| Due to its unique structure,two-dimensional(2D)carbon materials exhibit a variety of physical and chemical properties that are different from their three-dimensional counterparts,such as excellent mechanical,electrical,optical and thermal properties.The properties of 2D carbon materials are closely related to their atomic structure(such as bonding types and layer numbers),as a result,it is of great significance to reveal the mechanical properties and failure mechanisms of 2D materials with different structures.However,the current researches rarely focus on the subject.To change that,two typical 2D carbon materials(graphene(GR)which is bonded by sp~2 atoms and penta-graphene(PGR)which is bonded by the mixture of sp~2 and sp~3atoms)are studied by molecular dynamics method.In this paper,the mechanical responses of them under the spherical indenter are compared,and the difference of intrinsic mechanical properties and failure mechanisms have been also discussed in detail.The main contents are as follows:(1)Monolayer 2D materials.The mechanical responses under nanoindentation and intrinsic mechanical properties of monolayer GR and PGR have been fully discussed in this paper.Elastic modulus and breaking strength are calculated to be 278.14 N/m and 29.23 N/m for PGR,which are slightly less than the values of graphene,the latter has a high elastic modulus of 313.40N/m and breaking strength of 30.90 N/m.Load-displacement curve demonstrates that the PGR has a larger critical failure depth and higher critical load than GR before the fracture.On the one hand,the single bonds(for sp~3 hybridized type)are easy to rotate during the deformation,which results in a better deformation capacity of the PGR by continuously adjusting the bond angle and bond direction,and maintains its intactness.On the other hand,figures of the atomic structures show that the sp~2 and sp~3 hybridization types have greatly change the failure mechanisms of two kinds of materials.The sp~2 bonded GR generates a central crack under critical load and gradually expands to form the main crack,whereas the failure process of PGR with two bonding types starts from the first breaking of sp~3 bond,and the nanoscale cracks are generated dispersely and gradually converge into the primary crack.During the process,the recombination of the breaking sp~3 bonds are observed,which is the fundamental reason why PGR has a superior failure resistance compared to GR.(2)Multilayer 2D materials.The mechanical properties of PGR ranging from 2 to 10 layers are simulated and compared with the results of multilayer graphene reported in the literature.The result shows that GR and PGR possess distinct dependence relationships with the layer number,because the slippage in GR will aggravate the uneven distribution in each layer.The simulation results of multilayer PGR show that the maximum deformation for each layer continuously increase,and the stress concentration is dispersed by interlayer interaction.The fracture strength of the PGR finally reaches a peak value at the ninth layer(70.48N/m),which is 2.41 times that of the monolayer PGR.At the same time,its elastic modulus also increases with a maximum increase of 22.1%.The analysis of the atomic structure of the middle layer and the first layer indicates that the multilayer PGR has a combining failure mechanism of the single atomic chain(single layer GR)and the recombination between sp~3 broken bonds(single layer PGR).Most importantly,the movement of the single atomic chain promotes the rebonding between sp~3 broken bonds after the secondary fracture.Besides,the larger the indenter radius,the less stress concentration,and the higher the critical failure stress and critical failure depth of the films;the elastic modulus of multilayer PGR is greatly reduced as temperature rises,but it has no significant effect on fracture strength;the study of the stacking arrangement shows that the multilayer PGR with AA stacking pattern has higher elastic modulus and fracture strength compared to AB stacking pattern. |
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