Comparison of implantation with Ni2+ and Au2+ ions on the indentation response of sapphire

The mechanical behavior of sapphire modified by bombardment with energetic Ni2+ and Au2+ ions (E=3 MeV, fluence of 2×1016 cm−2) was examined by means of ultra micro-indentation in the 101̄0 plane of virgin and ion-treated crystal. The variation of hardness and Young’s modulus with depth of penetration of indenters with pointed or spherical tipped was determined. The spherical tip indenter revealed a dramatic difference in the response of the virgin (overload driven ‘pop-in’ of the plastic deformation) and implanted (smooth transition from the elastic to plastic response) sapphire. Microscopic observations of the indented surface revealed slip/twin traces close to the impressions in the unimplanted material, but not in ion-modified crystals. The results were rationalized in terms of the ease of plastic deformation or twinning as influenced by irradiation. It was concluded that the paucity of defects in virgin sapphire leads to the overload condition for the nucleation of plastic deformation at stresses approaching the theoretical strength of 15–20 GPa. Extensive plastic deformation sets in at a stress of 6.25 GPa in Ni-implanted crystal, and large dislocation pile-ups are responsible for the significant hardening observed, while the Au-implanted material suffered severe lattice damage resulting in a significant reduction of modulus and contact pressure to initiate plastic deformation at 2.25 GPa. The results point towards the appropriateness of indenters with very small spherical tips as the means to characterize the mechanical properties of ion-beam modified surfaces.