Microstructure of hardened and softened zirconia after xenon implantation
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William Hertl Corning, Inc., Corning, New York 14831
C. Barry Carter and James W. Mayer Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 (Received 27 February 1991; accepted 29 April 1991)
Ion-channeling and transmission electron microscopy (TEM) techniques were used to examine the microstructure of single-crystal Y 2 O 3 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 x 1015 Xe + /cm 2 , 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 x 1015 and 3 x 1016 Xe + /cm 2 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 x 1016 Xe + /cm 2 , 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 x 1017 Xe + /cm 2 . 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.
I. INTRODUCTION High dose ion-implantation can amorphize some ceramics, thereby causing a reduction in hardness. Lower dose implants or annealing of higher dose implants often results in a modified but crystalline microstructure characterized by high residual compressive stresses and increased hardness.1 In other cases, ion beams may actually cause recrystallization instead of amorphization.2'3 Naguib and Kelly3 give two criteria for determining if an ion beam will recrystallize or amorphize a material. One criterion is that if the ratio Tc/Tm, the crystallization temperature divided by the melting temperature of the implanted material, is greater than 0.30, the material is likely to amorphize. The initial impact of heavy ions sends the bombarded matrix atoms into motion, creating a thermal spike equivalent to a small liquid region surrounded by crystal. If Tc is low, the region has time to reorder before dropping below that temperature,
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'Present address: Materials Research Society, 9800 McKnight Road, Pittsburgh, Pennsylvania 15237. J. Mater. Res., Vol. 6, No. 9, Sep 1991 http://journals.cambridge.org
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resulting in recrystallization. For materials requiring a high temperature
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