Carbide Dissolution/Carbon Loss as a Function of Spray Distance in Unshrouded/Shrouded Plasma Sprayed Cr 3 C 2 -NiCr Coa
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JTTEE5 24:552–569 DOI: 10.1007/s11666-014-0210-2 1059-9630/$19.00 Ó ASM International
Carbide Dissolution/Carbon Loss as a Function of Spray Distance in Unshrouded/Shrouded Plasma Sprayed Cr3C2-NiCr Coatings 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. In Part 1 of this two-part article series, the effect of spray distance on the extent of carbide dissolution and carbon loss during high energy plasma spraying was investigated. The effectiveness of solid shield and gas shrouding is contrasted, and the mechanisms by which they influence the degree of decarburization discussed.
Keywords
carbide dissolution, Cr3C2-NiCr, decarburization, gas shrouding, shrouded plasma spray, spray distance
1. Introduction Cr3C2-NiCr thermal spray coatings are widely used to mitigate wear and corrosion at elevated temperatures where more wear resistant WC-Co cermets cannot be used due to their poor oxidation performance (Ref 1-3). It has been shown across several studies that thermally sprayed carbide composite coatings that retain a high carbide content and undergo minimal carbide degradation inflight, typically show superior wear resistance (Ref 3-6). Therefore, it has become a conventional practice in the production of carbide composite coatings to optimize the deposition parameters to melt the metallic binder, in order to achieve high deposit efficiency, while at the same time keeping the particle temperature low and the in-flight time short to minimize carbide dissolution and decarburisation. For these reasons, HVOF spraying has become the industry standard for producing carbide composite thermal spray coatings.
S. Matthews, School of Engineering and Advanced Technology, Massey University, Private Bag 102904, Auckland 0745, New Zealand. Contact e-mail: [email protected].
552—Volume 24(3) February 2015
During heat treatment of HVOF sprayed Cr3C2-NiCr coatings, it has been observed that zones of high carbide dissolution precipitate high concentrations of small carbide grains which quickly develop into finely structured and interconnected carbide networks (Ref 5-8). The significant variation in carbide structures developed as a function of carbide dissolution and subsequ
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