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The effects of load, grain size, and grain boundaries on the hardness of alumina

: Krell, A.

Ceramic engineering & science proceedings 19 (1998), Nr.4B, S.159-168
ISSN: 0196-6219
Annual Cocoa Beach Conference <22, 1998>
Fraunhofer IKTS ()
Härtemessung; Keramik

A model describes the load (size) effect in hardness assuming an increasing microplasticity when for the extension of the plastic zone growth and multiplication of pre-existing dislocations are more effective than the generation of new lattice defects in the surrounding virgin material. If similar scale limitations for the microplastic deformation apply in polycrystals due to small grain sizes as in single crystals at small plastic zone sizes (small loads), the size effect in sapphire is analogous to the grain size effect of the polycrystalline material: in sapphire the hardness depends on a changing microplasticity associated with known sizes of the plastic zone in the same way as it depends, in sintered alumina, on the grain size. Contrary to the size effect in sapphire, however, the grain size effect in polycrystals includes grain boundary contributions to the deformation. A large hardness difference is observed in plastically deformed volumes of sapphire and in sintered alumina wit h grains of the same size. This difference reveals substantial inelastic grain boundary deformation at room temperature even for coarser microstructures.