COMPARISON OF IMPLANTATION WITH NI2+ AND AU2+ IONS ON THE INDENTATIONRESPONSE OF SAPPHIRE

The mechanical behavior of sapphire modified by bombardment with energ etic Ni2+ and Au2+ ions (E = 3 MeV, fluence of 2 x 10(16) cm(-2)) was examined by means of ultra micro-indentation in the (10 (1) over bar 0 ) plane of virgin and ion-treated crystal. The variation of hardness a nd Young's modulus with depth of penetration of indenters with pointed or spherical tipped was determined. The spherical tip indenter reveal ed a dramatic difference in the response of the virgin (overload drive n 'pop-in' of the plastic deformation) and implanted (smooth transitio n from the elastic to plastic response) sapphire. Microscopic observat ions of the indented surface revealed slip/twin traces close to the im pressions in the unimplanted material, but not in ion-modified crystal s. The results were rationalized in terms of the ease of plastic defor mation or twinning as influenced by irradiation. It was concluded that the paucity of defects in virgin sapphire leads to the overload condi tion 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 dis location pile-ups are responsible for the significant hardening observ ed, while the Au-implanted material suffered severe lattice damage res ulting in a significant reduction of modulus and contact pressure to i nitiate plastic deformation at 2.25 GPa. The results point towards the appropriateness of indenters with very small spherical tips as the me ans to characterize the mechanical properties of ion-beam modified sur faces. (C) 1998 Elsevier Science S.A. All rights reserved.