as CNT turfs for thermal transfer media.

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The onset of plasticity during nanoindentation of a tungsten single crystal was examined as a function of pre-existing dislocation density. Vickers indentations were used to generate a spatially varying dislocation density, and nanoindentation was then carried out at regions of high and low dislocation densities. Even with dislocation densities as high as 1.8 · 10 13 m -2 , a sharp elastic-plastic transition was observed during some indentations. At lower dislocation densities, 3.5 · 10 9 m -2 , the shear stress at the elastic plastic transition increased and approached the theoretical shear stress of the crystal. A first-order model that predicts the load required for the onset of plasticity during nanoindentation from the activation of a dislocation source within a critical volume of material, rather than homogeneous dislocation nucleation, is developed. The model correlates well with experimentally measured loads at the onset of plasticity for dislocation densities of 10 12 m -2 and higher for these nanoindentation conditions.

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When grown via chemical vapor deposition carbon nanotubes (CNTs) may take on the form of a "turf", consisting of many CNTs with a complex interconnectedness attached to an inflexible substrate. These turfs can be formed over large areas and with a range of heights (between 1 to 100 µm), and grown on photolithographically patterned catalysts to form different aspect ratios. This study focuses on the indentation and permanent deformation of CNT assemblages under applied contact loading. Nanoindentation was conducted on CNT turfs and the properties, nominally the turf's elastic modulus and hardness, were 14.9 MPa ± 5.7 MPa and 2 MPa respectively. The onset of permanent deformation during indentation occurred at applied stresses of 2.5 MPa. The turf's collective permanent deformation under applied compressive loading was also studied. A model predicting the buckling stress of CNT turfs is also described.