Prismatic WC Research Articles

Nanomechanical response of tungsten carbide single crystals in extreme conditions: Temperature and strain rate dependence

The mechanical response of tungsten carbide (WC) single crystals under extreme conditions was investigated by means of pendulum-based nano-impact testing, from ambient to high temperatures. Dynamic hardness measurements and damage accumulation observations were conducted by single and multiple impact tests, respectively, at high strain rates (∼ 103-104 s-1). Large WC single crystals with different orientations, namely, (0001) basal and (101¯0) prismatic crystals, were studied. The results obtained from impact testing were compared to results acquired by high temperature nanoindentation in the quasi-static regime (10−2 s−1), at a similar length scale, up to 600 °C. Whilst basal WC crystals were shown to exhibit a higher dynamic hardness at high strain rates across the entire range of temperatures, the hardness of prismatic WC crystals at high strain rates were only found higher at temperatures below 200 °C and comparable at elevated temperatures. Post-mortem topographical scans revealed the generation of substantial plastic deformation during impact and large pile up formation in the vicinity of the impact imprints in prismatic crystals. Electron Channelling Contrast Imaging (ECCI) of the repetitive impacts suggested preferential planes for the accommodation of stress through plastic flow at high strain rates, as previously reported in the quasi-static regime. The results shed light on the mechanical behaviour of WC in extreme conditions, evidencing anisotropic mechanical behaviours.

Facet-dependent interfacial segregation behavior of V-doped WC-Co cemented carbides

Understanding the interfacial segregation behavior in V-doped WC-Co hard metals is necessary to demystify the effect of dopant on microstructural evolution and mechanical properties. Herein, aberration-corrected scanning transmission electron microscopy was utilized to investigate the atomic level segregation mechanism at WC-Co phase boundaries and WC grain boundaries in V-doped WC-Co composites systematically. A key finding is that the segregation behavior largely depends on the orientation of low-index planes, particularly the basal and prismatic facets that prevail in WC-Co. V solute atoms show a relatively weak segregation at the WC prismatic plane-terminated interfaces, occurring largely within a monolayer with ∼10 at% V at the outermost surface. In contrast, segregation at the basal facets of WC grains distributes within a bilayer with a higher concentration of ∼25 at%. Additionally, this study shows that the facet-dependent interfacial segregation behavior is universal for both phase boundaries and grain boundaries, which often have at least one basal or prismatic WC facet. Our study presents a general solute segregation mechanism at an atomic level and demonstrates that the terminal planes of interfaces determine the atomic structure and segregation profile, while the interfacial orientation has little influence.

Effect of carbon content on the properties of inhomogeneous cemented carbides with fine-grained structures produced via one-step transformation

Inhomogeneous cemented carbides with fine-grained structure produced via one-step transformation were prepared using coarse WC, WO3, C and Co as raw materials. The effect of carbon content on the properties of inhomogeneous cemented carbides was studied. The results indicated that the peak intensity of Co (1 1 1) had a positive relationship with carbon content. The coarse WC grains were irregularly-shaped with some growth steps and the fine grains were near-spherical shape in the alloy with low carbon content. Upon increasing the carbon content, the coarse WC grains tended to be planar and the number of growth steps decreased gradually. The number of fine WC grains increased at higher carbon contents in the WC-Co3W3C-Co alloys, resulting in a decrease in the average WC grain size. Then, the number of fine WC grains decreased and the coarse WC grains grew upon increasing the carbon content in the alloys without the Co3W3C phase, which increased the average WC grain size. The mechanical property variation of the alloys was derived from the phase composition, morphologies and the size of WC grains. Thus, the alloy with 7.24 wt% carbon displayed the maximum hardness and transverse rupture strength due to its high number of prismatic WC grains and lack of free carbon.

Development of a method for producing submicron cemented carbide from a powder obtained by electrical discharge erosion of scrap in oil

The work is devoted to the production of powder by electrical discharge erosion (EDE) in oil from WC-10Co cemented carbide scrap, post processing of the obtained powder and sintering of a submicron alloy from it. As a result of EDE of WC-10Co alloy in oil, a powder with an average particle diameter of 12.7 μm, was obtained. Pyrolysis of the oil prevented the loss of carbon by tungsten carbide and led to an increase in the carbon content of the powder from 5.5% to 8.6%. A technique of heat treatment of the obtained powder in CO2, for the removal of excess carbon (3.1%) was developed. On sintering the powder at 1370 °C, a WC-10Co alloy with a stoichiometric carbon content (5.5%) was obtained, in which plate-like and prismatic WC grains an average diameter of 0.58 μm were formed. The alloy combines high hardness (1520 HV) and fracture toughness (14.6 MPa√m).

MD study of C diffusion in WC/W interfaces observed in cemented carbides

WC/Co tool used in the turning of Ti-alloys are subject to rapid crater wear due to chemical processes at the tool chip interface in the form of dissolution/diffusion. It has been observed that a thin layer of bcc W forms on the outermost WC grains in contact with the Ti workpiece meaning that C has diffused away from the WC. The rates involved in this process are of interest for formulating a theory of how wear progresses during turning of Ti-alloys. In this work we investigate the rates involved in this diffusion process by means of classical MD simulations on 6 different WC/W interfaces, 3 with a basal WC surface and 3 with a prismatic WC surface, as a function of C depletion in the outermost WC layer. The results show that all interfaces are stable and that principally no diffusion events of C occur at temperatures below 1373 K for C depletion levels below 30 at.%. At 50 at.% depletion, C diffusion starts occurring regularly and at 70 at.% depletion and above the majority of the C atoms are diffusing except at the lowest temperatures. Additionally, any difference in diffusion rate observed between the basal and prismatic interfaces in their pristine states have vanished at the point of 50 at.% depletion. This all points to a process which is initially slow for each layer of the WC but which increases in speed substantially as C atoms are removed.

Influence of Ni and ZrO2 contents on sintering and mechanical properties of WC-2wt.%ZrO2-1wt.%Ni composites

WC-2wt.%ZrO2-1wt.%Ni composites were prepared by vacuum pressureless sintering (VPS) and post-hot isostatic pressing (post-HIP). The microstructure, phase composition, densification, hardness, fracture toughness, and flexural strength of composites with different Ni and ZrO2 contents were systematically investigated. The results show that WC-Ni and WC-ZrO2-Ni composites prepared by VPS can be densified by the addition of a small amount of Ni as the binder phase. Moreover, the densities of the composites can be further enhanced i.e. the composites are rendered nearly fully dense following HIP while the grains remain fine without obvious growth. The binder Ni and ZrO2 phases are uniformly distributed in the WC matrix and exhibit high bonding strength with it. The hardness, fracture toughness, and flexural strength of the WC-ZrO2-Ni composites following HIP could reach 22.4 GPa, 12.0 MPa m1/2, and 1101.2 MPa, respectively. Based on the influence of the Ni and ZrO2 contents on the microstructures and mechanical properties of the WC-2wt.%ZrO2-1wt.%Ni composites, the fracture mechanism was determined to be governed by the phase transformation of ZrO2 that led to the development of some micro-cracks followed by deflection, bridging, and branching of the cracks to improve fracture toughness. The composites are mainly composed of elongated triangular prismatic WC grains and ZrO2 phases, and hence the fracture mode can change such that transgranular fracture becomes the main fracture mode accompanied by a small amount of intergranular fracture. Thus, the flexural strength of the composites can be improved.

Achieving high transverse rupture strength of WC-8Co hardmetals through forming plate-like WC grains by plasma assisted milling

WC-8Co hardmetals with prismatic WC grains and plate-like WC grains were respectively produced through sintering the W-C-8Co elemental powder mixture, prepared by dielectric barrier discharge plasma assisted milling (denoted as DBDP-milling). The results show that the microstructure, hardness and transverse rupture strength (TRS) of the sintered hardmetals are related to the morphology and size of milled W, which is dependent on the DBDP-milling time. When using W of different particle sizes as raw materials, prismatic WC grains of corresponded different size tend to form in the DBDP-1h milled W-C-Co powders contain granular W particles. While the DBDP-3h milled powders contains flake-like W flakelets that generate plate-like WC grains with different degree of preferential orientation when the size of raw W materials are different. Compared with triangular prismatic WC, a suitable range of the degree of preferential orientated plate-like WC in hardmetals has the advantage for improving the TRS while maintaining excellent hardness.

Properties of WC–8Co hardmetals with plate-like WC grains prepared by plasma-assisted milling

WC–8Co hardmetals with different proportions of prismatic WC grains and plate-like WC grains were directly produced through sintering the W–C–8Co elemental powder mixture which was fabricated by dielectric barrier discharge plasma (DBDP)-assisted milling. The morphology of prepared WC–8Co hardmetals, geometry and the preferential orientation of plate-like WC were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The results demonstrate that the microstructure and mechanical properties of the sintered hardmetals are related to the morphology of W grain which is dependent on DBDP-milling time. The DBDP for 1 h (DBDP-1 h)-milled W–C–Co powder contains granular W particles that tend to form prismatic WC grains, while the DBDP for 3 h (DBDP-3 h)-milled powder contains lamellar W particles that generate plate-like WC grains. By adjusting the weight ratio of DBDP-1 h powder and DBDP-3 h powder in W–C–8Co mixture, the proportion of plate-like WC in the hardmetals can be controlled, and relatively high transverse rupture strength (TRS) is obtained as the proportion of plate-like WC grain in the hardmetals is about 35 % in present experimental condition. The DBDP-1 h-milled W–C–8Co powder (Sample A) contains granular W particles that tend to form prismatic WC grains, while the DBDP-3 h-milled powder (Sample B) contains lamellar W particles that generate plate-like WC grains. When the weight ratio of Samples A to B powder is 1:1, Sample A1B1 has relatively high transverse rupture strength (TRS) along both Sections V and P, with proportion of plate-like WC grain of ~35 %.

Abrasion wear behavior of WC–10Ni3Al cermet with plate-like triangular prismatic WC grains

The abrasion wear behavior of WC–10Ni3Al cermet with plate-like triangular prismatic WC grains has been evaluated using a ball-on-flat tribometer. WC–8Co has also been tested under the same conditions for the purpose of comparison. The results showed that WC–10Ni3Al has lower friction coefficient and wear loss than WC–8Co under the same contact load. Comparing with WC–8Co, neither binder phase extrusion nor surface delamination is observed on the worn surface of WC–10Ni3Al under the examined loads. The higher abrasion wear resistance of WC–10Ni3Al is attributed to the synergistic mechanisms of its high hardness and sub-surface crack resistance induced by the intermetallic alloy Ni3Al binder and the existence of a large amount of plate-like triangular prismatic WC grains.

Growth Behavior of WC Grains in Ni<sub>3</sub>Al Matrix during Liquid Phase Sintering

Growth behavior of WC grains with two different sizes of 10 μm and 4 μm in the same liquid matrix has been studied in the WC–40vol.%Ni3Al system. The samples were sintered under a carbon saturated condition. Surfaces of the coarse WC grains are strongly micro-faceted and stepped. With the increase of sintering time, the average size of the fine WC grain increases continuously and very large plate-like triangular prismatic WC grains appear. The present investigation thus shows that the growth behavior of fine WC grain is affected by lower solubility of W in Ni3Al matrix and the abnormal growth of WC grains is suppressed. However, the growth behavior of coarse WC grain is governed only by the difference in energy between the facets.

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.

Preparation and mechanical properties of WC-10 Ni3Al cemented carbides with plate-like triangular prismatic WC grains

The elemental powder mixtures of Ni, Al, Fe, Cr, Zr and B in the proportion Ni-10.50 Al-10.90 Fe-8.12 Cr-0.89 Zr-0.21 B(wt.%) were synthesized into Ni3Al by high-energy ball milling. The alloyed Ni3Al powders were then added to WC powders with a nominal stoichiometry of WC-10wt.% Ni3Al and underwent further milling. Finally the milled WC-10wt.% Ni3Al powders were fabricated into unusual alloys with obviously oriented plate-like triangular prismatic WC grains by spark plasma sintering (SPS) in the sintering temperature range of 1300–1400°C. The effect of sintering temperature on the microstructures and mechanical properties was also investigated. The results indicate that a proper temperature of 1350°C will help to obtain obviously oriented plate-like triangular prismatic WC grains with a high axial ratio. The obtained bulk WC-10wt.% Ni3Al possesses a high hardness (HV10), transverse rupture strength and fracture toughness of 17.76GPa, 2092MPa, 21.56MPam1/2, respectively.

Effects of Starting Powder Size and Ball-Milling Time on Microstructure of WC-Co Base Cemented Carbides Prepared from W+C+Co Base Mixed Powder

We have already developed a new WC-Co base cemented carbides with highly oriented plate-like triangular prismatic WC grains by using W+C+Co base mixed powder instead of usual WC+Co base mixed powder, and then proposed the mechanism for the generation of such WC grains.In this report, the results on the effects of the starting-powder size and ball-milling time on the orientation degree of plate-like WC grains were mainly described. The orientation degree of plate-like WC grains became larger in the cases of coarser W and finer Co powders, and longer ball-milling of W, C and Co. These results could be explained, based on our previously proposed mechanism for the generation of highly oriented plate-like WC grains.

High Temperature Strength of WC-Co Base Cemented Carbide Having Highly Oriented Plate-Like Triangular Prismatic WC Grains.

We have already reported that room temperature fracture toughness of our newly developed WC-Co base cemented carbide (alloy) having highly oriented plate-like triangular prismatic WC grains are larger than that of the conventional alloy at an identical hardness of alloy. In this study, high temperature mechanical properties such as transverse-rupture strength (TRS), creep-rupture time, etc., of the new alloy at 1273 K were clarified to be superior than those of the conventional alloy: TRS of the new alloy was higher by 10-20% and creep rupture time was longer by 50-100% than the conventional alloy. The cause for such superiority was attributed to the difficulty in the generation and propagation of stable cracks in the new alloy compared with in the conventional alloy, probably due to more rigid skeleton of WC which were generated and grown by the reaction of CoxWyCz with graphite.

Microstructural Development and Properties of New WC-Co Base Hardmetal Prepared From CoxWyCz + C Instead of WC.

We had already reported that WC-Co base hardmetal (W+Graphite+Co alloy) prepared from W+Graphite mixed powder instead of conventional WC powder has a large amount of highly oriented plate-like triangular prismatic WC crystal grains, and that both fracture toughness and transverse-rupture strength are higher than those of the conventional hardmetals at an identical hardness. In this study, we prepared another new WC-Co base hardmetal by using Co, W, C, complex carbide such as CO3W9C4 (K) and Co2W4C (θ) as the main starting material. It was clarified that WC crystal grains in the new hardmetal are also plate-like triangular prismatic, but are not oriented, differing from W+Graphite+Co alloy. The fracture toughness, however, was higher by 10-15% than that of the conventional alloy at an identical hardness, while the transverse-rupture strength was comparable with that of the conventional alloy.

Mechanisms for Formation of Highly Oriented Plate-Like Triangular Prismatic WC Grains in WC-Co Base Cemented Carbides Prepared from W and C Instead of WC.

We have developed an economical new fabrication process of WC-Co base cemented carbides with highly oriented plate-like triangular prismatic WC grains by using special W+C mixed powder instead of conventional WC powder, and already reported that the new alloy has superior room-temperature mechanical properties, compared with the conventional alloy.In this study, the mechanism of such WC grain formation was investigated in detail. It was clarified that the formation has its origin in both the flattening of W granular particles to plate-like shape by ball-milling and their highly unidirectional orientation by die-pressing, and is assisted by both the formation of highly unidirectional plate-like Co-W-C compounds (CoxWyCz) particles and its change to highly unidirectional plate-like triangular prismatic WC grains during solid state heating. Such formation of WC grains is attributed to both the higher nucleation and/or growth rate of WC grains having (0001) crystal plane parallel to the geometrical basal plane of plate-like CoxWyCz particles. Furthermore, the formation is also assisted by non-collapse of such WC grains configuration during the appearance and solidification of Co-W-C liquid phase.

Microstructure and Mechanical Properties of New WC-Co Base Cemented Carbide Having , Highly Oriented Plate-Like Triangular Prismatic WC Grains.

We developed a process for fabrication of new WC-Co base cemented carbides with highly oriented plate-like triangular prismatic WC grains. The characteristic of the fabrication process is that W +graphite mixed powder is used as the starting material for WC phase, instead of WC powder; the other fabrication conditions are the same as conventional ones. A large amount of triangular planes, i.e., (0001) crystal planes of WC grains in the sintered alloy were highly oriented vertically to the powder die-compaction direction. The new alloy showed generally higher fracture toughness and transverse-rupture strength than conventional alloy at an identical Co content, mean free path of binder phase or hardness, especially when the crack propagation direction is parallel or vertical to the powder diepressing direction.