L. M. Berger, C. C. Stahr, S. Saaro, Fraunhofer Institute for Material and Beam Technology, Dresden, Germany; J. Dubsky, K. Neufuss, Institute of Plasma Physics ASCR, Praha 8, Czech Republic
In result of the spray process normally a phase transformation from alpha-alumina (corundum) to predominantly gamma-alumina takes places. 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 consists of corundum. It is expected that the prevention of this phase transformation will significantly change the mecahnical, electrical and other properties of alumina coatings.There are contradictionary informations about the possibility of stabilization of alpha-alumina by additions of chromia in the literature. In this work the stabilization by use of different spray processes (water stabilized plasma WSP, gas stabilised plasma APS, HVOF) was studied. Mechanical mixtures of alumina and chromia were used, as well as prealloyed powders consisting of mixed crystals. The coatings were studied by XRD, metallograhic cross-sections and hardness measurements. It was shown that in the case of the mechanically mixed powders the stabilization predominantly depends on the enthalpy of the spray process applied. The stabilization by use of the WSP process was confirmed.
Summary: The phase content of alumina coatings determines their properties. In result of the spray process normally a phase transformation from alpha-alumina (corundum) to predominantly gamma-alumina takes places. There are contradictionary informations about the possibility of stabilization of alpha-alumina by additions of chromia in the literature. In this work it was shown that the stabilization depends on the spray process applied.
