Structure and properties of nanocrystalline zinc films

We have developed a unique processing technique to fabricate Zinc (Zn) nanocomposites with controlled microstructures. In this method, pulsed laser deposition of Zn in conjunction with a few monolayers of tungsten (W) is used to control the grain size of nanocrystalline composites. The grain size of Zn was controlled by the amount of Zn and the substrate temperature. The Zn islands of lower surface energy nucleate on W-layer of high surface energy, which is also insoluble in Zn resulting in lower interfacial energy. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 30 nm down to 6 nm. The hardness of these nanocrystalline films increases with the decrease in grain size, following approximately Hall-Petch relationship. The most interesting observation is the decrease in hardness below a critical size, which is explained on the basis of grain boundary deformation/sliding. The role of W in grain boundary deformation is of particular interest in strengthening and stabilizing the nanocrystalline composites. The potential of this technique to attain even lower grain sizes is discussed.