Substitution of ThO2 by La2O3 additions in tungsten electrodes for atmospheric plasma spraying

Abstract ThO 2 additions are commonly used in tungsten-based electrodes for plasma spraying due to the excellent electron emissivity, improved arcing behavior, higher strength, and better machinability. Because of their radioactive potential, which makes handling, use, recycling, and disposal more difficult, alternative additives are required that provide the same advantages as thoriated tungsten, but without environmental hazards. Within this work, tungsten cathodes with 2 wt.% ThO 2 and 1 wt.% La 2 O 3 were compared with respect to their arc ignition behavior, plasma stability and arc erosion. Both, cyclic and continuous plasma spraying experiments were carried out. In addition, structure and mechanical properties of Al 2 O 3 coatings sprayed on Mo substrates were evaluated. La 2 O 3 is characterized by a similar plasma ignition and operation behavior as well as a comparable coating quality with respect to ThO 2 additions. Further, La 2 O 3 additions caused a reduced degradation of the cathode material, which is attributed to the lower cathode temperature, giving rise to an expected longer lifetime.