Plasma spray coatings in different nanosize alumina
Introduction
Materials with fine-scale structures have long been recognized to exhibit remarkable and technologically attractive properties. Over the past decade, interest has been growing in a new class of materials that are composed of ultrafine grains or particles. Much of the research activity is focused on the synthesis and processing of nanostructured materials [1], [2], [3]. The process more likely to benefit from a short term is the deposition of coatings by thermally activated process. There are two problems to prepare the nanocoating by thermal spray. One is how to feed the nanopowder. The nanopowders usually adhere to the walls of feeding system and are very difficult to arrive at the plasma torch due to its high specific area. The other is how to keep the nanostructure in the coating after the thermal spraying process. The thermal spray process is a high-temperature one, the grain growth of starting powders is inevitable. The spray parameters should be optimized to control the growth of starting powders. Therefore, how to solve both of problems is the key to prepare successfully the nanocoating. In the present work, the alumina coating, which has been one of the most important materials used for plasma spray due to its high corrosion and erosion resistance and hardness, is studied on the basis of our previous works [4], [5]. This paper compares the properties of alumina coatings obtained by thermal spraying in three different nanostructured powders, utilizing the air plasma spray (APS) system.
Section snippets
Experiment
The A-2000 atmospheric plasma spraying equipment (Sulzer Metco, Switzerland) was used to deposit nanostructured alumina coatings. The spray powders used in this work were A powder, T powder and F powder. The morphology of these powders was spherical and ellipsoidal, as shown in Fig. 1. The sizes of these powders are different. The average size of F powder is about 60 nm, those of A powder and T powder are 100 and 200 nm, respectively. The powders were fed with a Twin-System 10-V
Results and discussion
The patterns of X-ray diffraction (XRD) of these starting powders (Fig. 3) indicate that the A powder and T powder are composed of pure α-Al2O3 and the F powder consists of only γ-Al2O3. The patterns of XRD of coatings (Fig. 4) shows that the phase compositions are changed after the plasma spray process when comparing with those of starting powders. Unlike the starting powder, all of these coatings are composed of two phases—α-Al2O3 and γ-Al2O3. However, the contents of γ-Al2O3 phase in these
Conclusion
In conclusion, thermal spray technology is a good means of manufacturing the nanostructured materials. The grain in the as-sprayed coatings could keep similar size with starting powder. The dense nanostructured F alumina coating is deposited by air plasma spray. It possesses higher γ-Al2O3 phase percentage, larger interplanar distance and smoother surface when comparing with other two coatings with larger size starting powders.
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