Deposition of Al 2 O 3 -TiO 2 Nanostructured Powders by Atmospheric Plasma Spraying
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E. Sa´nchez, E. Bannier, V. Cantavella, M.D. Salvador, E. Klyatskina, J. Morgiel, J. Grzonka, and A.R. Boccaccini (Submitted September 7, 2007; in revised form April 9, 2008) Al2O3-13%TiO2 coatings were deposited on stainless steel substrates from conventional and nanostructured powders using atmospheric plasma spraying (APS). A complete characterization of the feedstock confirmed its nanostructured nature. Coating microstructures and phase compositions were characterized using SEM, TEM, and XRD techniques. The microstructure comprised two clearly differentiated regions. One region, completely fused, consisted mainly of nanometer-sized grains of c-Al2O3 with dissolved Ti+4. The other region, partly fused, retained the microstructure of the starting powder and was principally made up of submicrometer-sized grains of a-Al2O3, as confirmed by TEM. Coating microhardness as well as tribological behavior were determined. Vickers microhardness values of conventional coatings were in average slightly lower than the values for nanostructured coating. The wear resistance of conventional coatings was shown to be lower than that of nanostructured coatings as a consequence of Ti segregation. A correlation between the final properties, the coating microstructure, and the feedstock characteristics is given.
Keywords
Al2O3-TiO2, nanostructured coatings, thermal spraying
1. Introduction Nanostructured materials represent an enhanced version of their conventional counterparts (Ref 1, 2) and usually exhibiting better performances (Ref 3-5). Bulk nanomaterials are being extensively studied but also nanostructured coatings on conventional materials are investigated as they have the potential of providing several industries with novel materials with improved properties and prolonged lifetime (Ref 6-8). Thermal spray techniques such as plasma spraying, in which a powder is injected into plasma, molten, and accelerated toward a substrate where it impacts creating a coating, are widely used in industry (Ref 9). The production of nanostructured coatings using such conventional techniques would be especially interesting if only changes of the raw material would be required, as no expensive investment would be necessary. Unfortunately,
nanoparticles cannot be sprayed because of their low mass and poor flowability. However, several studies have shown that this problem can be solved by agglomerating them in micrometer-sized aggregates, which can be sprayed as easily as conventional powders (Ref 10-13). Commercial agglomerated nanopowders are now available. However, the process must be carefully controlled in order to keep the initial nanostructure in the final coating (Ref 14-16). Typical nanostructured ceramic coatings obtained by plasma spraying from agglomerated powders include alumina (Ref 17), alumina-titania (Ref 18, 19), cemented tungsten carbides (Ref 20, 21), or zirconia (Ref 22, 23). In this work, alumina-titania ceramic coatings were obtained by atmospheric plasma spraying (APS) using commercial micrometric and nanostructured powde
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