Nanocrystalline Materials

Chapter 7: Effect of the Grain Size and Interfaces on the Properties of Bulk Nanomaterials

When interpreting the experimental results, obtained for bulk nanomaterials, it is important to be able to separate grain boundary (associated with interfaces) from volume (associated with small grain size) effects. This problem is far from being solved because at present investigation of bulk nanomaterials is still in the stage of collecting experimental results. For this reason, the level of understanding of the structure and properties of the bulk nanocrystalline materials is considerably lower in comparison with isolated nanoparticles. The properties of bulk nanomaterials in relation to the particle size and the state of the grain boundaries were discussed in reviews [1,2].

7.1 MECHANICAL PROPERTIES

Because of the application of bulk nanocrystalline materials in practice, it has been necessary to investigate in detail their hardness, strength, elasticity, plasticity and other mechanical properties.

In the group of the mechanical properties of nanocrystalline materials, special attention must be given to the very high hardness. Hardness characterises the resistance of materials to plastic deformation during indentation of a harder body into it. Diamond is used generally as an indentor material. When measuring hardness by the Vickers method, the effects associated with the difference in the elastic properties of the materials are almost completely excluded because the size of the indentation is measured after removing the stress, i. e. in the absence of elastic loading. The experimentally measured values of hardness are subjected to the effect of various secondary factors, such as the non ideal surface of the material, deviation from the perpendicularity...