Compositional Development as a Function of Spray Distance in Unshrouded/Shrouded Plasma-Sprayed Cr 3 C 2 -NiCr Coatings
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S. Matthews (Submitted July 22, 2014; in revised form October 23, 2014) Thermal spraying of Cr3C2-NiCr composites generates varying degrees of carbide dissolution into the Ni binder. During high-temperature exposure, the carbide dissolution zones precipitate high concentrations of small carbides which develop into finely structured networks. This raises the possibility of producing unique tailored carbide composite structures through the generation of controlled carbide dissolution and appropriate heat treatment. The first step in this process is to produce a supersaturated Ni-Cr-C solid solution from which the carbide phase could be precipitated. In a previous work, a broad range of plasma parameters were trialed to assess their effect on the degree of carbide dissolution at a fixed spray distance of 100 mm. The current two-part work builds on the most promising plasma parameters from those trials. Part 2 of this article series investigated the effect of spray distance on the compositional development in Cr3C2-NiCr coatings during high-energy plasma spraying. The coating compositions were analyzed in detail and quantified through Rietveld fitting of the coating XRD patterns. Coating microstructural features were correlated with the observed variations in composition. The effect of the spray parameters and spray distance on the equilibrium coating compositions is discussed.
Keywords
amorphous Ni alloy, carbide dissolution, Cr3C2NiCr, Cr7C3, metastable (Cr,Ni)7C3, shrouded plasma spray, spray distance
1. Introduction Thermal spraying of Cr3C2-NiCr coatings generates varying degrees of carbide dissolution into the Ni alloy binder. It is conventional practice to minimize such carbide dissolution to retain the highest carbide content possible, since high carbide content cermet coatings exhibit superior wear resistance (Ref 1-5). During high-temperature exposure, the zones of carbide dissolution precipitate high concentrations of small carbide grains which develop into finely structured and interconnected carbide networks (Ref 3, 5-7). The variation in these carbide structures that develop as a function of initial carbide dissolution, heat treatment temperature, and treatment time raises the possibility of tailoring the carbide phase morphology to exploit the beneficial wear resistance of submicron carbide grains, or the development of unique carbide composite structure functionalities.
S. Matthews, School of Engineering and Advanced Technology, Massey University, Private Bag 102904, Auckland 0745, New Zealand. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
The first step in exploring this concept for Cr3C2-NiCr cermet coatings is the production of a coating with a supersaturated solid solution of Ni-Cr-C from which high concentrations of carbides could be precipitated with heat treatment. In a marked deviation from convention, the approach taken in this project to generate such coatings was to spray a conventional Cr3C2-NiCr agglomerated and sintered powder under high-power plasma cond
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