Effect of biaxial strain on grain growth in nanocrystalline films: Coupling between grain-boundary energy and strain energy

A primary shortcoming of nanocrystalline materials is that they tend to coarsen at elevated temperatures, which limits their practical application. Nowadays, it is crucial to develop a non-alloying strategy to improve the thermal stability of nanocrystalline materials without changing their chemical composition. This work has demonstrated that varying the biaxial strain in nanocrystalline films can drastically change the grain-growth rate in nanocrystalline Ni films and even enable the tailoring of stable nanograin sizes in nanocrystalline Ni-Mo films. The biaxial strain can be adjusted, in this case, by introducing different thermal strains via using substrates with different coefficients of thermal expansion. Thus the thermal stability of nanocrystalline Ni and Ni-Mo films on Cu substrates is higher than that on W or Ni substrates. The underlying mechanism of this exceptional substrate-dependent thermal stability was revealed by performing thermodynamic model calculations, which provided insights into the effects of coupling between the grain-boundary energy and strain energy on the thermal stability of nanocrystalline films. In addition, the kinetic reasons for the different thermal stabilities of the Ni and Ni-Mo films were assessed, and the effect of the biaxial strain on the kinetic stabilizations was discussed. This work presents a strain-induced stabilization strategy for nanocrystalline materials, which is significant for enhancing the thermal stability of nanocrystalline materials without requiring alloying elements.