On the thinnest Al2O3 interlayers in Al-based nanolaminates to enhance strength, and the role of constraint

Physical vapour deposition combined with atomic layer deposition was exploited to design a model sys-tem of UFG aluminium with a narrow grain size and shape distribution, including two types of inter-faces (Al-Al & Al-Al2O3), with Al-Al grain boundary orientations exclusively parallel to the loading axis. This enabled isolated study of the strengthening mechanisms that ultrathin oxide layers would provide in a metal multilayer structure. The Al/Al2O3 crystalline/amorphous multilayers with 240 nm metal lay-ers and oxide thicknesses in the range < 1 nm-12 nm (i.e. to below the natural oxidation thickness), were microcompressed, yielding a pseudo-macroscopic yield strength of 532 MPa - over 100 MPa higher than the literature-conforming oxide-free reference. The homogenous co-deformation of the structure, with barrelling of the individual metal layers at the micropillar edges, results from the high bonding strength of the metal with its native oxide, meaning no failure or sliding at the interface, unlike previ-ous Al/ceramic multilayer studies. Only the thicker ( >= 5 nm) oxide layers fractured in-plane: at locations coincident with vertical Al-Al grain boundaries. An analysis of contributions to the strength of these crys-talline/amorphous metal/ceramic hybrid multilayers is carried out, identifying the Al-Al2O3 interface to be the crucial factor, rather than the in-plane tensile stiffness and considerable plasticity of ALD Al2O3 itself. The strengthening effect of the oxide layer was effective down to a layer thickness of just 0.5 nm. (c) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )