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2824 |
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Influence of chromia alloying on the characteristics of APS and HVOF sprayed alumina coatings |
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Kari Niemi* / Tampere University of Technology
Department of Materials Science, Finland Johanna Hakalahti / Tampere University Of Technology, Department of Materials Science, Finland Leo Hyvärinen/ Tampere University Of Technology, Department of Material Science, Finland Jussi Laurila/ Tampere University Of Technology, Department of Material Science, Finland Petri Vuoristo/ Tampere University Of Technology, Department of Materials Science, Finland Filofteia-Laura Toma/ Fraunhofer Institute for Material and Beam Technology, Germany Lutz-Michael Berger/ Fraunhofer Institute for Material and Beam Technology, Germany |
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Abrasion wear characteristics of thermally sprayed alumina coatings can be significantly improved by using high velocity spray methods, like HVOF, instead of atmospheric plasma spraying (APS). Corrosion resistance of the high velocity oxygen fuel (HVOF) sprayed alumina coatings is, however, not necessarily improved. In order to improve both corrosion and wear resistance characteristics of the alumina coatings chromia alloying of alumina was carried out. Alumina and alumina-chromia powders with different chromia contents were deposited by TopGun HVOF spraying and APS. The coatings were evaluated by optical microscopy, microhardness measurements, XRD and SEM. Abrasion wear resistance of the coatings was tested with the rubber-wheel abrasion test. Corrosion characteristics of the coatings were evaluated by the exposure tests at low and high pH conditions. The coatings were immersed in electrolytes with different pH values at the temperature of 85 oC for 310 hours. The electrolytes were then analysed by ICP to detect which elementary elements were dissolved from the coating material. The amount of dissolved elements in solution was found to be a direct indication of the corrosion resistance of the coating in specific electrolyte. Coatings deposited by TopGun HVOF spray method resulted in denser microstructure, higher microhardness and significantly improved abrasion wear resistance as compared to corresponding coatings deposited by APS. Corrosion resistance of the alumina coatings was low for both HVOF and APS coatings at tested conditions but was greatly improved by chromia alloying for both spray methods. |