We have synthesized nanocrystalline thin films of Cu, Zn, TiN, and WC having uniform grain size in the range of 5 to 100 nm. This was accomplished by introducing a couple of manolayers of materials with high surface and have a weak interaction with the substrate. The hardness measurements of these well-characterized specimens with controlled microstructures show that hardness initially increases with decreasing grain size following the well-known Hall-Petch relationship (H∝d−½). However, there is a critical grain size below which the hardness decreases with decreasing grain size. The experimental evidence for this softening of nanocrystalline materials at very small grain sizes (referred as reverse Hall-Petch effect) is presented for the first time. Most of the plastic deformation in our model is envisioned to be due to a large number of small “sliding events” associated with grain boundary shear or grain boundary sliding. This grain-size dependence of hardness can be used to create functionally gradient materials for improved adhesion and wear among other improved properties.

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