Molecular Dynamics Simulations For Deformation And Failure Mechanisms Of 3C-SiC Nanofilms

Due to the excellent mechanical properties such as high strength,high hardness and high wear resistance,Silicon carbide(SiC)materials have been widely used in many fields.The research on their mechanical properties has been a hot topic in the last decades.In order to achieve high-performance nanostructured SiC materials,their deformation and failure mechanisms should be analyzed.Molecular dynamics(MD)simulation,which is a powerful method to reveal the mechanical performance of nanostructured materials,has been receiving increasing attention.It can provide in-depth the formation and evolution of nanoscale defects and their interaction,and reveal the deformation and failure mechanisms of materials.In this dissertation,the deformation and failure mechanisms of zinc-blende structure 3C-SiC were studied with MD simulation.The main progress was achieved as follows:(1)The nanoindentations of 3C-SiC single crystal samples were simulated with MD,to study the plastic deformation mechanism and indentation anisotropy.The main results are as follows.The main plastic deformation mechanisms include nucleation,growth and motion of 1/2 <110> dislocations and the formation of prismatic loops.There are two mechanisms for the formation of prismatic loops: the "lasso"-like mechanism and extended "lasso"-like mechanism.The key point of both mechanisms is that the two screw segments of a dislocation,which have opposite direction,move towards each other by cross-slip,merge and annihilate.The plastic deformation mechanisms corresponding to the indentation on different crystal planes are similar,but the dislocation distributions are obviously different due to the difference between the rotational symmetries of different crystal planes,implying anisotropic mechanical properties.(2)The uniaxial compressive properties of bicrystalline SiC(BC SiC)samples as well as their interfacial properties were investigated.Compared with that of their counterpart single crystal,the elastic moduli and critical stresses of the BC SiC decrease,and the plastic deformation mechanism of the BC SiC samples with different inclination angles is nucleation of dislocation from grain boundary,propagation in grain and the interaction between dislocations,and between dislocation and grain boundary.(3)The responses of nanotwinned 3C-SiC under the uniaxial tension/compression and nanoindentation were simulated to study the effects of twin boundaries(TBs)and thickness.The simulation results show that brittle fracture is the main failure mode of twin 3C-SiC during tension;the nucleation and propagation of dislocations and the interactions between TBs and disloations are the main deformation mechanism during compression.During the indentation with a cylindrical indenter,plastic deformation is dominated by the interaction between dislocation and TBs,i.e.,TBs hinder the dislocations movement,and the TBs can also be the source of dislocation.During the indentation with a spherical indenter,the mechanism of hardening is also that the TBs block the propagation of dislocations,while the movement of the dislocations parallel with the TB and the nucleation of dislocations from TB result in softening.(4)The MD simulations for uniaxial tension/compression and nanoindentation of 3C-SiC single crystals with voids were performed to study the effects of void.The following results are obtained.During tension crack initiates from void surface.During compression dislocation nucleates from void surface;while during indentation dislocation loops nucleate beneath the indenter,propagate to void surface and are "engulfed" by it.If the porosity is constant,the smaller the void size,the larger the critical stress;if the void size is constant,the elastic modulus and critical stress of the material decrease with the increase of the porosity.During nanoindentation,the distance from void center to sample surface plays an important role in plastic deformation,and there exists an effective range,within which the hardness of the material decreases with the increase of the void radius.In this dissertation,the mechanical properties and deformation mechanisms of zinc-blende structure SiC materials were investigated and revealed at nanoscale,which could provide available information for the design and optimization of such kind of materials,as well as their performances.