Strength hierarchy for nano-sized crystals Sergiy Kotrechko 1* , Igor Mikhailovskij 2 , Tatiana Mazilova 2 , Oleksandr Ovsjannikov 1 1 G.V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of the Ukraine, Acad. Vernadsky Blvd., 36, Kiev UA-03680, Ukraine 2 National Scientific Center, Kharkov Institute for Physics and Technology, National Academy of Sciences of the Ukraine, Academicheskaja, 1, Kharkov 61108, Ukraine * kotr@imp.kiev.ua Keywords: nanoneedles, nanopillars, graphene, nanotubes, carbon atomic chains, strength. Abstract A concept of atomic mechanisms governing strength of nano-sized defect-free crystals is presented. It is exhibited that these mechanisms consist in local instability of the lattice. Two main reasons for localization of instability in three-dimension (3D) crystals are analyzed, namely, (i) fluctuation of local stresses induced by thermal vibrations of atoms, and (ii) non-uniform distribution of local stresses caused by a surface tension. Based on this conception, explanations of both the temperature dependence of strength of 3D nanocrystals and scale effect are given. Ideas on the reasons for and regularities of change in strength at transition from 3D to 2D (graphene) and 1D (monatomic chain) crystals are represented. It is shown that dimensionality of crystal is one of the main factors governing strength of defect-free crystals. Experimental values of the strength of carbon monatomic chains are given, which 2  times exceeds the strength of graphene and is the highest attainable level of strength in the world. Introduction Development of nanotechnologies creates new type of nano-sized objects (graphene, nanopillars, nanotubes, nanoneedles). Their strength significantly exceeds those for „ordinary“ single- and polycrystalline materials, which is due to difference in mechanisms governing their strength. It most cases, they are defect-free crystals so, initially it was believed that their strength must tend to the ideal strength and be the intrinsic material trait much like the elastic constants. However, it was appeared that the strength of nano-size crystals changes within the wide range depending on their sizes, temperature, loading rate, etc. The explanation of above effects requires development of the theory of limit state of nano-size crystals under mechanical loading. Recently, great attention is focused on two-dimension (2D) and one-dimension (1D) crystals along with three-dimension (3D) nanocrystals. It is because of their fascinating properties from a basic scientific viewpoint as well as from their great potential in forthcoming technological applications. For example, strength of the carbon atomic chains is considered as the highest one, and claims to be the absolute upper strength limit [1]. This work is aimed at both presentation of atomic mechanisms governing strength of nano- sized 3D-crystals and analysis of their transformation at transition to 2D- and 1D-crystals. Strength of nano-sized 3D-crystals A concept of "ideal" strength can be used as a starting point for the analysis of atomic mechanisms governing the strength of nano-sized crystals. The ideal strength of a material is defined to be the maximal homogenous stress that a perfect crystal, without structural or compositional defects, can withstand. Reaching of the limit state of such crystal is related to uniform and simultaneous break of Key Engineering Materials Vols. 592-593 (2014) pp 301-306 Online available since 2013/Nov/15 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.592-593.301 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 194.44.31.34, National Academy of Sciences of Ukraine, Kiev, Ukraine-20/11/13,14:32:43)