3.4. Strength of nanopillars and nanoneedles

Nowadays, nanoneedles and nanopillars are the most common nanosized specimens, which strength can be determined experimentally. Two main kinds of nanopillars exist, namely: (i) nanopillars obtained by focused ion beam (FIB) technology [43, 44], and (ii) nanopillars obtained by etching of nanocomposites [3]. Surface layer of specimens obtained by FIB technique contains a great number of defects in sub-surface layer, and so their strength does not exceed 1.0–1.5 GPa [43]. The second kind of specimens has no such shortcomings, and so, their strength is much higher. For instance, strength of Mo nanopillars of diameter 300–1000 nm is approximately 9 GPa. This is 6 times greater than strength of nanopillars obtained by FIB technique. However, this strength value is more than 2 times less than the evidence on MD-simulation of compression of defect-free Mo nanopillars of this crystallographic orientation [100]; according to our data, this value is ≈ 20 GPa. This difference can be caused by buckling instability, since at compression tests of nanopillars, it is quite difficult to provide the ideal conditions of uniaxial compression.

we have additional effect of a surface roughness. This effect may give rise to a reduction in strength by at least 30–40%, and it is the cause for considerable scatter of experimental data. The

Figure 12. Size effect for nanowires, nanoneedles and nanopillars: W (a) and Mo (b); are nanopillars manufactured (prepared) by FIB technology (built by the evidence of [43]); are obtained by etching (built by the evidence of [3]).

Atomic Mechanisms Governing Strength of Metallic Nanosized Crystals

http://dx.doi.org/10.5772/intechopen.75159

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1. On the atomic scale, the strength of nanosized crystals (NSC) is governed by two modes of lattice instability. This is instability with respect to the action of tensile stresses and shear instability. In nanosized crystals of fcc metals, the second mode of instability predominates. BCC metals are characterised by a competition between these two modes of instability, depending on the loading conditions of the crystal. Instability on the Bain path is realised at uniform triaxial (hydrostatic) tension. At uniaxial tension, even under cryogenic conditions, the strength of the bcc nanocrystals is controlled by shear instability (instability

2. Localisation of instability in a limited volume of a nanospecimen is the distinguishing feature of the mechanism of nanosized crystal instability. It occurs due to two main factors: (i) local stresses fluctuation caused by thermal vibrations of atoms and (ii) the effect of

strength of nanopillars is significantly influenced by the buckling effect.

4. Conclusions

on the orthorhombic path).

A high-field technique of tensile testing in-situ of nanoneedles is free of such disadvantage. In addition, it makes it possible to test the specimens of significantly smaller diameters—from 20 till 125 nm. Data presented in Figure 12 enable to estimate experimentally strength of Mo and W nanocrystals by the value of upper scatter limit of experimental evidence on failure of nanoneedle specimens. This value for W is 23.2 GPа, and for Mo, it is 20.0 GPа. These figures also present the results of MD simulation of tension of cylindrical specimens in the direction [110]. Calculated strength values are approximately 1.4 times greater than the maximum experimental values of the nanopillars strength. At high-field treatment of the tip of nanoneedle specimen, its working volume is "cleaned" from dislocations [4, 19], and so, both decrease the values of strength of nanoneedle specimens, and their scatter may be because of stress raising related to rough lateral surface, which is formed by electrochemical polishing. The results obtained make it possible to identify the main factors leading to a decrease in the strength of nanosized crystals. According to these data, tensile strength of defect-free nanowires is always less than ideal strength. This is due to both surface tension effect (effect of a physical surface) and thermally induced local stress fluctuation (temperature effect). At transition to nanoneedles,

Figure 12. Size effect for nanowires, nanoneedles and nanopillars: W (a) and Mo (b); are nanopillars manufactured (prepared) by FIB technology (built by the evidence of [43]); are obtained by etching (built by the evidence of [3]).

we have additional effect of a surface roughness. This effect may give rise to a reduction in strength by at least 30–40%, and it is the cause for considerable scatter of experimental data. The strength of nanopillars is significantly influenced by the buckling effect.
