Mechanism of Suppressing Oxidation of FeAl Molten Droplet by Adding C to Powder and Its Effect on Microstructure and Properties of Plasma-sprayed Coating

doi:10.11933/j.issn.1007-9289.20220506004

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China Surface Engineering ›› 2023, Vol. 36 ›› Issue (1) : 44-56. DOI: 10.11933/j.issn.1007-9289.20220506004

Mechanism of Suppressing Oxidation of FeAl Molten Droplet by Adding C to Powder and Its Effect on Microstructure and Properties of Plasma-sprayed Coating

ZHOU Zhe, ZHANG Li, DONG Xinyuan, LUO Xiaotao, MAHRUKH M, LI Changjiu

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Abstract

The severe oxidation of metal elements leads to a large amount of oxide inclusions that hinder the formation of metallurgical bonds across intersplat interfaces during thermal spraying in an ambient atmosphere, which further degrades the performance of thermally sprayed metal coatings. Controlling the oxidation of metal elements during spray deposition remains a significant challenge. In this paper, the thermodynamic and kinetic requirements of in-situ in-flight deoxidation by adding deoxidizer elements (diamond particles) to Fe / Al spray powders to achieve oxide-free molten droplets in an ambient atmosphere were investigated. Fe / Al / 2.5C composite powders containing 2.5wt.% diamond particles were prepared by mechanical alloying. Atmospheric plasma spraying (APS) was used to generate an oxide-free molten spray of FeAl particles at temperatures higher than 2 000 °C. This was achieved by the in-situ self-oxide cleaning effect of in-flight particles by carbon, which prevented metal droplets from oxidizing in an open atmosphere. For comparison, a FeAl coating was also deposited using Fe / Al composite powders prepared by mechanical alloying. The particle velocity and temperature were measured by a commercial thermal-spraying particle diagnostic system. The microstructures of the coatings were characterized by scanning electron microscopy and X-ray diffraction. The chemical compositions of the coatings were measured by inductively coupled plasma spectroscopy. The adhesive strength and hardness of the plasma-sprayed FeAl-based coatings were tested. The results show that under the combined effects of plasma-jet heating and the FeAl exothermic reaction, the heated FeAl spray particles could reach a high temperature of over 2 100 °C, which meets the thermodynamic conditions for carbon in-situ deoxidation. Because oxidation occurs from the surface of an in-flight molten droplet, to maintain continuous deoxidation of the molten FeAl droplet, the rapid transfer of carbon from the interior of the molten droplet to the surface is necessary. It was confirmed that the strong convection flow of the molten metal droplets satisfied the kinetic conditions of carbon in-situ in-flight deoxidation. The oxygen content in the APS-sprayed FeAl coatings increased with the increase of the spray distance. In contrast, the oxygen content in the APS-sprayed FeAl / 2.5C coatings decreased from 1.01 wt.% to 0.48 wt.% as the spray distance increased from 70 mm to 150 mm. This was accompanied by the decrease in carbon content from 1.41 wt.% to 0.31 wt.%, indicating the effective protection of Fe and Al from oxidation by the carbon in the droplet. Compared with the oxygen content of 3.67 wt.% in traditional FeAl coating, the oxygen content of the FeAl / 2.5C coatings was reduced to less than 0.5 wt.%. Considering that the carbon content of the coatings decreased as the spray distance increased, the results confirm that the in-flight oxidation of Fe and Al was suppressed by the preferential sacrificial oxidation of carbon in the molten droplets. Thus, the oxide in the FeAl / 2.5C coatings was mainly attributed to post-deposition oxidation. Moreover, the FeAl droplet reached the temperature to realize the spread-fusing self-metallurgical bonding effect for FeAl splats. The coatings exhibited a dense microstructure with an apparent porosity of less than 0.5%. The adhesive bonding strength of the prepared coatings exceeded 59.2 MPa owing to metallurgical bonding. The hardness of the coatings reached 590 HV_0.3, which was approximately twice that of traditional FeAl coatings. This was attributed to the reinforcement provided by the Fe₃AlC_x cementite phase resulting from the residual carbon in the coating.

Key words

atmospheric plasma spraying； FeAl / 2.5C coating； microstructure and property； in-situ deoxidation； self-metallurgical bonding

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ZHOU Zhe, ZHANG Li, DONG Xinyuan, LUO Xiaotao, MAHRUKH M, LI Changjiu. Mechanism of Suppressing Oxidation of FeAl Molten Droplet by Adding C to Powder and Its Effect on Microstructure and Properties of Plasma-sprayed Coating[J]. China Surface Engineering, 2023, 36(1): 44-56 https://doi.org/10.11933/j.issn.1007-9289.20220506004

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Supported by National Natural Science Foundation of China (U1837201, 52031010) and National Science and Technology Major Project of China(2019-VII-0007-0147).

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2023-01-19
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