Mechanical properties of thin films are to a great extend measured by indentation techniques. By scaling down both the indenter size and the grain size, size effects are evoked originating from the nature of the nc-structure below the indenter and from the ratio between the indenter size and the grain size.

Large-scale molecular dynamics simulations are employed to investigate the deformation properties of a thin nanocrystalline (nc)-Au film. Two deformation mechanisms are observed in the nc-structure: (1) dislocations emitted under the indenter and absorbed in neighbor grain boundaries and (2) grain boundary sliding.
In earlier work [1] it was observed that GBs act as sinks for dislocation nucleated under the indenter. In the present work it is shown that the sink efficiency of the grain boundaries is highly dependent on the stress distribution in the GBs and that dislocations can be as well absorbed as be repelled by the GBs.
By changing the size of the indenter it is shown that the characteristic features of the indentation curve are dependent on the ration between the indenter and the grain size by means of the number of grain that are below the indenter. Other size effects on as well the elastic as the plastic behavior resulting from the curvature of the indenter are also discussed.

[1] D. Feichtinger, P. M. Derlet, and H. Van Swygenhoven, Phys. Rev. B 67, 024113 (2003).

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