Thermal stress resistance of ion implanted sapphire crystals

Monocrystals of sapphire have been subjected to ion implantation with 86 keV Si^- and 80 keV Cr^- ions to doses in the range of 5×10¹⁴-5×10¹⁶ cm^-2 prior to thermal stress testing in a pulsed plasma. Above a certain critical dose ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The effect of ion dose on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is shown to be much lower than that for Cr^- ions. However, for doses exceeding 2×10¹⁶ cm^-2 the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density of the implantation-induced defects are considered to be the major factors determining the stress resistance of sapphire crystals irradiated with Si^- and Cr^- ions.