An alternative method to produce bulk nanocrystalline materials and avoid the powder compaction step is to produce amorphous material by rapid solidification followed by controlled heat treatment to introduce nanocrystalline structure. The extremely high cooling rates in plasma sprayed particles give rise to formation of nonequilibrium phases, which may become amorphous for certain materials. Five different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia and silica. The powder feedstock materials have been plasma sprayed using water stabilized plasma torch (WSP) and subsequently heat-treated to prepare nanocomposite materials with varying nanocrystallite size. The as-sprayed materials have very low open porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950℃ with an associated volume shrinkage of 1-2%. The resulting structure is best described as nanocomposite with very small crystallites (12 nm on average) embedded in inter-crystallite network. Role of the silica compound on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. Elastic properties were measured for the nanocrystalline materials. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat treated nanocrystalline samples.
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