The effect of argon ion implantation on the nano-machanical properties of single crystal Si was examined making use of nano-indentation and nano-scratch tests. The morphologies of the scratched tracks of the unimplanted Si and that implanted at a moderate Ar+ fluence were observed on a scanning electron microscope, while the changes in the microstructure of the single crystal Si by Ar+ implantation were investigated on a transmittance electron microscope. It was found that the implantation of Si at a small or moderate fluence of Ar+ below I X 10(15) ions/cm(2) had little effect on the surface roughness and a minor effect on the surface nano-hardness. At the same time, the implantation of Si with Ar+ at a moderate fluence up to I X 10(16) ions/cm(2) led to a significant increase in the critical load. This was attributed to the desired changes in the microstructures of the single crystal Si by Ar+ implantation at a proper fluence. Namely, the Si surface implanted with Ar+ at a moderate fluence was composed of nanosized polycrystalline Si uniformly distributed in amorphous Si matrix, which contributed to significantly increase the nano-scratch resistance and surface toughness of the single crystal silicon. It was suggested to implant the single crystal Si at an Ar+ fluence of I x 10(16) ions/cm(2) so as to acquire the optimized modification effect. (c) 2004 Elsevier B.V. All rights reserved.