Microstructure of hardened and softened zirconia after xenon implantation

Abstract

Ion-channeling and transmission electron microscopy (TEM) techniques were used to examine the microstructure of single-crystal Y₂O₃ stabilized cubic zirconia (YSZ) after implantation with 240 keV Xe⁺ ions. The observed microstructure was related to Knoop indentation hardness measurements. These measurements showed an increase in hardness for low ion-doses, reaching some maximum value, then a decrease in hardness at higher doses. In the hardening regime, below 7.5 × 10¹⁵ Xe⁺/cm², point defects and dislocation networks were observed by TEM. Ion-channeling showed a corresponding increase in damage as a function of ion-dose. For doses between 7.5 × 10¹⁵ and 3 × 10¹⁶ Xe⁺/cm² the hardness falls, and the amount of damage, measured with ion-channeling, reaches a limiting value at less than complete damage. In this dose range the Xe concentration continues to increase beyond the dose where the amount of damage saturates. For high doses, greater than 3 × 10¹⁶ Xe⁺/cm², where softening of the zirconia occurs, additional reflections appear in the electron diffraction pattern that are consistent with the lattice parameter of solid Xe. A diffuse ring is also visible; this is believed to be due to the presence of fluid Xe. Both ion-channeling and TEM show that a significant amount of monocrystalline zirconia remains even up to doses of 1 × 10¹⁷ Xe⁺/cm². There is also evidence for the presence of recrystallized zirconia at the high doses. Since so much crystalline material remains, it seems that amorphization of the zirconia is not the dominant cause of the softening at high doses.