Correlation of laser-reflection and thermionic emission of thermally loaded coatings under UHV conditions

Abstract

In (ultra) high vacuum conditions the evaporation of materials at high temperatures is an important issue, which may significantly reduce the lifetime of thin coatings. To analyze the behavior of film evaporation at high temperatures, the surface evolution has been monitored in-situ using thermionic emission and a laser reflection setup. The temperature during the investigations was regulated by a pyrometer on a designated spot via direct resistive sample/substrate heating. The identified correlation between these two signals showed the capabilities of the measurement system and technique to develop new materials for high temperature applications, such as thermal barrier or thermionic emission coatings. Within this study different oxide coatings have been analyzed, based on reactive magnetron sputtered ZrO2 and Y2O3 films on tungsten substrates. Depending on the thermal stability of the respective materials, the coatings of various thickness were tested at temperatures in the range of 1200-1800°C while keeping the total pressure below 10-5 Pa. Even though the thermionic emission of the oxide coatings provides information regarding the coating breakdown, the reflection signal is more decisive. The reflection signal shows a strong dependence on the thickness of the “transparent” oxide coatings, allowing to obtain close information on the film evolution. For instance, the evaporation rate of ZrO2 at 1700 °C was determined to be appr. 10 nm/h for pressures below 10-5 Pa. The combination of the reflection signal and thermionic emission on the other hand allows an observation of the chemical stability of the film. The investigated oxide coatings thereby maintain their chemical composition throughout the high temperature process and eventually evaporate completely.