Revealing the pressure-induced softening/weakening mechanism in representative covalent materials

Diamond, cubic boron nitride (c-BN), silicon (Si), and germanium (Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated. It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond (c-BN), and weakening in Si (Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.Web of Science385art. no. 05610

Structural Nanocrystalline Materials: Fundamentals And Applications

Nanocrystalline materials exhibit exceptional mechanical properties, representing an exciting new class of structural materials for technological applications. The advancement of this important field depends on the development of new fabrication methods, and an appreciation of the underlying nano-scale and interface effects. This authored book addresses these essential issues, presenting for the first time a fundamental, coherent and current account at the theoretical and practical level of nanocrystalline and nanocomposite bulk materials and coatings. The subject is approached systematically, covering processing methods, key structural and mechanical properties, and a wealth of applications. This is a valuable resource for graduate students studying nanomaterials science and nanotechnologies, as well as researchers and practitioners in materials science and engineering

Designing ultrahard nanostructured diamond through internal defects and interface engineering at different length scales

Nanocrystalline diamonds (NCDs) are promising structural materials due to their extraordinary mechanical properties such as ultrahigh hardness and excellent toughness, however, a rational design rule in plasticity and fracture through controlling nanostructures at different length scales is far from being explored. By means of atomic simulations and plasticity theory in the present paper, we comprehensively explored the plastic deformation behaviors of a series of well-defined NCDs by varying amorphous interfacial layers (AILs) and internal defects, e.g., twin boundary, stacking fault, p-bonded interface, and fivefold twin. It was observed that the effect of internal defects on the mechanical response of NCD can be attributed to the competition between dislocation blocking process and interface sliding process. The introduction of AIL at grain boundary (GB) is found to provide an effective solution to decrease both dislocation nucleation and penetration at GBs. These findings provide not only a mechanistic insight into the unique strengthening and toughening in various NCDs, but a rational guidance in designing novel superhard carbon materials with superior performance by engineering internal defects and GB structures at different length scales.Web of Science17040239

Ultrastrong pi-bonded interface as ductile plastic flow channel in nanostructured diamond

A combinational effect of nanostructured crystallites and it-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 pi-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 pi-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 sp(2) bonds across the pi-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.Web of Science1234142413