Behaviour of α-AI₂O₃ insulator surfaces under electron irradiation

Abstract

Cathodoluminescence (CL), Auger electron spectroscopy (AES) and direct crack measurements were performed onα-Al₂O₃ samples in order to relate chemical, electrical and mechanical effects induced by electron irradiation of the surface. Electrical discharges and visible luminescence were observed during the excitation of the samples with a 80 keV electron beam. Changes of the surface state and of the toughnessK _Ic were subsequently detected. The results suggest that charging of the sample is related to the presence of defects and corresponding trap levels in the energy gap. The concentration of defects (oxygen vacancies or associated F, F⁺ centres) may be enhanced, especially in the vicinity of the surface, by the electrical discharges induced by the electron irradiation. This leads to an increase of mechanical stresses in the brittle material: a striking example was the fracture of a corundum single crystal in the electron microscope, which cannot be explained by the direct heating effect of the primary electron beam. On the contrary, an advantageous situation for the mechanical properties of the material may be achieved when the defects have a blocking effect on the crack propagation; a subsequent increase of the toughnessK _Ic is then recorded.