Post-treatment of Plasma-Sprayed Cr 2 O 3 with Methane-Containing Gas for Conversion to Binder-Free Cr 3 C 2
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Michael C. Brupbacher
, Dajie Zhang, William M. Buchta, Yo-Rhin Rhim, Dennis C. Nagle, and James B. Spicer (Submitted May 18, 2015; in revised form October 2, 2015)
In most applications, the performance of thermally sprayed Cr3C2-NiCr cermet coatings is known to be adversely affected by the presence of the NiCr binder phase. A processing technique for the rapid synthesis of Cr3C2 on industrial-scale components could improve the functionality of these coatings by eliminating the metallic binder phase. To form a thick, continuous surface layer of adherent, binder-free Cr3C2, the reduction of plasma-sprayed Cr2O3 with methane-containing gas was investigated. Conversion of the plasma-sprayed Cr2O3 to carbide resulted in a significant increase in coating porosity, yielding a highly microporous Cr3C2 surface layer. The physical characteristics of the reduction process appear to be dependent on the coating defect structure at the reduction temperature. Phase morphology and porosity evolution throughout the reduction process were qualitatively examined using x-ray diffraction and scanning electron microscopy. The utility of the resultant Cr3C2 coating is discussed with respect to these microstructural characterizations and microindentation hardness measurements.
Keywords
atmospheric plasma spray (APS), chromium carbide, oxides
1. Introduction The reaction mechanisms and the kinetics of the reduction of chromium oxide (Cr2O3) powder and pressed pellets with methane-containing gas have been studied extensively, and the body of related work is a testament to the industrial importance of this process (Ref 1-5). Although the details of the proposed reaction mechanisms describing reduction vary, one central notion prevails—namely, a particular phase of chromium carbide, Cr3C2, is the end product of complete conversion. Also, there is an agreement on the pronounced effect that methane concentration has on the rate of reduction—above a critical methane concentration, the deposition of carbon can strongly retard Cr3C2 formation. In addition, the relatively low temperatures and short reaction times observed by many authors for complete conversion to Cr3C2 compared to solid-state carbothermal reduction processing (Ref 6, 7) highlight the usefulness of exploiting a
Michael C. Brupbacher, Dajie Zhang, and James B. Spicer, Department of Materials Science and Engineering, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218; and William M. Buchta, Yo-Rhin Rhim, and Dennis C. Nagle, The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723. Contact e-mails: [email protected], [email protected], mark. [email protected], [email protected], dennis.nagle@ jhuapl.edu, and [email protected].
Journal of Thermal Spray Technology
vapor-phase reducing agent. Beyond the production of Cr3C2, this material plays a central role as a component in feedstock powder for cermet thermal spray coatings. In order to facilitate bonding between the feedstock powder and the substrate during
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