[en] Monocrystals of magnesium oxide and sapphire have been subjected to ion implantation with 86 keV Si^- ions to a dose of 5x10¹⁶ cm^-2 and with 3 MeV H⁺ ions to a dose of 4.8x10¹⁷ cm^-2 prior to thermal stress testing in plasma. Fracture and deformation characteristics of the surface layer were measured in ion implanted and unimplanted samples using optical and scanning electron microscopy. 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 thermal stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance is determined for sapphire crystals. In comparison with 86 keV^- Si ions the high energy implantation of sapphire and magnesium oxide crystals with 3 MeV H⁺ ions results in the formation of large-scale defects, which produce the low density crack system and cause a considerable reduction in the resistance to damage. Fracture mechanics principles are applied to evaluate the size of the implantation-induced microcracks which is shown to be comparable with the ion range and the damage range in the crystals tested. Possible mechanisms of crack nucleation for a low and high energy ion implantation are discussed