Simultaneous stiffening and strengthening of nanodiamond by fivefold twins

Cyclic fivefold twinning in metallic systems was shown to effectively improve the mechanical strength and stiffness, but this remains unknown yet for strong covalent diamond. Here, tensile properties of fivefold twinned nanodiamonds (5FTNDs) are investigated for the first time via classic molecular dynamic simulations. It is predicted that mechanical properties of the nanodiamonds are greatly dominated by fivefold twins and cross-sectional geometry. 5FTNDs with star-shaped cross section exhibit superior tensile Young’s modulus and strength over their intrinsically strong bulk counterpart; however, pentagon-shaped 5FTNDs are tensile softer and weaker than their bulk counterpart. Moreover, with reduction in the axial radial size, tensile Young’s modulus and strength of star-shaped 5FTNDs are nonlinearly enhanced, but are mechanically degraded for pentagon-shaped 5FTNDs. Such geometry-induced conflict tensile properties are explained by the contour maps of residual stresses, stiffnesses, and strengths over the transverse cross section. The discovery provides a step forward in the search for superstrong/stiff covalent structures. Impact statement Twinning boundaries (TBs) have been regarded as an effective way in improving the mechanical properties of metallic materials because energy-favorable TBs suppress the nucleation and motion events of dislocations. Recently, it was revealed that high-density TBs produce unprecedented mechanical hardness of covelent-bonded diamond up to 200 GPa. To date, it is still pending as to if fivefold TBs are able to strengthen and stiffen intrinsically strong diamond, although fivefold TBs have been shown to be a more effective way in enhancing the mechanical properties of metallic systems than lamellar TBs. The present work uses classic molecular dynamics simulations to show that star-shaped nanodiamond with fivefold TBs are mechanically stiffer and stronger than their bulk counterpart, and the enhancement in mechanical properties becomes more pronounced with decreasing the dimension of cross section. The work provides a new way in the design of mechanically stronger covalent diamond. Graphic abstract