In-situ Measurement of the Thickness of Aluminum Oxide Scales at High Temperature

Two ways for measuring thickness of aluminum oxide which forms upon the surface of a heating element metal Fecralloy at high temperature over 1000°C are presented. One is LRI (Laser Reflection Interferometer). That is, a He--Ne laser incident to the specimen reflects on the top surface of the aluminum oxide and also on the interface with the substrate. Then these two reflections interfere to yield undulation of the intensity as the oxide grows. As the oxide gets thicker, however, the surface gets rougher and transparency decreases, which results in substantial scatter and low intensity in reflection, respectively. To overcome this problem, a spatial filter was employed on the light path. The other way is EDM (Emissivity Difference Method) which applies the phenomenon that the thermal emissivity of the surface depends upon the oxide thickness and the wavelength of the radiation. The oxide thickness is evaluated from the: difference in temperature indications measured by two infra-red thermometers which respond to different wavelengths, λ = 8--15 μm and λ = 1.55 μm, respectively. The validity of LRI and EDM has been proved by experiments, and some empirical relations are presented. The features and limitations are also discussed.