Understanding the deformation behaviour of nanostructured surfaces has become essential for understanding the mechanical properties of new, advanced materials. Application of a device can only be successful if its mechanical properties are well-known. Surface mechanical properties are expected to control the mechanical response of nanostructures. To study those properties, nanoindentation has proved to be a valuable technique [1]. In earlier work we have shown that nanoindentation, carried out with STM, can be used to elucidate the mechanisms taking place at the initial stages of plastic deformation of metals [2]. In the present work we examine the mechanical properties by using the atomic force microscope (AFM). The main goal of the use of the AFM is that, together with the possibility of imaging the surface before and after the test, the tip is used as a very-well controlled nanoindenter where load-depth curves can be obtained.
We have carried out a comparative analysis of load–depth curves and defects imaged thereafter for three sets of gold surfaces: (a) 'As-cleaned' flat single crystal surfaces, both with a (100) and a (111) orientation (b) The same surfaces after controlled ion irradiation with high doses, resulting in well ordered nanostructures on defect-rich surface and sub-surface regions and (c) vicinal surfaces (788) slightly miss-oriented with respect to the (111) flat surface. This latter surface is rich in steps but is short of surface and sub-surface radiation damage. We report that the experimental values of the surface stiffness and hardness show significant variations from the conditions (a) to (c). From image analysis and atomistic simulations we obtain additional information about the different mechanical mechanisms involved in each case. We explain our results in terms of a model in which the surface mechanical properties result from a balance between the effect of surface steps and the hardening arising from the radiation damage.
[1] J. D. Kiely and J. E. Houston, Phys. Rev. B 57 (1998) 12588; J. Li et al., Nature 418 (2002) 307.
[2] E. Carrasco et al., Phys. Rev. B, 68 (2003) 8102(R); A. Asenjo et al., Phys. Rev. B, 73 (2006) 75431.
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