The European Ceramic Society
YCN Newsletter : Women in Ceramics by Marina Ciurans Oset
Plasma Centrifugal Atomization of Cast Tungsten Carbide for Wear Resistant Coatings: Properties and Applications
Oil drilling, ore processing and metal shaping applications, among other, require highly wear-resistant tool materials capable of withstanding harsh working conditions under heavy loads. Transition d-metal carbides containing elements from groups IV-VI are ideal materials for such applications owing to their exceptional physical and mechanical properties.
Among those compounds, non-stoichiometric carbides possess the highest melting temperatures and the highest hardness. However, extremely high production and processing costs make these materials prohibitively expensive for the intended large scale industrial applications. In this context, tungsten carbides become particularly outstanding. These compounds exhibit sufficiently high hardness from room temperature and up to ~1300 K, high Young’s modulus, low thermal expansion coefficient and high temperature and chemical stability.
Stoichiometric WC, which is widely used in the production of wear resistant alloys, is the only tungsten-carbon compound stable at room temperature. However, metastable W2C is progressively attracting more interest owing to suspected higher hardness. Cast tungsten carbide (CTC, also referred to as fused tungsten carbide) results from the eutectoid decomposition of metastable cubic WC1-x carbide containing ~3.95 wt.% C and consists of WC lamellae inserted in a W2C matrix. The presence of metastable W2C phase, together with the biphasic structure of CTC, results in improved hardness and fracture toughness with respect to pure WC or W2C.
Due to excessive brittleness, extremely high sintering temperatures and high material costs, cutting tools are not manufactured from bulk carbides. Instead, carbide powders are used for depositing metal matrix composite (MMC) hardfacing overlays onto steel tools by means of conventional welding processes. To ensure the mechanical integrity and minimize stress build-up within the matrix, spherical CTC powders are preferred over angular CTC powders. Cold crucible atomization, rotary electrode atomization or direct melting, among other, have been reported suitable for the production of spherical CTC. Resulting powders, however, exhibit particle sizes >200 µm, irregular shape and relatively high volume fraction of porosity. A plasma-assisted centrifugal atomization system suitable for processing of CTC was developed by Metasphere Technology AB, currently Höganäs Sweden AB. In this process, angular CTC with particle sizes <3 mm is fed as feedstock material into a rotating crucible, melted by the glow discharge of a plasmatron and ejected into the chamber as a dispersion of fine droplets. Spherical CTC powder obtained after solidification exhibits particle sizes in the range 40-1000 µm with outstanding shape uniformity, exceptionally fine microstructure and significantly higher mechanical properties than conventional CTC.
The main goal of my research project is to better understand the interplay between plasma centrifugal atomization of CTC and the properties of the powders thus produced, ultimately allowing for the development of spherical CTC powders with highly tailored properties. To this end, my current research focuses on extensive mechanical and microstructural characterization of CTC powders, as well as the study of microstructural evolution upon heat treatment and dissolution phenomena of CTC upon coating deposition. This project is run in collaboration between Höganäs Sweden AB and Luleå University of Technology, co-funded by the Swedish Foundation for Strategic Research (SSF) industrial PhD program.
Explanatory text (graphical abstract):
Schematic representation of the plasma centrifugal atomization process, from coarse angular CTC into fine spherical CTC with refined microstructure.
Marina Ciurans Oset
Division of Materials Science, Department of Engineering Sciences and Mathematics
Luleå University of Technology, SE-971 87 Luleå, Sweden
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