Radiation effects and gas cavities in pyrolytic graphite implanted with helium ions

Changes in the microstructure of edge oriented (EO) and basal oriented (BO) pyrolytic graphite (PG), implanted with 40 keV and 3.5 MeV helium ions, respectively, (in both cases at 300 and 770 K) have been investigated by transmission electron microscopy. In the as-received specimens lenticular cavities and other imperfections were found. Ion irradiation at 300 K significantly modifies the microstructure, mainly due to submicroscopic defect accumulation, giving rise to radiation-induced swelling, curing of original cavities and strengthening of the matrix. With increasing dose pressurized helium bubbles of lenticular and ellipsoidal shape appear in EO specimens while disk-shaped helium filled cavities are observed in BO PG. In BO PG flaking occurs at high doses, preceded by a latent exfoliation which originates along the interface between the radiation-strengthened, helium-rich layer and the virgin matrix. At 770 K blistering occurs in BO PG while the microstructural modifications are small compared to the effects at 300 K. Finally, experimental evidence is presented which shows that ion irradiation even at room temperature up to doses of 10 dpa does not cause amorphization in graphite.