Effect of low temperature ion irradiation on the microstructure of nitride ceramics

Cross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si₃N₄) and aluminum nitride (AlN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×10²⁰ ions/m² (maximum damage of approx. 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si₃N₄ in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si₃N₄ specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si₃N₄ amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as approx. 0.6 dpa. The volumetric swelling associated with the amorphization of Si₃N₄ is <10%. Amorphization was not observed in any of the irradiated AlN specimens. A moderate density of small (approx. 3 nm) defect clusters were observed in the crystalline damaged regions of both the Si₃N₄ and AlN specimens at both irradiation temperatures. Aligned, network dislocations were also observed in the AlN specimen irradiated to high dose at 80 K.