A combinational effect of nanostructured crystallites and π-bonded interfaces is much attractive in solving the conflict between strength/hardness and toughness to design extrinsically superhard materials with enhanced fracture toughness and/or other properties such as tunable electronic properties. In the present work, taking the experimentally observed π-bonded interfaces in nanostructured diamond as the prototype, we theoretically investigated their stabilities, electronic structures, and mechanical strengths with special consideration of the size effect of nanocrystallites or nanolayers. It is unprecedentedly found that the π-bonded interfaces exhibit tunable electronic semiconducting properties, superior fracture toughness, and anomalously large creep-like plasticity at the cost of minor losses in strength/hardness; such unique combination is uncovered to be attributed to the ductile bridging effect of the sp2 bonds across the π-bonded interface that dominates the localized plastic flow channel. As the length scale of nanocrystallites/nanolayers is lower than a critical value, however, the first failure occurring inside nanocrystallites/nanolayers features softening and embrittling. These findings not only provide a novel insight into the unique strengthening and toughening origin observed in ultrahard nanostructured diamonds consisting of nanotwins, nanocomposites, and nanocrystallites but also highlight a unique pathway by combining the nanostructured crystallites and the strongly bonded interface to design the novel superhard materials with superior toughness.

Center for Integrated Computational Materials Engineering Beihang University Beijing 100191 P R China

Department of Chemistry Technical University Munich Lichtenbergstrasse 4 D 85747 Garching Germany

Department of Vascular Intervention Tenth People's Hospital of Tongji University Shanghai 200072 China

IT4Innovations VSB Technical University of Ostrava 17 listopadu 2172 15 708 00 Ostrava Czech Republic

Nanotechnology Centre VSB Technical University of Ostrava 17 listopadu 2172 15 708 00 Ostrava Czech Republic

National United Engineering Laboratory for Biomedical Material Modification Dezhou Shandong 251100 P R China

School of Materials Science and Engineering Beihang University Beijing 100191 P R China

Theoretical Division Los Alamos National Laboratory Los Alamos New Mexico 87545 United States

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