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About the dependence of the stabilization of alpha-alumina on the spray process
Über die Abhängigkeit der Stabilisierung des Alpha-Aluminiumoxids vom Spritzverfahren
In result of the spray process normally a phase change from alpha-alumina (corundum) in the feedstock powder to predominantly alpha-alumina in the coating takes place. This is a well known but often neglected fact in the preparation and use of alumina coatings. This is of special interest since the high usage properties of thermally sprayed alumina coatings are generally derived from sintered alumina ceramics which consist of corundum. It is expected that the prevention of this phase transformation will significantly change the mechanical, electrical and other properties of thermally sprayed alumina coatings. There is controversial information about the possibility of stabilization of alpha-alumina by additions of chromia in the literature. In this work the stabilization using different spray processes (water stabilized plasma WSP, gas stabilized plasma APS and HVOF) was studied. Mechanical mixtures of alumina and chromia were used, as well as pre-alloyed powders consisting ofsolid solutions. The coatings were studied by Xray diffraction and metallographic cross-sections. It was shown that in the case of the mechanically mixed powders the stabilization predominantly depends on the applied spray process. The stabilization by use of the WSP process was confirmed.
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