Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide (original) (raw)
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Reduction of carbide grain growth in WC–VC–Co by sintering in a nitrogen atmosphere
International Journal of Refractory Metals and Hard Materials, 2009
In order to produce a harder and more wear resistant material than WC-Co, a WC-VC-Co hardmetal was sintered in nitrogen at a pressure of 1 bar to reduce the growth of (W,V)C grains created during sintering. The hardmetal was analysed using scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffractometry (XRD). The cubic carbonitride (W,V)(C,N) grains were found to be of two types, those with higher vanadium content and the others with lower vanadium content. The presence of nitrogen restricted the growth of the cubic carbide grains giving a much narrower grain size distribution than in a material produced from an identical powder but sintered in vacuum. The nitrogen potential was higher in the bulk than in the sintering atmosphere resulting in outward diffusion of nitrogen and inward diffusion of V. This created a gradient zone of approximately 50 lm depleted of cubic carbonitride and enriched with binder phase. Remarkably, the WC grain size in the gradient zone was not larger than in the bulk.
Sintering of binderless tungsten carbide
Ceramics International, 2010
The most widely used mold materials for optical glass molding processes are cemented tungsten carbide and silicon carbide. In this research, tungsten carbide with minor addition of TiC and TiN as the second phase has been studied. The powders were ball-milled and pre-formed under a temperature of 200 8C and a pressure of 130 MPa. The specimens were sintered in a graphite lined furnace at a temperature of 1600 8C. A density of 15.43 g/cm 3 , a Vickers hardness number of 23.14 GPa, and a fracture toughness of 6.56 MPa m 1/2 was found for the sintered specimen fabricated by this process. The result of X-ray analysis revealed no trace of precipitated graphite during sintering, nor the brittle eta-phase as a result of decarburization. Through scanning electron microscopy, spherical air bubbles have been found to precipitate inside the grains, because the activation energy for grain-boundary diffusion is lower than that of the air inside the grains. Therefore it is advisable that the pre-form process is carried out in vacuum. Crown
VC, Cr 3C 2 and NbC doped WC–Co cemented carbides prepared by pulsed electric current sintering
International Journal of Refractory Metals & Hard Materials, 2007
WC-12 wt.% Co grade cemented carbides doped with 0.9 wt.% VC, NbC or Cr 3 C 2 grain growth inhibitor were consolidated by pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), in the solid state at 1240°C for 2 min. The microstructure and properties of the PECS material grades are compared with those of pressureless sintered grades, liquid phase sintered at 1420°C for 1 h. Microstructural and hardness characterization revealed that both the chemical composition and sintering technique play an important role on the WC grain growth and Wnal mechanical properties. To obtain a nanometer sized WC-Co microstructure, it is essential to carefully select the grain growth inhibitor in addition to the application of a fast thermal densiWcation cycle by means of spark plasma sintering.
The effect of consolidation parameters on the mechanical properties of binderless tungsten carbide
International Journal of Refractory Metals and Hard Materials, 2011
This paper discusses the effect of the process parameters on the mechanical properties of binderless pure tungsten carbide during a GPS (gas protection sintering) process. The result of experiments reveal that the mechanical properties of the material increases with raising the sintering temperature and extending the retention time; however a decreased hardness was observed as a result of abnormal grain growth under higher sintering temperatures. The results of XRD and EDS analyses confirmed the absence of brittle phases such as W 2 C or impurity phases in the microstructure. The optimized process parameters for GPS process are identified as: a mean particle size of 1.03 μm, a sintering temperature of 1860°C and a retention time of 60 min; the resulting mechanical properties are: a relative density of 95.1%, a micro-hardness of 1718 kgf/ mm 2 and a fracture toughness of 5.97 MPa m 1/2. The width of particles size distribution has a significant effect on the density and hardness of the sintered material however the width of particles size distribution is dependent on the original particle size. Finally, ultra-fine particles increase the chance of conglomeration and sub-micron structures. The conglomeration of ultra-fine particles hinders the filling of porosities during sintering and lowers the density and hardness of the material.
International Journal of Refractory Metals and Hard Materials, 2018
Tungsten is a refractory metal, suitable for a broad range of high-temperature applications, due to its high melting point and high-temperature strength. Recently, tungsten alloys became of great interest in thermonuclear fusion research. The alloys maintain low sputtering yield and high melting point, but greatly improve some drawback properties comparing to pure tungsten. These alloys are commonly prepared by mechanical alloying and compacted using Spark Plasma Sintering/Field Assisted Sintering. Powder in a graphite die is consolidated by applying pulsed electric current and pressure. Graphite foil is commonly placed between the die and the powder, to prevent adhesion. However, for number of applications it is necessary to avoid carbon contamination. In the present study we have examined carbon contamination mechanism of W-10 wt.%Cr-1 wt.%Hf alloy during Field Assisted Sintering along with the alloy's properties. The experiments have shown that the graphite foil leads to significant dealloying via mechanism of liquid phase sintering. As a result, tailored properties of the material are deteriorated after several minutes of the consolidation process. Therefore, application of carbon diffusion barrier was proposed, examined and compared with the alloy sintered with graphite foil.
Microstructures of binderless tungsten carbides sintered by spark plasma sintering process
Materials Science and Engineering: A, 2003
Pure WC was sintered by spark plasma sintering (SPS) process for a binderless cemented carbide application. The relative density of spark plasma sintered WC was over 98% when the SPS temperature was 1700 8C under 50 MPa pressure. Grain growth of WC could be suppressed with full densification by shortening sintering time. When the initial WC powder size was varied from 0.57 to 4.06 mm, the sintered density decreased with decreasing WC powder size. However, WC with an initial powder size of 0.57 mm could be sintered to densification by addition of free carbon. The higher amount of surface oxide in the finer WC powder is considered to have caused decarbonization during the sintering process. For WC powders of 4.06 mm, abnormal grain growth occurred when the sintering time was over 1 min if the sintering temperature was over 1700 8C. Also, with the sintering temperature and sintering time where no abnormal grain growth occurred, abnormal grain growth occurred by addition of carbon. This result shows that the abnormal grain growth in WC sintered by SPS process can be controlled by carbon addition. The abnormal grain growth in sintered WC somewhat increased fracture toughness by crack deflection. #
An investigation into the effects of HIP after sintering of WC-ZrC-Co-Cr3C2 cemented carbides
International Journal of Refractory Metals and Hard Materials, 2019
The sintering behaviour of cemented carbides based on WC-ZrC-Co-Cr 3 C 2 powder mixtures have been analyzed by dilatometric and calorimetric methods for different cobalt contents and WC/ZrC ratios. As expected, powder oxide reduction in these compositions is mainly of carbothermic nature. However, depending on the milling conditions, some highly stable Zr-rich oxides are retained in the binder phase after sintering. Hot isostatic pressing (HIP) cycles have been successfully applied for closing residual porosity after vacuum sintering. For a fixed amount of binder phase and a WC/ZrC ratio, the hardness of these materials depends on the amount of residual porosity and WC grain growth control. The best combination of hardness and toughness is found for alloys with 8 wt%Co and WC/ZrC wt. ratios of 6.46. HIP treatments induce the formation of a compact and well adhered layer mainly comprised of Zr oxides and WC grains. The cobalt binder phase migrates from this layer towards the sample bulk likely due to the loss of wettability on these Zr rich oxides. Hot hardness is higher for the alloy with higher WC/ZrC ratio suggesting that this property depends on both the volume fraction of (Zr x W 1-x)C and WC phases and their degree of contiguity.
Materials and Manufacturing Processes, 2014
WC powders with an average crystallite size of 10nm were successfully prepared by ball milling of micron-sized tungsten carbide powders. Grain growth inhibitors (VC and Cr 3 C 2 ) with concentrations of 0.6 wt% each were added to nanocomposites of WC-9Co and WC-12Co, in both as-received and milled WC.Powder mixtures were then consolidated using spark plasma sintering technique (SPS) at 1200 0 C and 1300 0 C for 10 min under high vacuum and pressure of 50 MPa. The influence of WC crystallite size, Co content, and sintering temperature over microstructure and mechanical properties of the resulting compositeswere studiedthrough XRD and FESEM. Densification and attained grain sizes of the sintered products were measured by Archimedes principle and Scherrer procedure, respectively. Moreover, micro hardness (H v 30) and fracture toughness were measured and compared for each composition to comparatively assess the individual effect. It was observed that the addition of VC and Cr 3 C 2 resulted in decreased densification of the synthesized composites. These grain growth inhibitors were found to limit grain sizes to 131nm with an average hardness of 1592 H v30 and fracture toughness of 9.23 Mpam 1/2 .
The influence of sintering in nitrogen gas on the microstructure of a WC–VC–TiC–Co cemented carbide
International Journal of Refractory Metals and Hard Materials, 2008
This paper investigates the effects of nitrogen-sintering on the V-rich cubic phase grain size in a material with large additions of VC and TiC. Nitrogen transport is made the rate determining factor for the retardation of grain coarsening during sintering. A compacted sample comprising TiC, VC, WC and Co powders was sintered in 1 bar nitrogen gas.
Ultrahigh Temperature Flash Sintering of Binder-Less Tungsten Carbide within 6 s
Materials
We report on an ultrarapid (6 s) consolidation of binder-less WC using a novel Ultrahigh temperature Flash Sintering (UFS) approach. The UFS technique bridges the gap between electric resistance sintering (≪1 s) and flash spark plasma sintering (20–60 s). Compared to the well-established spark plasma sintering, the proposed approach results in improved energy efficiency with massive energy and time savings while maintaining a comparable relative density (94.6%) and Vickers hardness of 2124 HV. The novelty of this work relies on (i) multiple steps current discharge profile to suit the rapid change of electrical conductivity experienced by the sintering powder, (ii) upgraded low thermal inertia CFC dies and (iii) ultra-high consolidation temperature approaching 2750 °C. Compared to SPS process, the UFS process is highly energy efficient (≈200 times faster and it consumes ≈95% less energy) and it holds the promise of energy efficient and ultrafast consolidation of several conductive re...