Nondestructive thickness measurement of silica scale using cathodoluminescence

The knowledge of the composition, morphology, and thickness of surface protective scales, such as SiO₂ and Al₂O₃, is important to control the performance of heat-resistant alloys operated at high temperatures above 1000 °C. Cathodoluminescence (CL) analysis is one of the most promising methods to acquire such information nondestructively. Unlike Al₂O₃ scale, the use of the brightness of CL image to determine SiO₂ scale thickness is difficult because the brightness and color of its acquired CL image depend not only on the thickness of the scale but also on its oxygen potential and impurity concentration. In this study, we investigated the CL spectra of SiO₂ scales formed on Fe-20%Si alloy, Si, and MoSi₂ to present a method for measuring the thickness of SiO₂ scale on silica-forming materials from their CL spectra. A linear calibration curve was obtained from the intensity of the CL peak at 445 nm which originated from the intrinsic defects in SiO₂ and the thickness of the SiO₂ scale for each heat-treatment temperature (1000 °C, 1200 °C, 1300 °C, and 1400 °C). Data in this study can be adequately employed to derive CL calibration curves at different temperatures because the CL intensity and heat-treatment temperature were fitted according to Arrhenius relationship at different SiO₂ scale thickness, regardless of the type of base materials. These results indicated that the thickness of the SiO₂ scale formed on silica-forming alloys can be determined by acquiring their CL spectra. Therefore, CL analysis has the potential to be employed as a novel analytical method to control the performance of silica-forming alloys.