Most PVD fabricated multilayers with nanoscale layer thickness have microstructures like columnar boundaries, so that the cross-sectional textures are always like what is shown in the attached figure. The columnar boundaries are always perpendicular to the layers and seems to be distributed periodically in the microstructures. This feature is detrimental to the mechanical performance, since it is weakly bonded. Does anyone know the formation mechanism of the columnar boundary in PVD multilayers? and How to modulate or eliminate it by changing the PVD conditions?
If you are doing Nitride based metal like (TiN, CrN........) then you just increase the percentage of nitrogen from 20 to 35 percent. It will change the columnar structure. By the way, columnar structure is not that much bad for the mechanical property its depend on which type of metal you are depositing in multilayer. As in my case, Ti/TiN multilayer the corrosion property increases but hardness is around 14 GPa. Individual TiN have 24 GPa and Ti ~4 GPa. If you put first layer metal and last layer ceramic then it will increase your property.
The columnar structure could be a problem when it is in shear or tension. In compression like indentation it is not a big problem. I have done some work on micro-tensile tests of Al/SiC multilayers, with the applied force parallel to the layers, trying to study the co-deformation of both layers. But due to the existence of the columnar boundary, the tested samples were always failed by cracking of the columnar boundary in the elastic regime. I understand that it is the difference in the crystal growth velocity that result in the formation of the columnar structure. If there are some ways to eliminate them, thus to achieve a perfect nano-layered structure, it would be really nice to explore the co-deformation of different layers in other loading states, like in tension and shear. Compression is rather well studied.
So, how you are finding deformation using indentation or something else...
In compression like indentation, the columnar boundary would play a minor role in the deformation of the multilayers because of the strong constraint under the indent.
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