Session BB5.4

4:30 PM BB5.4
ALUMINA BASED MATERIALS FOR USE WITH WATER-STABILIZED PLASMA SPRAY SYSTEM. Jan Ilavsky, Pavel Chraska, Jiri Dubsky, Blahoslav Kolman, Karel Neufuss, Inst. of Plasma Physics, Prague, CZECH REPUBLIC.

Availability of a high-throughput water-stabilized plasma spray system with its significantly better economics brings about a number of emerging engineering applications, in which use of deposits becomes economically feasible. However, this reduction in cost of depositing a unit (kilogram) of material opens field for (new or old) materials with better economics. Such materials are alumina based feedstock powders which have been used in plasma spraying for a long time. Alumina based materials show complex behavior, which has always limited their use especially at elevated temperatures. The processes occurring at temperatures above 1000$^{\circ}$C include complex phase transformations accompanied by change in alumina density as well as sintering. Extensive work has been done in last years on the subject of understanding and controlling of the phase transformation temperatures by addition of other oxides. Even better, it was found possible to control phase composition of the as-sprayed deposits by variations of feedstock chemistry and by manufacturing deposits in the stable alpha phase avoid the phase transformations entirely. As-sprayed deposits are composed of metastable delta phase; if TiO₂ is present gamma phase which transforms at temperatures below 1000$^{\circ}$C into delta phase. Stable alpha phase is formed between 1050$^{\circ}$C and 1200$^{\circ}$C, which is sometimes accompanied by intermediate theta phase. Addition of TiO₂ into the alumina lowers the alpha phase formation temperature and theta phase is not formed at all or its content is significantly reduced. Addition of Cr₂O₃ increases alpha phase formation temperature and also increases amount of alpha phase present in as-sprayed deposits. Larger amounts of Cr₂O₃ stabilize alpha phase in the as-sprayed deposits avoiding phase transformations entirely. The presentation reviews progress in understanding of temperatures of the phase transformations in plasma sprayed deposits, accompanying changes in microstructure and methods of controlling the temperatures.