Formation of Grain Boundaries in Liquid-Phase-Sintered WC–Co Alloys

The origin of WC/WC grain boundaries in liquid-phase-sintered WC–Co alloys has been investigated in a WC–Mo₂C–Co model system using coarse WC polygrain powder. The evolution of grain shape during liquid phase sintering was able to be identified by observing a growth layer that contained Mo. During liquid phase sintering, most of the grain boundaries in the powder were penetrated by a Co liquid but some of them were not. Electron backscattered diffraction analysis confirmed that some boundaries in the powder, in particular, Σ2 twist boundaries and Σ97 special boundaries, remained intact during liquid phase sintering. These experimental results confirm that the grain boundaries of WC grains in liquid-phase-sintered WC–Co alloys originated from those present in the starting powder.     

Microstructure and Abrasive Wear of Binderless Carbides

The microstructure and the abrasive wear characteristics of two binderless carbides (WC–Mo₂C and WC–TiC–TaC) have been studied. The microstructural analysis identified differences in the amount of mixing between the γ-phase and WC. Mo₂C and WC showed a large tendency to form mixed carbides, whereas WC and TiC did not. In both materials grain boundary segregation of metallic species was found. Compared with alumina ceramics and WC–6 wt% Co the WC–Mo₂C material showed the lowest abrasion rate and WC–TiC–TaC an intermediate. The wear mechanism was surprisingly ductile for WC–Mo₂C, whereas WC–TiC–TaC suffered from grain pullout of TiC.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

Discussion of Nonconventional Effects in Solid-State Sintering of Cemented Carbides

WC–Co materials are usually produced through a powder metallurgy route, including a liquid-phase sintering step in the 1350°–1450°C temperature range. However, it is well established that a large part of sintering already occurs in the solid-state for micrometer or submicrometer WC particles. Solid-state spreading of the Co-rich binder phase and local rearrangement of WC particles are responsible for the compact densification. But important issues still remain unexplained. First, densification by pure rearrangement should stop at a critical packing fraction of the WC refractory phase. Second, a strong influence of the C/W ratio on the spreading and sintering kinetics is observed experimentally. Both these effects are discussed in this paper, based on experimental dilatometric results, microstructural investigations by SEM and transmission electron microscopy, and considerations about interfacial energies in the system.     

Influence of Orientation on Properties of Grain Boundaries in NaCl

Simple (100) tilt, twist, and double-tilt bicrystals of NaCl, grown by the Kyropoulos technique from melts of high-purity NaCl, alone and with controlled impurity additions, were examined for mechanical strength and structure of the grain boundaries. Grain boundary fracture strengths, measured in three-point bending, showed that high-purity bicrystals with simple tilt orientations were stronger than those with simple twist at high mismatch angles (37° to 45°). The results did not show a functional dependence of strength on angle of mismatch in either tilt or twist bicrystals. Pips observed on parted grain boundaries of high-purity NaCl (100] twist and double-tilt bicrystals were believed to represent regions of continuity across the boundary. This feature was rare in similar-purity NaCl (100) tilt bicrystals. Separate additions of 100 ppm SiO₂, CaCl₂, FeCls, and KCl to the melt had no apparent effect on the character of the grain boundary. However, an addition of 1000 ppm CaCl₂ nearly doubled the strength of a (1001) 30° twist bicrystal, whereas the same addition weakened a (100) 45° tilt bicrystal. Sodium chloride (100) tilt grain boundaries, examined in situ under dark-field illumination, showed randomly distributed spots, believed to be impurity segregations, with their maximum density at the boundary. The spot densities increased with increasing tilt angle for angles of 15° and greater. The spots were not observed in the low-angle tilt boundaries (<15°) and were seen only in intermittent clusters in the few twist bicrystals examined.     

High-Temperature Reaction Between Cemented Tungsten Carbide and Copper

Interdiffusion in the system cemented tungsten carbide-molten copper has been studied in the range ≤1120°C with special emphasis on the effects of WC grain size and Co content. Techniques used for analyzing the diffusion layers obtained are EPMA, optical microscopy, and microhardness measurement. A Cu-bonded WC layer develops with simultaneous diffusion of Co from the cemented carbide into the bulk copper. The Cu-bonded WC layer grows until a Co-rich layer forms at the Cu/WC-Co interface; further heating pushes the Cu-bonded WC layer deep into the bulk cemented carbide without any significant change in layer thickness. When the WC grain size is reduced and the cobalt content increased, the penetration of copper into cemented carbides increases. A tentative mechanism of interdiffusion has been proposed based on the experimental results.     

Structural Analysis on Planar Defects Formed in WC Platelets in Ti-Doped WC–Co

Platelet-reinforced WC–Co alloys are processed by liquid-phase sintering from very fine-grained WC powders in the presence of small amounts of TiC. Large and flat WC grains develop in the material. The microstructure of these platelets is investigated by high-resolution electron microscopy in order to obtain information on their formation mechanism. Inside the grains, an extended defect parallel to the basal plane is observed. It can be described by a pair of stacking faults with a shear vector equal to 1/3 〈0-110〉 occurring in two successive (0001) planes. At the level of the faults, the plane spacing is slightly reduced. The defect area is similar to a thin cubic layer about 0.5 nm thick at the interior of the platelet. The enhanced grain growth of the platelets is likely related to the presence of the defect area.     

Etching for Microstructural Observation of Cemented Submicrometer-Sized Carbides

When carbide grains in a metal matrix are very small (less than ∼1 μm), the microstructure is difficult to observe and characterize, because the grain interfaces cannot be distinguished easily via scanning electron microscopy (SEM) when the material is etched conventionally in a Murakami solution or in H₂O-diluted HCl. This difficulty can be overcome by etching in a newly developed etchant: 90H₂O₂–10HNO₃ (by vol%). After an etching of a WC-Co sample that contained submicrometer-sized grains, the individual grains were distinctly observable via SEM. During the etching, the dissolution rates of WC grains were different, depending on their crystallographic plane, which allowed the grain boundaries to be distinguished through observation via SEM. In addition, dissolution of the cobalt matrix occurred much faster than did the etching of the WC grains. The WC/Co interface also was revealed clearly under high magnification, because of minimization of the electromagnetic interaction between the cobalt and the electron beam of the SEM apparatus.     

Stability and Surface Energies of Wetted Grain Boundaries in Aluminum Oxide

The stability of a calcium-aluminum-silicate liquid film between two near-basal plane surfaces of sapphire at 1650°C was studied. Samples were prepared having an average basal misorientation across the interface of 6–7° about < 〈10 1 0〉. The interfaces varied in orientation from 0° to ∼38° to the [0001] direction. Three types of interfaces were observed: faceted, solid-liquid interfaces; low-angle grain boundaries consisting of aligned arrays of dislocations; and boundaries consisting of alternating regions of dislocations and faceted solid-liquid interfaces. The type of interface observed depended on the orientation of the interface and could be predicted by using a construction based on Wulff shapes. Because the type of interface depends on crystal alignment and interface angle, these results suggest an absolute method of determining the surface free energy of wetted boundaries.     

Elastic after-effect in WC/Co cemented carbide

Free-decay internal friction measurements as a function of temperature and frequency have been used to study the inherent viscous behavior of the Co binder present at grain boundaries of a WC/Co cemented carbide material. The presence of a relaxation peak was related to the micromechanism of anelastic grain-boundary sliding and a peak-shift analysis was presented as a function of frequency. Using the mechanics of the elastic after-effect, both the viscosity and the activation energy of the intergranular Co binder could be determined. A comparison of the intergranular viscosity data was carried out with data obtained on bulk Co metal both in the high-purity state or in solid solution with a small volume fraction of WC. The viscosity of the Co binder, as determined by internal friction, showed good agreement with the bulk Co/WC solid solution, whose chemical composition was presumably the closest to the intergranular binder in the cermet material. Based on the agreement between intergranular and bulk viscosities as well as activation energy values, it is considered that the present method can be more generally used in ceramic/metal materials.     

Facet–Defacet Transition of Grain Boundaries in Alumina

Grain boundaries in pure alumina powder compacts sintered at 1400°C are smoothly curved, indicating that they have atomically rough structures. When these specimens are heat-treated at temperatures between 900° and 1100°C, a small fraction of the grain boundaries develop either hill-and-valley or kinked shapes with flat segments. Some of these flat boundary segments lie on the {011[Twomacr]} plane of one of the grain pairs. These grain boundaries thus appear to become singular at these temperatures. When a corundum crystal with a basal surface is sintered in alumina powder at 1400°C, all grain boundaries formed between the corundum basal surface and small grains, as well as those between the small grains, are smoothly curved, indicating their rough structure. When heat-treated at 900°C for 3 days, about 30% of the grain boundaries between the corundum basal surface and the small grains develop kinks with flat boundary segments, and some of these flat segments lie on the basal plane of the corundum. When heat-treated again at 1400°C, all grain boundaries are curved, indicating that they become reversibly rough. These observations show that at least some of the grain boundaries in alumina undergo roughening-singular transitions at temperatures between 900° and 1100°C.     

Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering

The structure and composition of sapphire bicrystal boundaries produced by liquid-phase sintering depended on the crystallographic misorientation of the crystals across the boundary and on the orientation of the boundary. Basal twist boundaries of 15° or 30° were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. For tilt boundaries of 7° or 12°, the glass wetted segments of boundaries that contained the basal plane of either crystal. Boundary segments with orientations of 40° or more from the basal plane, however, were dewetted (i.e., "dry"). Boundary segments oriented less than ∼40° from the basal orientation were partially wetted, consisting of segments of wetted and dry grain boundaries. For the 12° tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. For bicrystal boundaries having tilts of ≤4°, dewetting occurred for all observed boundary orientations. Basal-oriented segments in these small angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while nonbasal segments were apparently free of Ca and Si. Most results could be explained based on a combined Wulff plot construction, which predicts partially wetted grain boundaries and "missing" angles for unwetted grain boundaries. Results that could not be explained by the construction included growth step ledges bounded by nonequilibrium facet planes.     

Effect of carbon nanotubes additives on mechanical properties of WC–6Co cemented carbide

In order to increase the toughness of WC–6Co cemented carbide, different contents of carbon nanotubes (CNTs) were added to the WC–6Co alloy powder to prepare cemented carbide by low-pressure sintering. The results showed that some of the CNTs were embedded between the grains of WC–6Co cemented carbide, which would hinder the growth of WC grain boundary, thus leading to grain refinement. In addition, CNTs inhibited the formation of decarbonized phase and guided the deflection and bridge of crack to hinder the crack extension. With the increase of CNTs content, the density increased at first and then decreased, and the transverse fracture strength increased at first and then decreased. When the content was 0.2 wt.%, the alloy had the best performance. The density of the alloy was 99.67%; the transverse fracture strength was up to 2937.5 MPa, which is about 100% higher than that of cemented carbide without CNTs. The fracture toughness was 9.84 MPa m^1/2, and the hardness was 1924.8HV₃₀.     

Homogeneous WC–Co-Cemented Carbides from a Cobalt-Coated WC Powder Produced by a Novel Solution-Chemical Route

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.     

Role of Vanadium Carbide Additive during Sintering of WC–Co: Mechanism of Grain Growth Inhibition

In a WC–Co specimen, the shape of WC crystals was a triangular prism with truncated corners. When VC was added to inhibit grain growth, the crystal shape changed to a triangular prism without truncation. This shape change was related to the variation of edge energy, which has a significant influence on the coarsening process of WC grains.     

Strength and Toughness Degradation of Tungsten Carbide–Cobalt Due to Thermal Shock

WC–Co composites are widely used as cutting or drilling tools because of their high hardness, strength, and fracture toughness. The working temperature is generally in the range of 300° to 700°C, so thermal shock fracture of WC–Co can occur if the parts are suddenly cooled. In this study, changes in fracture strength and fracture toughness after thermal shock were observed.     

STEM Analysis of Grain Boundaries in Cemented Carbides

There is controversy over whether the cobalt binder of liquid-phase sintered WC forms a continuous skin over the WC particles or whether the carbide grains form a continuous skeleton. It is shown that the atomic Co/W ratio in the 20-Å grain boundaries is >3 times larger than that in the grains, supporting the former model.     

The two dimensional microstructure characterization of cemented carbides with an automatic image analysis process

The traditional two dimensional microstructure characterization of cemented carbide, based on stereology of linear intercept method, requires tedious and subjective manual measurements. In this study, an automatic image analysis procedure with two key techniques, i.e. maximum class square error method and watershed transformation method, has been successfully developed. The image analysis for WC-16Co cemented carbides with this procedure easily acquires consistent microstructure parameters. The analysis for area weighted WC grain size, as well as the subsequent mean free path of Co binder show quite different results compared with the conventional number weighted data. It is found that for both number weighted and area weighted data, the contiguity of WC/WC grains is insensitive to the variation of either mean WC grain size or mean free path of Co binder. The mean WC grain size is linearly related to the mean free path of Co binder. The hardness of cemented carbide, having a linear relationship with the inverse square root of mean WC grain size, conforms to Chermont and Osterstock's model. Although it is too early to conclude whether number weighted or area weighted WC grain size (and subsequent mean free path of Co binder) is better, this study shows that area weighted WC grain size and the corresponding mean free path of Co binder are more suitable for Chermont and Osterstock's hardness modeling compared with number weighted WC grain size. The area weighted WC grain size and subsequent mean free path of Co binder, which have rarely been considered for microstructure characterization of cemented carbide previously, could be the key parameters for a better understanding of the microstructure evolution, as well as a better mechanical behavior modeling for cemented carbide.     

The influence of grain boundary misorientation on ionic conductivity in YSZ

Molecular dynamics (MD) was used to investigate the structure and ion transport properties of three interfaces in 8 mol% yttria-stabilized zirconia (YSZ); namely the Σ5 (310)/[001] and Σ13 (320)/[001] tilt grain boundaries and Σ5 (111) 60° twist grain boundary. Atomic interactions were described by a potential function of the Buckingham form. Diffusion rates of oxide ions in the grain boundary containing systems showed that the tilt grain boundaries reduce the overall ionic conductivity relative to a single crystal of 8 mol% YSZ, while the Σ5 twist boundary is able to support rapid diffusion and increases the total conductivity. The effect of segregation of dopant ions to the boundary regions was also investigated.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.