FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings
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FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings Vasileios Katranidis1 • Sai Gu1 • David C. Cox2 • Mark J. Whiting3 Spyros Kamnis4
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Submitted: 23 January 2018 / in revised form: 11 April 2018 Ó The Author(s) 2018
Abstract The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W2C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape.
& Vasileios Katranidis [email protected] 1
Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, UK
2
Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK
3
Department of Mechanical Engineering Sciences, University of Surrey, Guildford GU2 7XH, UK
4
Monitor Coatings, 2 Elm Road, Tyne and Wear NE29 8SE, UK
Keywords decarburization \ properties focused ion beam \ testing HVOF \ processing imaging \ processing wear-resistant coatings \ applications WC-Co \ feedstock
Introduction Thermal spray coatings for wear/corrosion-resistant applications have seen increased adoption by industry over the past decades, proving themselves as effective alternatives to the toxic hard chrome plating (Ref 1). Such thermally sprayed coatings are typically based on tungsten or chromium carbides in a cobalt, or cobalt/nickel-based alloyed binder, commonly referred to as hardmetals (Ref 2). Highvelocity combustion thermal spray methods such as HVOF and HVAF are particularly effective for the deposition of hardmetals owing to the high velocity of the material jets and relatively low deposition temperatures. In the case of WC-Co coatings, low deposition temperatures are vital due to their sensitivity to thermal decomposition and decarburization. Meanwhile, high particle velocities are necessary to achieve the deformation of particles impinging on the target surface, forming the coating. The thermal dissolution and decarburization of WC-Co have been shown to embrittle the coatings and lower the carbide volume fraction (Ref 3). Ulti
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