Synthesis and processing of nanocrystalline tungsten carbide: Towards cemented carbides with optimal mechanical properties (original) (raw)
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International Journal of Refractory Metals and Hard Materials, 2016
In this paper the influence of the consolidation process and sintering temperature on the properties of near nanoand nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m 2 /g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr 3 C 2 in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.
International Journal of Refractory Metals and Hard Materials, 2001
Mechanical properties and microstructures of nanocrystalline WC±10Co cemented carbides were investigated. The nanocrystalline WC±10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375°C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC±10Co, commercial WC powders in a diameter range of 0.57±4 lm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr 3 C 2 and VC of varying amount were added into nanocrystalline WC±10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC±10Co cemented carbides, Co± W±C alloy was fabricated at the temperature of sintering process for the WC±10Co cemented carbides. The hardness of WC±10Co cemented carbides increased with decreasing WC grain size following a Hall±Petch-type relationship. The fracture toughness of WC±10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation. Ó
International Journal of Refractory Metals and Hard Materials, 2006
Microstructure and mechanical properties of WC-TiC-10 wt%Co cemented carbides fabricated by sintering with hot isostatic pressing (Sinter-HIP) process were investigated. The WC/TiC grain size ratio of WC-TiC-10 wt%Co cemented carbides was controlled by changing the average size of WC powders ranged from 0.5 to 4 lm, with keeping the average size of TiC powder as 1 lm. The microstructures of sintered WC-TiC-10 wt%Co cemented carbides were sensitively dependent on the WC/TiC grain size ratio. In WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio of 0.5, the TiC/(Ti, W)C core-rim phases were distributed in WC/Co matrix. While, in WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio above 0.8, the WC and TiC/(Ti, W)C core-rim phases were surrounded by Co binder phase. Hardness of WC-TiC-10 wt%Co cemented carbide increased with decreasing the WC/TiC grain size ratio from 4 to 0.8 following the modified Hall-Petch type equation. However, the hardness of WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio of 0.5 shows much higher values than that expected by modified Hall-Petch type equation. Transverse rupture strength of WC-20TiC-10 wt%Co cemented carbides increases with decreasing the WC/TiC grain size ratio.
Effect of Carbon Addition on Microstructure and Properties of WC–Co Cemented Carbides
Journal of Materials Science & Technology, 2012
Based on a unique method to synthesize WC-Co composite powder by in-situ reactions of metal oxides and carbon, the effects of the carbon addition in the initial powders on the phase constitution, microstructure and mechanical properties of the cemented carbides were investigated. It is found that with a suitable carbon addition the pure phase constitution can be obtained in the sintered bulk from the composite powder. The mechanical properties of the cemented carbides depend on the phase constitution and the WC grain structure. To obtain the excellent properties of the WC-Co bulk, it is important to obtain the pure phase constitution from the appropriate carbon addition in the initial powders and a suitable grain size.
Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders
Materials Science and Engineering: A, 2003
Microstructure and mechanical properties of WC Á/10Co cemented carbides fabricated by spark plasma sintering (SPS) process were investigated. Nanocrystalline precursor powders were prepared by spray drying process from solution containing ammonia meta-tungstate and cobalt nitrate, and followed by reduction and carbonization into nanocrystalline WC/Co composite powders by a mechano-chemical process. The WC particles of about 100 nm in diameter were mixed homogeneously with Co binder. The nanocrystalline WC Á/10Co powders were consolidated by SPS process at temperature ranged 900 Á/1100 8C and under a pressure of 50 or 100 MPa, respectively. Optimum consolidation conditions, such as temperature and pressure, were determined by analysing the dimensional changes of powder compact during SPS process. Hardness and fracture toughness of consolidated WC Á/10Co cemented carbide were measured by using a Vicker's indentation test. The solute content within the Co binder phase of WC Á/10Co cemented carbide was evaluated by measuring the saturated magnetic moment. It is found that the hardness of cemented carbide was dependent on the density and grain size of WC. The fracture toughness of cemented carbides increased with increasing the saturated magnetic moment, while decreased rapidly when the liquid Co phase was formed during sintering.
Microstructure and mechanical properties of nanocrystalline WC-10Co cemented carbides
Scripta Materialia, 2001
The intercritical annealing and isothermal bainitic processing response was studied for three Nb and V microalloyed Transformation-Induced Plasticity (TRIP)-assisted 980 MPa grade steels. Their mechanical and microstructural properties were compared to industrially produced TRIP 800 steel. Depending on the isothermal holding temperature and microalloying, the experimental steels reached properties comparable to the reference steel. The retained austenite content did not show direct correlation to elongation properties. Niobium was found to be more effective microalloying element than vanadium in increasing the elongation properties, which were investigated by measuring true fracture strain from tensile test specimens.
Journal of the American Ceramic Society, 2007
A solution chemical route to cobalt-coated WC-powder is described that allows for the preparation of WC-Co powders and compacts having a carbon content very close to the desired carbon content even under an inert atmosphere. The microstructural homogeneity in the sintered WC-Co composites when using the Co-coated grains was found to be superior as compared with conventionally mill-mixed powders, and the structural changes in the individual WC-grains were found to be much smaller, which is ascribed mainly to the fact that the coated grains do not require a grinding step leading to the formation of a tail of smaller WC grain sizes.
Analytical modeling to calculate the hardness of ultra-fine WC–Co cemented carbides
Materials Science and Engineering: A, 2008
An analytical model to calculate the hardness of ultra-fine WC-10Co cemented carbides was investigated. The nanocrystalline WC-10Co powders were manufactured using a spray conversion process and sintered at 1375 • C in a vacuum. Varying amounts of TaC, Cr 3 C 2 , and VC were added to nanocrystalline WC-10Co cemented carbides as grain growth inhibitors. The hardness of WC-10Co cemented carbides increased with a decreasing WC grain size from 5 m to 300 nm. An analytical model to calculate the hardness of WC-10Co cemented carbides was proposed under the assumption that the applied load is transferred from the WC to the Co binder phase. The analytically calculated hardness showed good agreement with the experimentally measured hardness of WC-10Co cemented carbides. In the proposed analytical model, the hardness of WC-10Co cemented carbides is similar to that predicted by the Hall-Petch relationship when the WC grain size is large. However, when the grain size is finer than a critical value, the predicted hardness of the WC-10Co cemented carbide becomes saturated.
Nanostructured WC -Co cemented carbides, combining high hardness and high toughness, are expected to be widely applicable. In this study, nanocrystalline WC -10Co -0.8VC -0.2Cr 3 C 2 (wt.%) composite powders, whose average grain size is bout 25 nm, were fabricated by a unique ball milling technique with variable rotation rate and repetitious circulation in 32 min. The high energy ball milling process was optimized and the as-prepared nanocrystalline powders were characterized and analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential thermal analysis (DTA). Furthermore, a low temperature, high heating rate, short soaking time and pressure sintering technique, that is, rapid hot pressing sintering, was used in the sintering process, by which nanostructured WC -Co cemented carbides with mean grain size of 250 nm were produced. The material exhibits high hardness of 93.6 HRA and transverse rapture strength of 2746 MPa. In the sintered composites, the distribution of elements W, Co, V and Cr is homogeneous, and it is small quantity of miropores and extraordinary coarse tungsten carbide phase that negatively influence the transverse rapture strength of nanostructured WC -Co cemented carbides. D
Influence of VC on the microstructure and mechanical properties of WC–Co sintered cemented carbides
International Journal of Refractory Metals & Hard Materials, 1999
The aim of this work was to evaluate the microstructural aspects and mechanical properties of a series of 90 wt% [1 À yWC± yVC]±10 wt% Co cemented carbides. The microstructural parameters of contiguity (g a ), binder mean free path (v b ) and grain size of carbide phases (v a ) have been evaluated. Contiguity has a maximum value of 0.61 at the 36WC±54VC±10Co (y 0.6) composition. The addition of 8 g 7 resulted in the formation of a c-(W x V y )C solid solution phase inhibiting the appearance of g-(W x Co y )C grains which embrittles the body. Although the indentation fracture toughness value of 18 MPa m 1a2 obtained for y 0.6 is acceptable, it is advisable to strictly control the predominant c±(W x V y )C double carbide grain size in order to achieve higher hardness values. Ó