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We modeled the degradation of a composite produced from an amine-cured epoxy resin and SiO2 nanoparticles with a silane coupling agent (SCA) under wet conditions using electron-level simulations. For the epoxy region, the amine group is protonated depending on its location under neutral or acidic conditions. Protonation substantially weakens the amine C–N bond. For SCA-connected HO–SiO2, terminal –OH of SiO2 is deprotonated under neutral or alkaline conditions, and the energetic barrier to break the Si–O bond is < 1 eV when 50% of the terminal –OH is deprotonated. These findings support the experimental data on pH-dependent degradation. [ABSTRACT FROM AUTHOR]

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO2) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO2 is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si3N4). Structural correlations in n-Si3N4 reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si3N4 is found to be six times larger than that of crystalline α-Si3N4. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si3N4 are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si3N4 nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m1/2, water molecules decompose to oxidize Si or break Si-Si bonds. [ABSTRACT FROM AUTHOR]