Plastic deformation in nanocrystalline TiN at ultra-low stress: An in situ nanoindentation study

It is well adopted that a majority of ceramic materials have low ductility and the plastic yielding in crystalline ceramics occurs succeeding elastic deformation. Here by using in situ nanoindentation we show, however, that in nanocrystalline TiN, massive non-elastic events, such as grain rotation and grain boundary (GB) sliding, occur at ultra-low stresses (<1 GPa), well below the yielding strength of the material (6-8 GPa). Furthermore, by performing in situ nanoindentations on TiN with different average grain sizes of 110 nm, 35 nm and 20 nm, we observed a prominent grain size effect: that is specimens with smaller average grain sizes possess greater plastic deformation than the ones with larger grains. This in situ nanoindentation study thus provides new perspectives on understanding plasticity of hard nanocrystalline ceramics at room temperature.