TY - THES

T1 - Substitution of Thoria Additions by Lanthanum-Oxide Doping in Electrodes for Atmospheric Plasma Spraying

AU - Heißl, Martin

N1 - embargoed until 17-11-2017

PY - 2013

Y1 - 2013

N2 - Thoria (ThO2) additions are used in tungsten-based electrodes for plasma spraying due to the excellent electron emissivity, improved arcing behaviour, higher strength and better machinability. Because of their radioactive potential, which makes handling, use, recycling and disposal more difficult, research is done on alternative additives that provide the same advantages as thoriated tungsten but without environmental hazards. One of the most competitive alternatives to replace thoria is lanthania (La2O3). Within this thesis, tungsten cathodes with 2 wt.% thoria (WT20) and 1 wt.% lanthania (WL10) were compared. Tests in terms of arc ignition, plasma stability and arc erosion, both cyclic and continuous plasma spraying experiments were carried out. In addition, the structure both of the cathode tips and the Al2O3 coatings sprayed on Mo substrates were evaluated in metallographic cross-sections. In cyclical as well as continuous spraying experiments, the dropping rate of the arc voltage is lower for WL10-electrodes; thus a longer service life can be expected. This is also evident from the observation of the erosion rate. Because of their lower work function, WL10-electrodes emit electrons more easily than WT20-electrodes. For this reason, less electrical power is required, because a lower temperature level is sufficient to maintain the plasma. Still, the net plasma power is the same, because in the case of WT20 more power is dissipated. The differences between the two materials can be seen not only in process parameters, but also in their microstructure. At the tip of the WT20-cathode no oxides are observed and the grain structure is coarse. Presumably, the temperature at the tip is sufficiently high to evaporate the oxides and to recrystallise the tip structure. In contrast, oxides are still present at the tip region of the WL10-cathode, providing enhancement of electron emission. The lanthania additions cause a reduced degradation of the cathode material, which is attributed to the lower cathode temperature. The increasing wear of the electrodes affects the quality of the coating. As the wearout is less in the case of WL10-cathodes, the coating quality is kept at a higher level for a longer period of time. The results show that it is not necessary to use thoriated electrodes, because lanthanum electrodes have a high potential to replace them.

AB - Thoria (ThO2) additions are used in tungsten-based electrodes for plasma spraying due to the excellent electron emissivity, improved arcing behaviour, higher strength and better machinability. Because of their radioactive potential, which makes handling, use, recycling and disposal more difficult, research is done on alternative additives that provide the same advantages as thoriated tungsten but without environmental hazards. One of the most competitive alternatives to replace thoria is lanthania (La2O3). Within this thesis, tungsten cathodes with 2 wt.% thoria (WT20) and 1 wt.% lanthania (WL10) were compared. Tests in terms of arc ignition, plasma stability and arc erosion, both cyclic and continuous plasma spraying experiments were carried out. In addition, the structure both of the cathode tips and the Al2O3 coatings sprayed on Mo substrates were evaluated in metallographic cross-sections. In cyclical as well as continuous spraying experiments, the dropping rate of the arc voltage is lower for WL10-electrodes; thus a longer service life can be expected. This is also evident from the observation of the erosion rate. Because of their lower work function, WL10-electrodes emit electrons more easily than WT20-electrodes. For this reason, less electrical power is required, because a lower temperature level is sufficient to maintain the plasma. Still, the net plasma power is the same, because in the case of WT20 more power is dissipated. The differences between the two materials can be seen not only in process parameters, but also in their microstructure. At the tip of the WT20-cathode no oxides are observed and the grain structure is coarse. Presumably, the temperature at the tip is sufficiently high to evaporate the oxides and to recrystallise the tip structure. In contrast, oxides are still present at the tip region of the WL10-cathode, providing enhancement of electron emission. The lanthania additions cause a reduced degradation of the cathode material, which is attributed to the lower cathode temperature. The increasing wear of the electrodes affects the quality of the coating. As the wearout is less in the case of WL10-cathodes, the coating quality is kept at a higher level for a longer period of time. The results show that it is not necessary to use thoriated electrodes, because lanthanum electrodes have a high potential to replace them.

KW - atmospheric plasma spraying

KW - thorium oxide

KW - lanthanum oxide

KW - tungsten electrodes

KW - Atmosphärisches Plasmaspritzen

KW - Thoriumoxid

KW - Lanthanoxid

KW - Wolframelektroden

M3 - Diploma Thesis

ER -