On hardness and toughness of ultrafine and nanocrystalline hard materials

The availability of nanocrystalline hard material powders offers a possibility to study the mechanical properties of extremely fine-grained sintered materials and polycrystals. In the micrometer range the hardness of polycrystalline materials usually increases with decreasing grain size. For the nanometer range this relation suggests extraordinarily high hardness values. But, at small crystallite size interfaces begin to play a growing role in deformation. Whole crystallites may be displaced instead of being plastically deformed due the huge number of atoms arranged in interfaces of a low atomic order and an increasing number of cracks along grain boundaries is formed. The hardness of hard metals and sintered carbides and carbonitrides with a grain size <1 mu m is measured and compared to the hardness of single crystals. In fact, hardness increases with decreasing grain size. For WC the hardness of the single crystal was achieved and slightly surpassed at small grain size. Nanocrystallites smaller than about 50 nm are Gee from dislocations. Deformation of polycrystals of grain size <0.1 mu m is expected to take place mainly via grain boundary gliding and crack formation. (C) 1999 Elsevier Science Ltd. All rights reserved.