Computer simulations were used to study the propagation of cracks coated with thin solid films in a 2D triangular Lennard-Jones solid. The key parameter investigated was the mismatch in the interatomic distances of the film and the substrate (the eleastic moduli were kept the same). The major result is that "Large Film Atoms," i.e. interatomic distance in the film larger than in the substrate, promote dislocation generation at the film-substrate interface while "Small Film Atoms" inhibit dislocation formation and promote brittle fracture in an otherwise ductile material. The critical applied load to cause fracture decreased significantly with increasing film thickness. These results are in qualitative agreement with some currently available experimental data. A model was developed which explains the simulation results in terms of the residual stresses resulting from the "atomic size" mismatch of film and substrate. The fact that cracks propagated brittly in the substrate after the film was ruptured is explained in terms of a model for dynamic embrittlement.
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