Deformation processes and the effects of microstructure in multilayered ceramics

This paper reviews how layered interfaces influence the deformation behaviour of thin ceramic films. It is shown that the substantial increases in hardness of approximately 30 GPa that have been reported are not generally observed, and the ideas to explain them, based on changes in dislocation line energy, are inconsistent with these observations. Cross-sectional transmission electron microscopy of TiN/NbN multilayers with wavelengths from 2 to 25 nm shows that flow occurs by dislocation motion across the layers and by the compaction of layers if they are porous. The hardness of the multilayers is approximately 6 GPa greater than that of the monolithic films using a rule of mixtures, whilst there is no significant variation in hardness with layer thickness. It is shown that these observations are significantly closer to the predictions made using analyses based on changes to the dislocation line energy and it is tentatively suggested that the high values previously reported were associated with high internal stresses.