WC Grain Growth Research Articles

Two-step hot-pressing sintering of nanocomposite WC–MgO compacts

Two-step hot-pressing sintering (TSS) was applied to consolidate nanocomposite tungsten carbide–magnesia (WC–MgO) powders. The first step sintering was employed at a higher temperature to obtain an initial high density, and the second step was held at a lower temperature by isothermal sintering for several hours to increase bulk density without significant grain growth. The experimental results showed the sintering temperature plays an important role in densification and grain growth of WC–MgO compacts. The optimum TSS regime consisted of heating at 1750 °C (1st step) and 1550 °C (2nd step), resulting in the formation of near full dense microstructure (0.99 TD) with suppressed grain growth (2.59 μm). Accordingly, the improvement on the mechanical properties, including increase in the hardness (from 16.7 to 18.4 GPa), fracture toughness (from 10.2 to 12.95 MPa m 1/2) and flexural strength (from 976.6 to 1283.7 MPa), was also observed due to the grain refining and full dense bulk.

Effects of Cr<SUB>3</SUB>C<SUB>2</SUB> and V<SUB>8</SUB>C<SUB>7</SUB> on the Microstructure and Mechanical Properties of WC-SiC Whisker Ceramics

WC-SiC whisker ceramics (5 vol% SiC) containing Cr 3 C 2 and/or V 8 C 7 with no metallic binders were prepared by resistance-heated hot pressing at 1650°C and were examined with regard to sinterability, microstructure and mechanical properties. The addition of Cr 3 C 2 or V 8 C 7 brought about an increase or decrease in the sinterability. For equal molar amounts of Cr and V, V 8 C 7 more strongly inhibited the growth of WC grains than Cr 3 C 2 . The hardness of the ceramics was dependent on the WC grain size which was influenced by the nature or quantity of the additives. The hardness values are consistent with the Hall-Petch-type relationship between hardness and WC grain size estimated for WC phases in WC-Co cemented carbides.

Preparation of WC-SiC Whisker Composites by Hot Pressing and Their Mechanical Properties

Composites of WC and SiC whisker (0–30 vol% SiC) were prepared at 1550 to 1800C by using a resistance-heated hot pressing technique called spark plasma sintering. The composites obtained were examined for reaction products and microstructure, and were characterized for mechanical properties. The addition of small amounts of SiC induced the marked grain growth of WC, and drove the densification of WC. Above 10 vol% SiC, dispersed SiC phases impeded the grain growth of WC. Increasing sintering temperature made SiC whiskers thick and lowered the aspect ratio of whiskers. The hardness of composites decreased with increasing average grain size of WC. The Young’s modulus of dense composites was decreased with the SiC content. The addition of 3 to 5 vol % SiC greatly increased the fracture toughness of WC.

Abnormal grain growth of WC with small amount of cobalt

Abnormal grain growth (AGG) often appears in the synthesis of cemented tungsten carbide (WC), especially where ultra-fine powder is used. In this study, it was observed that AGG was strongly affected by the amount of Co. The obvious AGG was found to occur when the amount of Co was between 0.2 and 0.9 wt%. Careful examination of the etched samples found crack-like defects in abnormal grains, which were indications of grain boundaries in these large grains. Therefore, it is suggested that the considerable AGG of WC at lower Co concentrations be attributed to a grain boundary re-entrant edge (GBRE)-assisted two-dimensional nucleation process. Grain coalescence during sintering was the source of the grain boundaries. Due to the selective wetting of (0001) WC planes with the Co liquid, those grain boundaries were parallel to the (0001) planes resulting in platelet-like large grains with high aspect ratio. The amount of Co would affect the number of the wetted (0001) planes and the probability of grain boundary formation, and hence affect the AGG. The correspondence of the calculated weight percentage of Co required surrounding all the planes of WC grains and the experimental upper limit of the range supported this mechanism.

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr3C2-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr3C2 was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.

Grain Growth, Microstructure and Property of Ultrafine WC-Co Alloy by Spark Plasma Sintering

In order to control WC grain size and get a microstructure with fine grain size during the sintering process, WC grain growth in WC-Co cemented carbide was investigated. The possible reason on the grain growth was showed which includes the normal grain growth (NGG) and the abnormal grain growth (AGG). We adopted the SPS (spark plasma sintering) to sinter nanometer WC-Co powder, the range of temperature was 1100 -1250 °C, the pressure was 40MPa, and sintering time was 5min. In this paper, we discuss the density, hardness, microstructures and grain sizes of the sintered samples. The results showed that during the process sintering WC-Co powder, there are two kinds of growths. XRD investigation of the WC–Co samples spark plasma sintered to full density revealed that no new phase was created. Through the observed microstructure of the sintered samples, we analyzed the reason of the abnormal grain growth of WC grain. The density and hardness of samples were increased with increasing temperature.

Effects of Cr3C2 additions on the densification, grain growth and properties of ultrafine WC–11Co composites by spark plasma sintering

Abstract Cemented carbide powders based on WC–11Co with a grain size of about 250 nm added various extents of Cr 3 C 2 were consolidated to full density by spark plasma sintering (SPS). The density, hardness and fracture toughness of the SPS consolidated samples were measured and the microstructure, fracture surfaces and grain size were observed by SEM. XRD investigation of the spark plasma sintered WC–Co samples revealed that no new phase formed. The WC–11Co composite powder were fully densified within 5 min by SPS at the temperature of 1200 °C, which is much lower temperature than that of conventional liquid phase sintering, under a pressure of 40 MPa. Addition of grain growth inhibitors Cr 3 C 2 reduced the grain size to 435 nm, but led to the lower density. The adding amount of Cr 3 C 2 should be controlled within a certain range, and the samples adding the appropriate proportion of Cr 3 C 2 can obtain higher hardness and toughness.

Formation of Twinned WC Grains during Carbonization of Eta Phase (W<sub>3</sub>Co<sub>3</sub>C)

Twinned WC grains are sometimes observed in WC powder and sintered WC-Co alloys. The present investigation has studied the formation of twinned WC grains during carburization of an Eta phase. Eta grains were carburized at 700-1450°C for 1 min to 9 h. Twinned WC grains formed during the carburization. Crystallographic characterization of the formed twins were made using SEM and TEM. The formation of twins was found to be affected by the carbon activity during carburization. The twins formed under high carbon activities while no twins formed under low carbon activities. Two kinds of twins with different orientations were observed. The present experimental observation suggests that the twins formed via 2-dimensional nucleation and layer-bylayer growth on small WC clusters under high supersaturation and high driving force for the growth of WC grains.

Effects of AGG on fracture toughness of tungsten carbide

The cobalt-bonded tungsten carbide has been regarded as a specific success in improving the fracture toughness of ceramics. However, the metal binder phase lowers the hardness and corrosion resistance of the composite and limits its applications. In this paper, the possibility of in situ-toughening tungsten carbide, making use of abnormal grain growth of WC grains to reinforce WC–Co composites, is investigated. The volume fraction of abnormally large grains in the samples is controlled by tailoring compositions and sintering conditions. The fracture toughness of the samples is evaluated using the indentation technique. The experiment results show that the fracture toughness of the sample could be improved by introducing a certain volume fraction of abnormal grains, but at some expense of the hardness. The highest value of fracture toughness, 7.34 MPa m 0.5, is achieved for WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 °C for 4 h. The main mechanisms for the improved toughness are cutting elongated grains, crack bridging and crack deflection, which could resist crack growth in the sample. Therefore, it is possible to in situ-toughen tungsten carbide using the abnormal grain growth of WC.

Effects of lower cobalt binder concentrations in sintering of tungsten carbide

Cemented tungsten carbides have received much attention because of their superior characteristics. Traditional cemented tungsten carbides usually contain 3–30 wt% binder phase. In this paper, WC with low Co concentration less than 3 wt% is studied using traditional powder metallurgy. The binder phase has tremendous effect on sinterability of WC. High sinterability and high hardness can be achieved for the WC (0.7 μm) with 0.5 wt% Co. Abnormal grain growth (AGG) is often observed in sintering WC with small amount of Co. It seems that AGG is affected by the concentration of Co and a range of Co concentrations may exist for the large amount of AGG. To control the grain size, VC is added to inhibit the grain growth of WC. It is observed that the hardness is affected by the amount of addition of VC. Controlling the ratio of C/W less than unity at low Co concentrations will result in the production of W 2C phase. The hardness of WC–Co is affected by the amount of W 2C phase in the sample and W 2C is stable during the normal cooling process.

Microstructural evolution and mechanical behaviors of graded cemented carbides

Graded cemented carbides have been developed by pre-sintering carbon-deficient compacts and subsequently carburizing in this work. The microstructures and the mechanical behaviors of the graded cemented carbides were studied. The formation mechanism of the graded structure can be attributed to the Ostwald ripening induced by the carbon diffusion. The difference of the Ostwald ripening rate in areas of different carbon contents leads to the variation in the WC grain-packing structure, hence driving the flow of the liquid Co from the surface to interior. Theoretical results based on the carbon diffusion in the liquid Co match well with the experimental measurements. The graded cemented carbides possess a good combination of high hardness and high toughness, and indicate an excellent performance in the engineering test. The results also show that a high carburizing temperature and a long soaking time lead to an excessive growth of WC grains in the outer layer, which will block the further diffusion of carbon into the alloy body, and will deteriorate the mechanical behaviors as well.

A research on the grain growth of WC–Co cemented carbide

In order to control WC grain size and get a microstructure with fine grain size during the sintering process, WC grain growth in WC–Co cemented carbide was researched. The research includes theoretical calculation and experiment. Through analysis, in the sintering process the important factors are the sintering temperature and time. The results of the calculation and experiment showed that the grain size increases with the high temperature and long time. A quantitative relationship between WC grain size sintered and sintering temperature and time was preliminarily obtained and Grain Growth Diagram (GGD) for WC–Co system was firstly built.

Carbon Nanotubes Strengthened Nanophase WC‐Co Hard Alloys

A novel approach employing C2H2 instead of CO as carburization agent has been applied to fabricate nanophase WC‐Co composite alloys being strengthened by carbon nanotubes and nanorods, which were formed in‐situ during C2H2 carburization process. The sintered composite alloys show exceptionally high microhardness. Carbon nanotubes are supposed to play roles on strengthening the matrix and prohibiting growth of WC grains, thus greatly improving the mechanical properties of the matrix.

Microstructure and Mechanical Properties of W-C-B Ceramics Containing VC

We prepared WC-based W-C-B ceramics by reactive resistance-heated hot pressing at 1600 to 1800°C from a B 4 C-5W-80WC (molar ratio) powder mixture containing a small amount of VC, and examined the effect of VC on sinterability and the mechanical properties of the ceramics. The sintered bodies were composed mainly of WC and small quantities of W 2 C, WB, W 2 B and VC. Slight amounts of V were detected at the boundaries between WC grains. VC raised the sintering temperature necessary for obtaining dense bodies, and strongly suppressed the growth of WC grains. Young's modulus for the dense bodies was slightly smaller than that without VC because of the low modulus value of VC The suppressive effect of VC on the growth of WC grains avoided reducing hardness with increasing sintering temperature.

Influence of NbC-Addition on Mechanical Properties of WC-Co

Cemented carbides have been intensively used as cutting tool through their high hardness, high fracture toughness and high wear resistance. A considerable amount of works has been developed in order to improve the mechanical properties of alternate cemented carbide systems. This work has the purpose to reports the first results obtained to WC-Co reinforced with 5 wt.% NbC. The mixture of powders was hot-pressed at 1250 °C in a inert atmosphere. Hardness and fracture toughness were carried out in a Vickers hardness testing machine. The results have showed that the addition of niobium carbide improves the hardness of tungsten carbide and inhibits the WCgrain growth.

Growth behavior of rounded (Ti,W)C and faceted WC grains in a Co matrix during liquid phase sintering

Growth behavior of two different types of grains, faceted and rounded, in the same liquid matrix has been studied in the 70(25TiC–75WC)–30Co (wt%) system. Powder samples were sintered above the eutectic temperature for various times under a carbon saturated condition. (Ti,W)C grains with a rounded shape and WC grains with a faceted shape coexisted in the same Co-based liquid. With increasing sintering time the average size of (Ti,W)C grains increased continuously and very large WC grains appeared. The growth of rounded (Ti,W)C grains followed a cubic law indicating diffusion-controlled growth. On the other hand, the growth of the faceted WC grains resulted in a bimodal grain size distribution, showing abnormal grain growth. With increased initial size, however, abnormal growth of WC grains was suppressed. This growth behavior of WC grains could be explained using growth theories of faceted crystals and was also confirmed by a calculation using their growth equations. The present investigation thus shows that the growth behavior of one type of grain is not affected by the other type of grain in the same matrix and is governed only by whether the grain shape is faceted or rounded.

An experimental study of the sintering of nanocrystalline WC–Co powders

Since nanocrystalline WC–Co powder was produced over a decade ago, the sintering of nanocrystalline powders remains a technological challenge. The goal of sintering nanocrystalline powders is not only to achieve full densification but also to retain nanocrystalline grain sizes. This is difficult because of rapid grain growth at high temperatures. Previous studies on the sintering of nanocrystalline WC–Co have shown that grains grow rapidly during the early stage of sintering. But there are few studies on the mechanisms of grain growth and densification during this stage. This paper presents the results of an experimental investigation on grain growth and densification of nanocrystalline WC–Co powders during heat-up at equilibrium solid-state temperatures. The results have shown that nanocrystalline WC grains grow rapidly during this period concurrently with rapid densification. The rapid densification and grain growth are partially attributed to the surface energy anisotropy of tungsten carbide. The effects of vanadium carbide on grain growth at solid state during heat-up are also discussed.

Effects of Carbon Nanotubes Incorporation on the Grain Growth and Properties of WC/Co Nanocomposites

Carbon nanotubes (CNTs) combining unique mechanical and physical properties could offer a kind of nanosized reinforcements for composite materials. Incorporating of CNTs to develop advance engineering composites has become an interesting concept, but the cermets based CNTs composites have been less focused. WC-Co-CNTs nanocomposites were consolidated by spark plasma sintering (SPS) to investigate the effects of CNTs incorporation on the grain growth and mechanical properties of WC-Co nanocomposites. Experimental results show that CNTs could preserve their tubular structures in high temperature SPS process, some CNTs are surviving in the WC-Co fracture surfaces featured bridging and pulling out manner. The interaction between the CNTs and the matrix has a retardation effect of grain growth of WC, but CNTs additions could be resulted in an increase of carbon content in the binder phase that causes enhanced tendency of grain growth, either of which plays the dominated role depending on the CNTs content. The WC-10Co-0.5wt%CNTs nanocomposites possess superior hardness to toughness combinations, which hardness is about 15% and fracture toughness is about 40% higher than that of the pure nano-WC-10Co cermets consolidated under the same process.

The effect of phosphorus additions on densification, grain growth and properties of nanocrystalline WC–Co composites

To control the grain growth of nanocrystalline WC–Co cemented carbides by phosphorus (P) doping was studied in spark plasma sintering (SPS). Experimental results show that the use of small amount of phosphorus dopant to chemically-activated sintering of the nanocrystalline WC–Co composites could enhance the sinterability, promote the low temperature densification, and decrease the sintering temperature of the WC–Co nanocomposites. With only 0.3wt.% P additions, full density WC–7Co cermets were obtained at temperature as low as 1000°C, which is 100°C lower than that of the undoped counterparts in spark plasma sintering of the nanocomposites. By phosphorus doping in WC–Co nanocomposites, the grain growth of WC–Co cemented carbides could be inhibited to some extent, as a result WC–7wt.% Co–0.3wt.% P cemented carbides with average WC grain size of 200nm were achieved, which can be attributed to the lower sintering temperature resulting in a decrease of grain boundary mobility. Mechanical properties measurements show that P doping could lead to a great increment of hardness, but at the same time a little sacrifice of fracture toughness of the nano-WC–Co cemented carbides.

Co 액상 내에 공존하는 (Ti,W)(C,N)과 WC입자의 성장 거동

Growth behavior of two different types of grains, faceted and rounded, in a liquid matrix has been studied in the (75WC-25TiCN)-30Co system. Powder samples were sintered above the eutectic temperature for various times under a carbon saturated condition. (Ti,W)(C,N) grains with a rounded shape and WC grains with a faceted shape coexisted in the same Co based liquid. With increasing sintering time, the average size of (Ti.W)(C,N) grains increased continuously and very large WC grains appeared. The growth of rounded (Ti,W)(C,N) grains followed a cubic law, <TEX>r^3-r^3_0$</TEX>=kt, where r is the average size of the grains, <TEX>$r_0$</TEX> the initial average size, k the proportionality constant and t the sintering time. indicating a diffusion-controlled growth. On the other hand, the growth of the faceted WC grains resulted in a bimodal grain size distribution, showing an abnormal grain growth. These observations show that the growth behavior of different types of grains is governed by their shape, faceted or rounded, even in the same liquid matrix.