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

• Comparison of arc ignition and erosion of tungsten cathodes with ThO2 and La2O3
• Less electrical power required for cathodes with La2O3 to sustain the plasma
• Reduced degradation and thus longer lifetime of cathodes with La2O3 additions
• Quality of sprayed Al2O3 coatings can be maintained for a longer spraying period.

ThO2 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.% ThO2 and 1 wt.% La2O3 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 Al2O3 coatings sprayed on Mo substrates were evaluated. La2O3 is characterized by a similar plasma ignition and operation behavior as well as a comparable coating quality with respect to ThO2 additions. Further, La2O3 additions caused a reduced degradation of the cathode material, which is attributed to the lower cathode temperature, giving rise to an expected longer lifetime.