Large-scale molecular dynamics simulations of nano-structured materials

2018-10-16Nanostructured materials have exceptional mechanical properties and have a wide variety of applications ranging from manufacturing, aerospace to healthcare. However, the service life of these materials is often limited due to the formation of cracks and defects that leads to their eventual failure, thus insights into atomistic mechanisms underlying these processes are valuable inputs for the design of next-generation nanostructured materials. In this research work, large-scale molecular dynamics simulation, combined with machine learning and data mining techniques is used to understand three basic problems related to nanostructured materials design with superior mechanical properties, which are: (1) ultra-light material design using hierarchical structures like kagomé lattices, (2) self-healing of cracks in alumina composite embedded with SiC/SiO₂ nanoparticles, and (3) toughing of MoWSe₂ monolayer during crack propagation by novel strain-induced structural phase transformation. ❧ Many natural and man-made materials exhibit hierarchical structures like for example bird wings, Eiffel tower, Garabit Viaduct. This hierarchy in structures can be used for materials design at the atomic level as it allows us to tune several materials properties like stiffness, compressive strength, fracture toughness by just changing the underlying structural arrangement of materials. In the first work, we have studied the deformation behavior of an ultralight architecture consisting of hollow Ni nanotubes and solid nanorods arranged as a 3-D kagomé structure. Our result shows that hollow kagomé structure have higher strength and stiffness in comparison of solid kagomé structures. The hollow kagomé structure shows two stage deformation mechanism where up to 11% strain, deformation is highly localized near the nodal region and at higher strain they show bending-dominated deformation. Whereas in solid kagomé structures, large defamations are observed in the entire structure even at lower strain. ❧ Designing material systems that can sense and repair damage is of great interest. Inspired by biological systems, material scientists are developing self-healing materials in which damage initiation triggers an automatic release and transport of healing agent to the damage site and repairs it. In the second work, we have investigated crack healing in a ceramic nanocomposite of alumina containing SiC/SiO₂ nanoparticles. Our result shows that the nanoparticles closest to the advancing crack in alumina distort to create nanochannels through which silica flows toward the crack and heals the damage. Under applied strain, alumina matrix also forms facets at interfaces. These facets act as nucleation sites for multiple secondary amorphous grains in alumina matrix. Nanovoids form along the grain boundaries but these nanovoids are again healed by molten silica released from the nanoparticles. The mechanisms of crack healing in alumina will help experimentalists to design novel self-healing ceramics for a broad class of energy technologies. ❧ Two-dimensional materials exhibit different mechanical property in comparison of their bulk counterpart owing to their monolayer atomic thickness. In fact, properties of these layered materials can be tuned by mechanical deformation. In the third work, we have examined the atomistic mechanisms of defect formation and crack propagation in a strained monolayer MoWSe₂ alloy using molecular dynamics. Our result shows that under strain, the crack meanders as it propagates through the system and branches into daughter cracks when it crosses the MoSe₂/WSe₂ interface and enters the WSe₂ patches. The most dramatic change occurs in the process zone around the crack tip, where large stress concentration and the resulting biaxial tensile strain trigger an irreversible local structural phase transformation from the ground state 2H crystal structure to the 1T crystal structure. This phenomenon of crack branching, crack closure and strain induced phase transformation in MoWSe₂ alloy increases its fracture toughness