WC Grain Growth Research Articles

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co, Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150°C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr3C2 in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.

Preparation and sintering of WC–Co composite powders for coarse grained WC–8Co hardmetals

Due to high chemical and geometrical irregularities of WC grain and high uncertainty of WC grain growth, it is hard to prepare coarse grained WC–Co hardmetals with high performance through sintering of WC–Co ball milling powder. A new method for preparing of WC–8Co composite powder is put forward, which merges the preparation of Co powder with the mixing of WC–Co. The compositions, phases, and shapes of WC–8Co composite powder were investigated by chemical element analysis, X-ray diffractometry (XRD), and scanning electron microscopy (SEM), respectively. The phase and shape of raw WC powder do not change during the preparation of WC–8Co composite powder. Microstructures and mechanical properties of WC–8Co hardmetals prepared by sintering of the composite powders were compared with those of WC–8Co hardmetals prepared by sintering of the ball milling powders. The results show that the new coarse grained WC–8Co hardmetals have a better performance than conventional WC–8Co hardmetals for their better uniformity of WC grains. The comparison of microstructures and mechanic properties of sintered alloys also confirms that VC further inhibits the growth of WC grain and improves the performance of WC–8Co hardmetals.

Phase Composition and Microstructure of Binderless WC-ZrC Cemented Carbides Fabricated by Spark Plasma Sintering

Binderless WC-based cemented carbides with different fractions (0-9 wt.%) of ZrC nanopowder were fabricated through spark plasma sintering at 1600 °C under a uniaxial pressure of 50 MPa. The addition effect of ZrC nanopowder on the phase composition and microstructure of the fabricated materials were explored with the help of X-ray diffraction and scanning electron microscope. The results indicated that W2C phase was detected in the samples with 0-3 wt.% ZrC nanopowder, but with further increase in ZrC added fraction, ZrO2 phase instead of W2C phase was detected. The apparent density decreased gradually with the increase in added fraction of ZrC nanopowder, while the relative density increased initially and then decreased, reaching its maximum of about 98.2% when the added fraction of ZrC nanopowder was about 3 wt.%, indicating that appropriate added fraction of ZrC nanopowder can improve the densification of binderless WC cemented carbides. Without ZrC nanopowder, the coarsening and abnormal growth of WC grains were serious, resulting in many large prismatic WC grains in the samples. However, Such phenomena could be suppressed by adding ZrC nanopowder, resulting in much finer and more homogenous microstructure after 1-3 wt.% ZrC nanopowder was added. When the added fraction of ZrC nanopowder was higher than 3 wt.%, the agglomeration of ZrC nanopowder became more and more serious.

The Effect of VC on the Sintering Ability and Microstructure of WC-Al<sub>2</sub>O<sub>3</sub> Composites

The effects of doping with VC (grain growth inhibitors) on the microstructural characteristics and mechanical properties of WC-Al2O3 compacts prepared by hot-pressing sintering were investigated. VC suppressed the rate of densification during isothermal sintering, but eventually increased the sintered density with extended holding time. Microstructure observation results revealed that the grain growth of WC was significantly retarded with the addition of VC. Meanwhile, the activation energy for grain growth during isothermal sintering increased from 380.75 to 446.68 kJ/mol, which is consistent with the inhibition mechanism of VC.

Spark Plasma Sintered WC-Ni Cemented Carbides with Various Contents of ZrC Nano-Powder

WC-10Ni cemented carbides with various contents (0-9 wt.%) of ZrC nanopowder were fabricated by spark plasma sintering at 1350 °C with a pressure of 50 MPa. The phase composition, microstructure and mechanical properties of the as-prepared samples were investigated. X-ray diffraction analysis revealed that excepting WC hard phase, a solid solution phase Ni (W,Zr) with different amounts of individual metal was formed with increasing ZrC nanopowder content. Scanning electron microscopy examination indicated that a proper addition content of ZrC can suppress the abnormal growth of WC grains and improve the relative density of WC-Ni cemented carbides. However, with further increase in the content of ZrC (more than 7 wt.%), the agglomeration of ZrC became more and more serious. The samples with 5 wt.% ZrC nanopowder possess a relative higher flexural strength (~1750 MPa) among all the investigated samples. When the addition content of ZrC nanopowder was 3 wt.%, the Vickers hardness of the samples reached its maximum value (~1810 HV10).

Zirconia-toughened WC with/without VC and Cr3C2

Abstract The effects of Cr 3 C 2 and VC on densification behavior, grain growth and mechanical properties of the WC-8 wt% ZrO 2 composites were investigated, and the relationship between the mechanical properties and microstructure was studied. The addition of VC and Cr 3 C 2 not only retarded densification of the WC–ZrO 2 material, but also significantly suppressed the growth of WC-grains and ZrO 2 -particles. For the WC–ZrO 2 composite without VC and Cr 3 C 2 , microstructure coarsening at elevated temperatures caused degradation in hardness, whereas the hardness maintained a high value of ~22 GPa for the composite with VC and Cr 3 C 2 additives as the microstructure remained fine. The fracture toughness of the WC-based materials toughened by 8 wt% ZrO 2 seemed insensitive to the coarseness of microstructure, and achieved a value of ~11 MPa m 1/2 when VC and Cr 3 C 2 were added.

Cr3C2 and VC doped WC–Si3N4 composites prepared by spark plasma sintering

Cr3C2 and VC doped WC–10wt.% Si3N4 composites were prepared by spark plasma sintering. The influences of Cr3C2 and VC on densification behavior, grain growth and mechanical properties of the WC–Si3N4 composites were investigated. During densification of the WC-based materials, the Cr3C2 and VC additives assist the preliminary densification of solid-state-sintering, and mitigate the subsequent densification of liquid-sintering. The addition of Cr3C2 and VC shows an inhibitory effect against WC-grain growth, resulting in higher hardness for the WC–Si3N4 composites. Moreover, the inhibitors do not hinder the transformation of α to β-Si3N4 and the growth of elongated β-Si3N4 grains, which act as toughening agent for the composites.

Ultrafine binderless WC-based cemented carbides with varied amounts of AlN nano-powder fabricated by spark plasma sintering

Ultrafine binderless WC-based cemented carbides with varied amounts of AlN nano-powder (0–16wt.%) were fabricated by spark plasma sintering at 1600°C under a pressure of 50MPa with the assistance of VC and TaC as WC grain growth inhibitors. The densification behavior, phase composition, microstructure and mechanical properties of the as-prepared samples were investigated. During sintering, rapid sample shrinkage started at about 1050°C and ended at about 1550°C. X-ray diffraction analysis revealed that an appropriate addition amount of AlN nano-powder would be of help to limit the formation of W2C phase and promote the formation of solid solution phase of (V,W)C2 during sintering. As the added fraction of AlN nano-powder increased, the relative density of the samples initially increased and then decreased, reaching its maximum of about 99.6% when 3–5wt.% AlN nano-powder was added. Scanning electron microscopy examination indicated that the average size of WC grains in the samples was about 0.7μm and with increasing added fraction of AlN nano-powder, the abnormal growth of WC grains was suppressed and their sizes were somewhat reduced. Both the sample hardness and flexural strength first increased and then decreased with increasing added fraction of AlN nano-powder, reaching the maximum hardness of about 2400 HV10 with 3wt.% AlN nano-powder and maximum flexural strength of about 1250MPa with 5wt.% AlN nano-powder, respectively. However, the fracture toughness slightly decreased from 7.5 to 6.3MPa·m1/2 with increasing added fraction of AlN nano-powder.

Effect of VC, TaC and NbC Additions on Microstructure and Properties of Ultrafine WC-10%Co Cemented Carbides

The ultrafine WC-10%Co cemented carbides were prepared by vacuum sintering processing. To inhibit the growth of WC grains during sintering, single VC, TaC and NbC were used as inhibitors. Effects of VC, TaC and NbC as grain growth inhibitor on the microstructure and properties of ultrafine WC-10%Co cemented carbides were investigated with different kinds of testing methods. With the transformation of doped inhibitors content, the results show that VC, TaC and NbC additions can refine the WC grains, and increase the mechanical properties of the WC-10%Co cemented carbides, respectively. And the efficiency of these three kinds of inhibitors to limit gain size has been compared after sintering for 1.5h at 1 400 °C.

The Study of Co-Doped NbC / Cr<sub>3</sub>C<sub>2</sub> on the Application of Ultrafine WC-10%Co Cemented Carbides

In this paper, ultrafine WC-10%Co cemented carbides were prepared by vacuum sintering. For inhibition of growth of WC grains during sintering, NbC and Cr3C2 were used as inhibitors. With the transformation of co-doped inhibitors content, the grain growth is inhibited and the grain size achieves 490 nm at 0.4% Cr3C2 / 0.3% NbC. The mechanism that co-doped NbC / Cr3C2 on the effective reduction of ultrafine WC-10%Co cemented carbides was analyzed. The 0.4% Cr3C2 / 0.3% NbC addition equipped with the best integrated mechanical properties was obtained in this study. And sintered samples were investigated with scanning and transmission electron microscopy (SEM).

Mechanical properties and erosion resistance of ceria nano-particle-doped ultrafine WC–12Co composite prepared by spark plasma sintering

Ultra-fine grained WC–Co composites possess higher hardness or strength and wear resistance, compared to their coarse counterparts. However, abnormal grain growth of ultra-fine WC particles often occurs during the traditional pressureless liquid sintering, which substantially impairs the mechanical properties and erosion resistance of the material. Thus, controlling the abnormal grain growth becomes one of the key issues in fabricating ultra-fine grained cemented carbides. In this study, ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. The results demonstrated that trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to uniform and fine microstructures. Such ultra-fine grained WC–Co composites have both improved hardness and fracture toughness, resulting in enhanced resistance to high-speed solid-particle erosion. However, when the added nano-ceria was more than 0.1wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance.

Microstructure and mechanical properties of hot-pressed WC–MgO composites with Cr3C2 or VC addition

The influence of Cr3C2 and VC addition on the microstructure and mechanical properties of WC–MgO composites hot-pressed at 1650°C for 90min was comprehensively investigated. The grain growth of WC was significantly retarded and the homogeneity of MgO particulate dispersion was effectively improved with the addition of 0.5wt.% Cr3C2 or 0.5wt.% VC. The indentation size effect (ISE) on hardness was restrained and the load-independent hardness was increased by doping grain growth inhibitors. Improvements on fracture toughness of hot-pressed samples were also observed due to the refined WC grains and uniformly dispersed MgO particulates. In addition, experimental results demonstrated that Niihara's equation was preferable for estimating the indentation fracture toughness, by comparing the fracture toughness evaluated using the single-edge V-notch beam (SEVNB) method with the values estimated through the Vickers indentation technique.

In-situ elongated β-Si3N4 grains toughened WC composites prepared by one/two-step spark plasma sintering

The WC–Si3N4 composites toughened by in-situ elongated β-Si3N4 grains were prepared by spark plasma sintering. By exploiting the difference in kinetics between WC grain-growth and β-Si3N4 grain-growth through sintering at an elaborate temperature or two-step sintering, composites with microstructure of in-situ elongated β-Si3N4 grains were grown sufficiently in the fine-grained WC matrix without fast-grain-growth being obtained. The relation of the microstructure and mechanical properties of the composites, as well as the toughening mechanisms, are investigated. Comparing with the fracture toughness of 6.69MPam1/2 for the pure WC, that of the obtained WC–Si3N4 specimen reaches 10.94MPam1/2.

Optimization of Processing Parameters for WC-11Co Cemented Carbide Doped with Nano-Crystalline CeO2

Nine types of trace nano-CeO2-doped WC-11Co cemented carbides designed by orthogonal method were prepared. The variance analysis results show that the relative density increases with increasing sintering temperature in the range of 1400-1450 °C; the hardness is highly dependent on grain size of WC which is closely related to the particle size of the initial WC powder and grain refinement produced by nano-CeO2; the bending strength of WC-11Co cemented carbide with less than 0.15 wt.% of nano-CeO2 is effectively improved because of its effect on the decreasing of porosity and inhibiting the grain growth of WC and martensitic phase transformation of Co; the fracture toughness rises at the beginning then drops later if the particle size of initial WC powder is in the range of 3-11 μm.

VC and Cr3C2 doped WC–MgO compacts prepared by hot-pressing sintering

The effects of doping with VC and Cr3C2 (grain growth inhibitors, GGIs) on the microstructural characteristics and mechanical properties of WC–MgO compacts prepared by hot-pressing sintering were investigated. GGIs suppressed the rate of densification during isothermal and non-isothermal sintering, but eventually increased the sintered density with extended holding time. Microstructure observation results revealed that the grain growth of WC was significantly retarded, and the homogeneity of MgO particulate dispersion and particulate/matrix interfacial coherence were improved effectively with the addition of GGIs. Meanwhile, the activation energy for grain growth during isothermal sintering increased from 426.85 to 492.53kJ/mol, which is consistent with the inhibition mechanism of VC and/or Cr3C2. The mechanical properties (hardness, fracture toughness and flexural strength) of compacts were enhanced by adding 0.25wt.% VC and 0.25wt.% Cr3C2.

Effect of VC/Cr<sub>3</sub>C<sub>2</sub> Additions on Microstructure and Mechanical Properties of WC-16TiC-XTaC-10Co Ultrafine Cemented Carbides

In this paper, the WC-16TiC-xTaC-10Co mixture mixed by WC 0.52μm, (W, Ti, Ta)C 2.9μm and Co1.36μm and prepared by high-energy ball milling, changed the VC and Cr3C2 adding amount. After ball milling for 60 hours, an average particle size of 220nm powder was prepared and it was cold isostatic pressed at 300MPa and vacuum sintered at 1410°C. The physical properties and the micrographs of samples were detected. The main conclusions are as follow: the coercivity and hardness increase and Cobalt magnetic decreases with the content of Cr3C2 increasing, the transverse rupture strength (TRS) does not vary. The VC and Cr3C2 inhibit the growth of WC grain, but can’t inhibite the (W, Ti, Ta)C grain growth effectively.

WC-18wt.% Co with simultaneous improvements in hardness and toughness derived from nanocrystalline powder

Abstract Most materials exhibit a decreased toughness as the hardness (or strength) increases. Here we report a new strategy to combat this problem for cemented carbides (WC-Co). Instead of suppressing the growth of WC grains during sintering, we allow nano-WC particles to grow in a controlled manner, leading to the formation of thin WC platelets with a high aspect ratio. The thin thickness offers high hardness, while the high aspect ratio provides superior toughness. As a result, the bulk WC-Co produced from the nanocrystalline powder bucks the unfavorable trend exhibited by most of the cemented carbides available today – showing simultaneous improvements in hardness and toughness. The principle discovered in this study has significant impact on WC-Co and offers new opportunities to obtain superior mechanical properties for other cermets and advanced ceramics as well.

Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.

Growth mechanisms of WC in WC–5.75 wt% Co

The grain growth mechanisms of WC in WC–5.75 wt% Co are studied in this work. The two well known growth modes of WC in cemented carbides (coalescence process and solution/reprecipitation process) are both observed and show temperature dependence. At low temperatures (1100 and 1200 °C), the growth of WC grains is mainly through coalescence. At intermediate temperatures (1300 °C), coarsening of most WC grains takes place via isotropic growth, accompanied by the shape relaxation leading to the faceting of round grains. At high temperatures (1400 °C), layer-by-layer structures, resulting from anisotropic growth, are observed. Both isotropic growth at 1300 °C and anisotropic growth at 1400 °C are assisted by the solution/reprecipitation process. This is the first time that layer-by-layer structures are observed for WC crystals in WC–Co. The formation mechanism of layer-by-layer structures has been discussed based on the 2D nucleation and growth mechanism and the diffusion rates of W and C atoms within the Co binder at different temperatures.

Sintering of WC-10%Co nano powders containing TaC and VC grain growth inhibitors

WC-10%Co (mass fraction) nanocomposite was synthesized by high energy planetary ball milling at balls to powder mass ratio of 15. For inhibition of growth of WC grains during sintering, TaC and VC were used as inhibitors. Hardness and fracture toughness results reveal that the addition of TaC increases the toughness of hard metal and VC increases the hardness of the sample due to the effective reduction of WC grain size. The effect of co-addition of 0.6% TaC and 0.7% VC on physical and mechanical properties of WC-10%Co sintered sample was investigated. The hardness of the sample is increased to HV 30 1 787, which is 24% more than that of the inhibitor-free sample; the fracture toughness achieves 8.7 MPa·m 1/2, 35% higer then that of the inhibitor-free sample; the grain growth is inhibited and grain size achieves 0.38 μm.