Crack propagation and fracture in ceramic films—million atom molecular dynamics simulations on parallel computers

Abstract

A highly efficient space-time multiresolution algorithm has been developed to carry out large-scale molecular dynamics (MD) simulations for systems with long-range Coulomb and three-body covalent interactions. The algorithm combines the fast multipole and reduced cell multipole methods, and multiple time-step approach. The multiresolution algorithm has been implemented on the 512 node Touchstone Delta at Caltech and 128 node IBM SP system at Argonne National Laboratory. The multiresolution MD approach has been used to investigate the structural properties and mechanical failure in microporous silica. Structural correlations are characterized by the fractal dimension, internal surface area, and pore surface-to-volume ratio. Critical behavior at fracture is analyzed in terms of pore percolation, and kinetic roughening of the fractured surface is also investigated. The roughness exponent for fracture surfaces, α = 0.87 .02, supports experimental claims about the universality of α. Enhancement of the vibrational density-of-states due to low-energy floppy modes in crystalline and glassy states of the high-temperature ceramic Si 3N 4 is investigated using the MD method. In the glass, floppy modes appear continuously as the connectivity of the system is reduced, but for the crystal floppy modes appear suddenly at 20% volume expansion. The density-of-states due to the floppy modes is linear in energy and gives rise to a large enhancement of the specific heat. Crack propagation in amorphous silicon nitride films is investigated, and a correlation between the speed of crack propagation and the morphology of fracture surfaces is observed. Initially, as the crack propagates quasi-statically, the roughness exponent for fracture surfaces is found to be 0.44. However, beyond a certain speed of crack propagation, the exponent crosses over to 0.8. This cross-over behavior resolves the experimental controversy regarding the value of the roughness exponent,