Preparation and Electrocatalytic Activity of Nanophase WC-Co Composite Powder and WC Powder with Spherical Shell Structure

Using ammonium metatungstate (AMT), soluble cobalt salt and organic carbon source as the raw materials, the W-Co precursor powder with spherical shell structure was first fabricated by spray conversion method. Then the nanophase WC-Co composite powder was fabricated via calcinations and low temperature reduction-carbonization methods. And the WC powder with the same spherical shell structure was prepared at last by dissolving the Co phase into H3PO4and H2O2. The phase composition, powder morphology, chemical components and its distribution of the samples at different stages were characterized by XRD, SEM and EDS. The results confirmed that the powders showing spherical shell structure after spray conversion, calcinations, reduction, carbonization and dissolution. While the amount of surface porosity changed after these treatments. The grain size of WC was about 50nm measured by FWHM of XRD. The electro catalytic activities of the powders towards methanol electro-oxidation were investigated by cyclic voltammetry with a three-electrode system in acidic solution. Compared with the granular WC powder, the electro-catalytic activity of WC-Co composite powder sample decreased, but the electro-catalytic activity of WC powder with spherical shell structure has been significantly improved. These results indicated that the electro catalytic activity of WC can be improved through the formation of spherical shell structure, while the existence and distribution of Co phase could hinder the electro-catalytic activity of WC.

Ultrafine WC-10Co cemented carbides fabricated by electric-discharge compaction

This research investigates the microstructure and mechanical properties of ultrafine WC-10Co cemented carbides fabricated by an electric-discharge compaction (EDC) process, from powder synthesized by a spray-conversion process (SCP). Due to a short holding time during EDC, a grain size as small as 120 nm can be achieved. We also found that dispersion of pores in WC-Co cemented carbides may contribute to fracture toughness, besides the bridging ligament mechanism.

Microstructure, Mechanical Properties and Wear Resistance of WC/Co Nanocomposites

ABSTRACTThe microstructure, mechanical properties, abrasion and wear resistance of WC-Co nanocomposites synthesized by the spray conversion technique by McCandlish, Kear and Kim have been investigated. The binder phase of WC-Co nanocomposites is enriched in W and C, compared to conventional cermets. Small amorphous regions exist in the binder despite the slow cooling after liquid phase sintering. Few dislocations are found in the WC grains. The increased WC content and the amorphous regions modify (i.e. strengthen) the binder phase of the composites. Vickers indentation measurements show a hardness of the nanocomposites reaching 2310 kg/mm2. While the toughness of conventional cermets decreases with increasing hardness, the toughness does not decrease further as the WC grain size decreases from 0.7 to 0.07 μm. but remains constant at 8 MPam1/2. Scratches caused by a diamond indenter are small, commensurate with their hardness. These scratches are ductile, devoid of the grain fracture that is observed with conventional materials. The abrasions resistance of nanocomposites is about double that of conventional materials, although their hardness is larger by 23% only. This is due to the lack of WC grain fragmentation and removal which takes place in conventional cermets. Sliding wear resistance of WC/Co is proportional to their hardness; no additional benefit of nanostructure is obtained. This results from the very small size of adhesive wear events in even large WC grains.

Synthesis and Processing of Nanograined Fe - (Fe, Mo)6C Composite Powders

ABSTRACTSpray Conversion Processing was used to synthesize high volume fractions (0.52 - 0.74) of nanograined (Fe, Mo)6C carbide dispersions in iron, starting from water soluble precursors. The essential features of the process are, (1) preparation of a chemically homogeneous precursor powder, and (2) thermochemical conversion of the precursor powder to the desired nanostructured composite powder through controlled gas-solid reactions. The thermodynamic and kinetic features of the gas-solid reactions, and the influence of various processing parameters on the structures developed, are discussed. The powders were consolidated to near theoretical density by pressureless sintering and hot pressing. All the consolidated samples had bicontinuous structures. Compared to M2 high speed tool steels, these high volume fraction carbide strengthened iron alloys display superior hardness values up to 500°C.