Atomic-scale understanding of the structural evolution of TiN/AlN superlattice during nanoindentation - Part 1: Deformation

At present, the theoretical predictions of the mechanical properties of transition-metal nitride (TMN) superlattices (SLs) are primarily based on the intrinsic properties of perfect epitaxial nanolayers. However, due to a lack of understanding of the specific strengthening mechanism, the experimentally determined strength, e.g., hardness, of TMN SLs often deviates significantly from the theoretical predictions. Here, by coupling FIB (focused ion beam) sectioning with TEM, we observe the structural evolution of two representatives TiN/AlN SL coatings, i.e., a single-crystalline and a polycrystalline SL, under identical loads. We found that in comparison with the polycrystalline SL, the indented single-crystalline SL forms a larger 'intermixed' region, within which the layer structure transforms into a solid solution under loads. Close TEM characterization demonstrates that the single-crystalline SL deformation is of variety, including the distortion of SL interfaces, polycrystalline deformation (grain rotation) in solid solution, and SL slip deformation. By contrast, columnar grain boundary sliding is the primary deformation mechanism in the polycrystalline SL. And, a relatively large solid-solution zone in single-crystalline SL is attributed to the severe interfacial deformation. The current research unravels TMN SL deformation behavior at the atomic scale. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

At present, the theoretical predictions of the mechanical properties of transition-metal nitride superlattices primarily rely on the intrinsic properties of perfect epitaxial nanolayers. However, the experimentally determined strength, such as hardness, often deviates significantly from the theoretical predictions due to a lack of understanding of the specific strengthening mechanism. In this study, by using focused ion beam sectioning combined with transmission electron microscopy, the structural evolution of two representative TiN/AlN superlattice coatings, namely single-crystalline and polycrystalline superlattices, under identical loads was observed. It was found that the single-crystalline superlattice formed a larger 'intermixed' region compared to the polycrystalline superlattice, where the layer structure transformed into a solid solution under loads. The deformation of the single-crystalline superlattice involved distortion of the superlattice interfaces, polycrystalline deformation (grain rotation) in the solid solution, and superlattice slip deformation, while columnar grain boundary sliding was the primary deformation mechanism in the polycrystalline superlattice. The relatively large solid-solution zone in the single-crystalline superlattice was attributed to severe interfacial deformation. This research reveals the atomic-scale deformation behavior of transition-metal nitride superlattices.