Oxidation control in plasma spraying NiCrCoAlY coating
Graphical abstract
Highlights
► The oxidation of NiCrCoAlY particles was studied during the spray process. ► Two different oxidation mechanisms involved in the in-flight oxidation. ► In-flight oxidation is the dominant mechanism for NiCrCoAlY particles in spray. ► Adding shrouding inert gas is an effective method to decrease the oxide content.
Section snippets
1.Introduction
The plasma spray process is carried out by feeding powders into a plasma torch where the material is melted and propelled as molten or semi-molten particles toward the substrate. The coating is built up on the substrate surface by successive impacts of molten metal droplets, originating from the powder particles fed into the plasma torch [1]. Generally, the plasma spray process is conducted in an ambient air environment. Turbulent flow is caused by the jet flow and the surrounding air with
Experimental
The oxidation of the coating test specimen is cut from the Haynes 230 nickel-based super alloy, the test sample is 15 mm × 10 mm × 4 mm, the nominal composition of the alloy (wt%): Cr 22%, W 14%, Mo 2%, other Ni. Grit blasting was used prior to plasma spraying, and then the sample was cleaned ultrasonically in an acetone bath. The spraying material used for the experimentation was NiCrCoAlY powder (China Institute of Mining and Metallurgy) having particle size in the range 44–104 μm, the main
The microstructure of particles
Fig. 2 shows the surface and the section morphology of the as-received NiCrCoAlY powder. It can be seen that the shape of the powder is irregular (Fig. 2a), and a small amount of residual oxide exists which is dissolved oxygen in the metal on the preparation stage. The oxide can be found neither on the surface nor on the interior of the particles (Fig. 2b). The morphology of the sprayed powder is shown in Fig. 3. The powder appears spherical after spraying. The morphology on the typical
Conclusion
- (a)
For NiCrCoAlY particles, convective oxidation is the dominating oxidation mechanism at the center of the plasma jet at a distance of 55 mm from the torch nozzle exit; diffusion oxidation is the dominating mechanism when the spray distance is longer than 55 mm, and the degree of particle oxidation increases with the distance beyond the nozzle.
- (b)
In-flight oxidation and after impact oxidation on substrate are two different mechanisms during the spray process. The dominant oxidation mechanism of APS
Acknowledgments
This research was supported in part by the Science and Technology Development Project KM200610005026 and JP009012200803 funded by Beijing Municipal Education Commission, PR China, and this project was also partially supported by the National Natural Science Foundation of China (50805002).
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