Abstract
This paper reports the chemical synthesis of tungsten carbide/cobalt (WC/Co) nanocomposite powders via a unique chemical processing technique, involving the using of all water soluble solution of W-, Co- and C-precursors. In the actual synthesis, large quantities of commercial-scale WC-Co nanocomposite powders are made by an unique combination of converting a molecularly mixed W-, Co-, and C-containing solutions into a complex inorganic polymeric powder precursor, conversion of the inorganic polymeric precursor powder into a W-Co-C-O containing powder intermediates using a belt furnace with temperature at about 500°C - 600°C in an inert atmosphere, followed by carburization in a rotary furnace at temperature less than 1000°C in nitrogen. Liquid phase sintering technique is used to consolidate the WC/Co nanocomposite powder into sintered bulk parts. The sintered parts have excellent hardness in excess of 93 HRA, with WC grains in the order of 200 - 300 nm, while Co phase is uniformly distributed on the grain boundaries of the WC nanoparticles. We also report the presence of cobalt Co precipitates inside tungsten carbide WC nanograins in the composites of the consolidated bulk parts. EDS is used to identify the presence of these precipitates and micro-micro-diffraction technique is employed to determine the nature of these precipitates.
Highlights
The tungsten carbide-cobalt (WC-Co) composite material, so called cermet or hardmetal, is the most widely used hardfacing materials in industrial cutting or boring tools applications
Soluble tungsten, ammonium metatungstate (AMT), and cobalt, cobalt nitrate, acetate or oxalate, were dissolved in water to make aqueous solution, followed by a spray conversion process to make W-Co-O containing precursor powder, with subsequent reduction, and carburization to convert it into WC/Co at temperatures of 700 ̊C to 900 ̊C, in a CO/CO2 gaseous environment in a fluidized bed
The belt furnace processing here serves the purpose of: 1) removal of residual moisture resulting from spray drying process and crystal water that are inherited from the chemical precursors of AMT, cobalt nitrate, and the aqueous carbon source; 2) conversion of the C-H from the carbon source into carbon, with minimal loss of the carbon due to reaction of carbon atoms with surrounding oxygen and hydrogen to form gaseous species including CO and steam, or some other gaseous species; and 3) removal of the N-H groups, forming an intermediate pre-composite of W-Co-O-C system, probably in the form of cobalt tungstate CoWO4/WO3 mixture, and fine dispersions of carbon black
Summary
The tungsten carbide-cobalt (WC-Co) composite material, so called cermet or hardmetal, is the most widely used hardfacing materials in industrial cutting or boring tools applications. Soluble tungsten, ammonium metatungstate (AMT), and cobalt, cobalt nitrate, acetate or oxalate, were dissolved in water to make aqueous solution, followed by a spray conversion process to make W-Co-O containing precursor powder, with subsequent reduction, and carburization to convert it into WC/Co at temperatures of 700 ̊C to 900 ̊C, in a CO/CO2 gaseous environment in a fluidized bed This process had been the classic method of making WC/Co nanocomposite, and had opened the window opportunity for obtaining nanocomposite materials with significantly improved structural and mechanical properties [20] [21]. The mechanism of chemical reactions in each steps of the synthesis and the detailed studies of Co nanoparticulate dispersions presented inside WC nanograins will be reported in subsequent publications elsewhere
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