CrN/AlN multilayers have been shown to exhibit a peak in hardness of ～40 GPa and simultaneously a low friction coefficient as a function of the bilayer period (Λ). These improvements in mechanical properties in comparison with their monolithic counterparts have a close relationship with the existence of a metastable face-centered cubic (fcc) AlN phase which can be epitaxially stabilized in thin films. Consequently, new physical phenomena and exciting material properties may be introduced by interfaces present in multilayers.
The CrN/AlN multilayers with a nominal bilayer period (Λ) of 6.0, 5.5, and 2.0 nm were used in this study. CrN/AlN multilayer films were prepared in an AJA Orion 5 lab-scale magnetron sputtering system.
It is revealed that the dislocation densities are higher in CrN than in AlN layers, and a relatively lower value along the growth direction than in in-plane direction when the bilayer period is small (Λ = 2.0 nm). The dislocation density decreases with decreasing the bilayer period due to the enhanced inter-diffusion across the interfaces. 2) Relative strain fields revealed by geometrical phase analysis oscillate, corresponding to the bilayer period, and their magnitude increases with decreasing bilayer period. 3) Interplanar spacing oscillations in cubic CrN/AlN multilayers were experimentally observed by using spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM), and were corroborated by first principles calculations. The oscillations are closely related to changes in the electronic structure. Electron spectroscopy and microscopy were employed to analyse the strain distribution in the multilayers and obtain generalized relationships between the electronic structure on the one hand, and (non-)stoichiometry or strains in the multilayers on the other hand. These observations were successfully interpreted by means of theoretical calculations. The present study provides atomic-scale insights in the mechanisms of extraordinary strength pertaining to the CrN/AlN multilayers.
The author would like to thank David Holec (Montanuniversität Leoben), Matthias Bartosik and Paul H.Mayrhofer (TU Wien) for helpful discussions and preparing the films.