11
Mechanical Properties of Nanoparticles
11.1
General Considerations
The huge interest in nanomaterials began with Gleiter et al.’s paper [1] on the plastic
deformation of CaF 2 at room temperature. The discussion of mechanical properties
of nanomaterials is, in general, only of quite basic interest, the reason being that it is
problematic to produce macroscopic bodies with a high density and a grain size in
the range of less than 100 nm. However, two materials, neither of which is produced
by pressing and sintering, have attracted much greater interest as they will
undoubtedly achieve industrial importance. These materials are (i) polymers that
contain nanoparticles, nanotubes, or nanoplates to improve their mechanical
behaviors, and (ii) severely plastic-deformed metals that exhibit astonishing properties. However, because of their larger grain size, the latter are generally not accepted
as nanomaterials. Experimental studies on the mechanical properties of bulk
nanomaterials are generally impaired by major experimental problems in producing
specimens with exactly defined grain sizes and porosities. Therefore, model
calculations and molecular dynamic studies are of major importance for an understanding of the mechanical properties of these materials.
It is common practice to characterize the mechanical properties of a material by its
stress–strain diagram, which can be determined either in tension or compression
experiments. A typical example of such a stress–strain diagram is shown in
Figure 11.1, where elongation of the specimen during increasing load is plotted
on the abscissa, while the stress is plotted on the ordinate. To be independent of the
geometry of the specimen, elongation of the specimen is plotted as strain e ¼ Dl/l,
where l is the length of the specimen and Dl is the elongation under load. Similarly,
the stress s is given by s ¼ P/A, where P is the load and A is the cross-section of the
specimen. At low stresses, the deformation of a specimen starts with a linear, elastic
range, where the deformation is fully reversible. After the elastic regime, plastic
deformation begins; the stress of the onset of plastic deformation is called the “yield
stress.” As experimentally it is almost impossible to determine the yield stress exactly,
in general the stress where 0.2% plastic deformation is observed is defined as the
yield stress. After reaching the yield point, characterized by the yield stress, the
range of plastic deformation begins. Plastic deformation is not reversible and during
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j299
Mechanical Properties of Nanoparticles
11.1
General Considerations
The huge interest in nanomaterials began with Gleiter et al.’s paper [1] on the plastic
deformation of CaF 2 at room temperature. The discussion of mechanical properties
of nanomaterials is, in general, only of quite basic interest, the reason being that it is
problematic to produce macroscopic bodies with a high density and a grain size in
the range of less than 100 nm. However, two materials, neither of which is produced
by pressing and sintering, have attracted much greater interest as they will
undoubtedly achieve industrial importance. These materials are (i) polymers that
contain nanoparticles, nanotubes, or nanoplates to improve their mechanical
behaviors, and (ii) severely plastic-deformed metals that exhibit astonishing properties. However, because of their larger grain size, the latter are generally not accepted
as nanomaterials. Experimental studies on the mechanical properties of bulk
nanomaterials are generally impaired by major experimental problems in producing
specimens with exactly defined grain sizes and porosities. Therefore, model
calculations and molecular dynamic studies are of major importance for an understanding of the mechanical properties of these materials.
It is common practice to characterize the mechanical properties of a material by its
stress–strain diagram, which can be determined either in tension or compression
experiments. A typical example of such a stress–strain diagram is shown in
Figure 11.1, where elongation of the specimen during increasing load is plotted
on the abscissa, while the stress is plotted on the ordinate. To be independent of the
geometry of the specimen, elongation of the specimen is plotted as strain e ¼ Dl/l,
where l is the length of the specimen and Dl is the elongation under load. Similarly,
the stress s is given by s ¼ P/A, where P is the load and A is the cross-section of the
specimen. At low stresses, the deformation of a specimen starts with a linear, elastic
range, where the deformation is fully reversible. After the elastic regime, plastic
deformation begins; the stress of the onset of plastic deformation is called the “yield
stress.” As experimentally it is almost impossible to determine the yield stress exactly,
in general the stress where 0.2% plastic deformation is observed is defined as the
yield stress. After reaching the yield point, characterized by the yield stress, the
range of plastic deformation begins. Plastic deformation is not reversible and during
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j299