Elastic Properties of Diamond-Like Amorphous Carbon Films Grown by Computer Simulation of Ion-Beam Deposition Process
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Elastic Properties of Diamond-Like Amorphous Carbon Films Grown by Computer Simulation of Ion-Beam Deposition Process A.Yu. Belov1 and H.U. Jäger Forschungszentrum Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Postfach 510119, 01314 Dresden, Germany, e-mail: [email protected] 1 Permanent address: Institute of Crystallography RAS, Moscow, Russia ABSTRACT Atomic-scale calculations were performed for the first time to investigate mechanical properties of amorphous carbon films grown by a realistic simulation of ion-beam deposition. The simulated films have a thickness of a few nanometers and reproduce the main structural features of real films, with the bulk content of sp3 bonded atoms varying from 35 to 95%, depending on the ion energy (E = 20-80 eV). Employing empirical interatomic potentials for carbon, the average bulk stresses as well as the atomic-level stress distributions were calculated and analysed. The bulk stresses were found to depend not only on the ion energy, but also on the film quality, in particular, on such structural inhomogeneities as local fluctuations of the sp3 fraction with the depth. The local variation of the bulk stress from the average value considerably increases as the local content of sp2 bonded atoms increases. Elastic constants of amorphous carbon films were also computed using the method of inner elastic constants, which allows for the stress dependence of elastic constants to be analysed. The variation of Young’s modulus as a function of the lateral bulk stress in an amorphous film is demonstrated.
INTRODUCTION The aim of this paper is to understand the correlation between the structural disorder and the average bulk stress in growing amorphous carbon films. The penetration of C+ ions beneath the exposed surface of a diamond substrate during ion-beam deposition induces lateral compressive stresses σ, promoting the tetrahedral, or diamond-like, amorphous carbon (ta-C) formation. The internal stress is not confined to the transitional layer between the growing amorphous film and the crystalline substrate. It also accompanies the subsequent growth of the ta-C film, increasing its elastic energy and thereby influencing its mechanical stability. In particular, this internal stress can both prevent the surface crack growth into the bulk, which is a valuable property for applications of ta-C films to hard coatings, and cause an instability of the system film-substrate, resulting in spontaneous film lift-off. An experimental estimate for σ gives about 10 GPa [1] for a {001}Si substrate and 88% of the sp3 fraction, which is a measure of the structural disorder in ta-C. In contrast to coherent heteroepitaxial systems, there is no apparent structural parameter, characterizing a 'mismatch' between a crystalline substrate and an amorphous film and describing the internal stress in this pseudoepitaxial system. Our atomistic simulations showed that ta-C films separated from a {111} diamond substrate can expand up to 3% to relieve the lateral stresses. In heteroepit
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