**2. Materials and Methods**

The PSZ specimens consisted of zirconia doped with 3 mol% yttria (3Y–PSZ, Tsukuba Ceramic Works) and were cut into 4.0 mm (length) × 10.0 mm (width) × 3.0 mm (depth) specimens. They were sintered at 1500 ◦C. The top and bottom surfaces were mirror polished. To remove the phase transition during the shape processing, the PSZ specimens were processed with a quench heat treatment at 900 ◦C and air cooled.

The specimens were irradiated with a 100 keV electron beam generated by a Cockcroft– Walton electron accelerator at Osaka Prefecture University (OPU). In order to avoid generating heat in a small area with the micro-focus beam, the electron beam was defocused to about 2 cmΦ. The area of irradiation was extended to 6 cm × 10 cm much more than the surface of the sample by the scanning coil. The required electron fluence was determined from the number of charges flowing to the ground collected in a Faraday cup and the flux was 5 <sup>×</sup> <sup>10</sup><sup>12</sup> cm2/s during the irradiation. Before and after irradiation, the specimens were subjected to X-ray diffraction (XRD), and the crystal structure was analyzed using the *θ*−2*θ* method. The XRD measurements were carried out using Cu Kα radiation.
