Morphology- and dehydrogenation-controlled mechanical properties in diamond nanothreads

Recently synthesized diamond nanothreads (DNTs), collecting desired properties of both inorganic nanostructures and hydrocarbon molecular structures, are an interesting group of carbon-based materials. Using full atomistic first-principles based ReaxFF molecular dynamics (MD) simulations, a comprehensive study on tensile and bending mechanical characteristics of fifteen energy-favorable DNTs is performed. All the DNTs show unique tensile and bending mechanical properties that markedly vary with morphology and arrangement of carbon polygons. A straight DNT composed of purely carbon hexagons shows brittle fracture in the temperature range of 1-2000 K, whereas with regard to another hexagon-dominated DNT and helically coiled DNT with the largest coiled radius, a thermal-induced brittle-to-ductile transition is uncovered at 2000 K. Particularly, the coiled DNT subjected to tensile loading/unloading shows a clear mechanical hysteresis loop. Dehydrogenation does not change the morphologies and stability of DNTs, but significantly affect the tensile mechanical responses; the tensile stiffness, toughness and ductility can be enhanced by approximately 1-fold, 2-folds and 3-folds as much of their pristine counterparts, respectively, however, the failure strain is reduced at any degree of dehydrogenation. Similarly, bending stiffness also closely connects with dehydrogenation. A transition of bending stiffness in two specific dehydrogenation-free DNTs occurring at critical curvatures is detected as a consequence of local bond transformations. Moreover, bending stiffness in different bending directions can differ by around 8-folds, originating from the distinct surface morphologies. The findings provide a critical knowledge of mechanical properties of DNTs for practical applications. (C) 2017 Elsevier Ltd. All rights reserved.