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 system of UFG aluminium with a narrow grain size and shape distribution, including two types of interfaces (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 layers 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 previous 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 crystalline/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.