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.     

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.     

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.     

Wear Mechanisms of TiB2 and TiB2–TiSi2 at Fretting Contacts with Steel and WC–6 wt% Co

Unlubricated fretting wear tests on TiB₂ and TiB₂–5 wt% TiSi₂ ceramics against two different mating materials (bearing grade steel and WC–6 wt% Co balls) were performed with a view to understand the counterbody-dependent difference in friction and wear properties. The fretting experiments were conducted systematically by varying load (2–10 N) at an oscillating frequency of 4 Hz and 100 μm linear stroke, for a duration of 100,000 cycles. Adhesion, abrasion, and three-body wear have been observed as mechanisms of material damage for both the TiB₂/steel and TiB₂/WC–Co tribosystems. The third body is predominantly characterized as tribochemical layer for TiB₂/steel and loose wear debris particles for TiB₂/WC–Co tribocouple. An explanation on differences in tribological properties has been provided in reference to the counterbody material as well as microstructure and mechanical properties of flat materials.     

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.     

Interface Character Distributions in WC–Co Composites

The geometric and crystallographic characteristics of interfaces in WC–Co composites with a range of grain sizes and carbide volume fractions have been comprehensively characterized. The carbide crystals are most frequently terminated by (0001) and     surfaces. The average number of carbide vertices per grain and the basal-to-prismatic face area ratio of the WC–Co interfaces increase with the carbide volume fraction. The three most frequently occurring WC/WC grain boundaries are 90° twist boundaries about     , 30° twist boundaries about [0001], and asymmetric 90° boundaries about     . The boundary populations do not vary with grain size or carbide volume fraction, suggesting that they are determined by the grain boundary energy anisotropy.     

Surface Integrity Effects on the Fracture Resistance of Electrical-Discharge-Machined WC–Co Cemented Carbides

The flexural strength evolution for two WC–16 vol% Co cemented carbides, with different mean carbide size, subjected to sequential and upgrading electrical-discharge machining (EDM) is studied. It is compared with the fracture behavior exhibited by a reference surface finish condition, attained through conventional mechanical grinding and polishing using diamond as abrasive. Considering that rupture is related to existing defects, either introduced during sample elaboration or induced by machining, a detailed fractographic examination by scanning electron microscopy is conducted to discern fracture origins. The experimental findings indicate that the flexural strength of WC–Co hardmetals may be strongly affected by EDM, depending on the correlation existing between natural defects, as given by particular microstructural parameters, and EDM-induced flaws. An analysis of the results using a linear–elastic fracture mechanics approach permits one to establish a clear connection between surface integrity and fracture resistance. Quantitative discrepancies between the estimated and the experimentally measured critical flaw sizes for all the EDM-related grades are rationalized through the existence of local residual tensile stresses of considerable magnitude at the shaped surface. Release of these stresses through final mechanical and annealing treatments is pointed out as a quite effective alternative for improving the fracture behavior of WC–Co cemented carbides shaped by EDM.     

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.     

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.     

Density Measurements of Single Granules Using the Atomic Force Microscope

The density of single spray-dried granules has been determined with a new method based on atomic force microscopy (AFM). Spherical granules with a well-defined diameter are attached to the AFM cantilever, which acts as a beam-type spring, and the mass of a granule is estimated from the shift in the resonant frequency. The error of the measurements associated with the method was estimated to vary between 1% and 5%, depending on the size and shape of the granule. Density measurements of spray-dried WC–Co granules are presented, and the effect of a polymeric binder and dispersant on the consolidation during drying is discussed.     

High-Toughness Silicon Carbide as Armor

Silicon carbide, with single-edge precracked beam (SEPB) toughness greater than 7 MPa·m^1/2, was made by hot-pressing using Al–B–C (ABC) or Al–Y₂O₃ (YAG) as additives. The hardness of SiC processed with a liquid phase was always less than SiC densified without a liquid phase despite having a similar or finer grain size. With increasing Al content, the ABC system changed from trans- to intergranular fracture with a drop in hardness and a two- to threefold increase in SEPB toughness. Strength and Weibull modulus for materials processed with a liquid phase were higher than those of solid-state densified SiC. Ballistic testing, however, did not show any improvement over SiC densified with B and C additives. Depth of penetration was controlled by hardness of the SiC-based materials, while V ₅₀ values for 14.5 mm WC–Co cored projectiles were in the range of 720–750 m/s for all materials tested.     

Microstructure of Ti(CN)–WC–NbC–Ni Cermets

An investigation of the microstructural evolution and dissolution phenomena in a Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system is reported. In Ti(C_0.7N_0.3)– y NbC–20Ni systems, a phase separation occurs between the Ti(CN) core and the (Ti,Nb)(CN) rim phases when the system contains >15 wt% NbC. This phase separation results from the increased misfit between the cores and the solid-solution rim phases with the addition of NbC. Based on data obtained from a previous study and compositional analyses of the rim structure of the Ti(C_0.7N_0.3)– y NbC–20Ni system, the average dissolution rates of WC and NbC appear to be approximately the same with respect to that of Ti(CN), under given sintering conditions (1510°C for 1 h). In addition, compositional changes in the rim structure of the Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system are compared with those for a Ti(C_0.7N_0.3)– x WC–20Ni system to explain the effect of NbC on WC dissolution in the Ti(C_0.7N_0.3)–WC–NbC–Ni system. The presence of NbC in the Ti(C_0.7N_0.3) –x WC–20Ni system is found to suppress the dissolution of WC.     

Graphene anchored Ce doped spinel ferrites for practical and technological applications

Graphene plays a remarkable role as a supporting material for the fabrication of a variety of nanocomposites. This work presents the fabrication of graphene-based Ce doped Ni–Co (Ni_0.5Co_0.5Ce_0.2Fe_1.8O₄/G) ferrite nanocomposites. Ni_0.5Co_0.5Fe₂O₄ and Ni_0.5Co_0.5Ce_0.2Fe_1.8O₄ were prepared using sol gel method. However, Ce doped Ni–Co spinel nanoferrite was chemically anchored on the surface of graphene. Different characterizations techniques were adopted to investigate the variations in the properties of ferrite composite due to the incorporation of graphene. Thermal analysis revealed 18% heat weight loss of Ce doped Ni–Co ferrite sample during treatment up to 1000 °C respectively. X-ray diffraction analysis depicted the presence of spinel phase structure of all synthesized nanocomposites. Fourier transform infrared analysis revealed two absorption bands of tetrahedral and octahedral sites of the spinel phase and presence of graphene contents in the Ni_0.8Ce_0.2CoFeO₄/G composite. FESEM images revealed an increased agglomeration due to the presence of graphene in the Ce doped Ni–Co ferrite composites. Graphene based Ce doped Ni–Co ferrite nanocomposite showed highest conductivity (4.52 mS/cm) than other ferrite composites. Magnetic characteristics showed an improvement in the Ni–Co ferrite sample by the substitutions of Ce³⁺ ions and graphene contents. The improvement in the properties of these nanocomposites makes them potential material for many applications such as fabrication of electrodes, energy storage and nanoelectronics devices.     

Fabrication and magnetic properties of granular Co/porous InP nanocomposite materials

A novel Co/InP magnetic semiconductor nanocomposite was fabricated by electrodeposition magnetic Co nanoparticles into n-type porous InP templates in ethanol solution of cobalt chloride. The content or particle size of Co particles embedded in porous InP increased with increasing deposition time. Co particles had uniform distribution over pore sidewall surface of InP template, which was different from that of ceramic template and may open up new branch of fabrication of nanocomposites. The magnetism of such Co/InP nanocomposites can be gradually tuned from diamagnetism to ferromagnetism by increasing the deposition time of Co. Magnetic anisotropy of this Co/InP nanocomposite with magnetization easy axis along the axis of InP square channel was well realized by the competition between shape anisotropy and magnetocrystalline anisotropy. Such Co/InP nanocomposites with adjustable magnetism may have potential applications in future in the field of spin electronics.     

Compaction as a Critical Factor for Success in the Sintering of Ultra-Fine WC–Co Powders

Heat treatment of ultra-fine WC–13Co powders was carried out prior to cold compaction, in an attempt to improve the sintered density. The findings indicate that a preheat treatment of ultra-fine or nano-sized powders significantly improves the densification process. This study showed that a low compacting pressure, <200 MPa, can be effectively used through this technique to retain ultra-fine structure. The densification behavior of preheated powder was compared with the samples prepared by a conventional technique and explained with size distribution, standard deviation, and surface effect.     

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.     

Modeling the Influence of Orientation Texture on the Strength of WC–Co Composites

Two-dimensional finite element simulations were used to study the effects of orientation texture on the transverse rupture strengths of WC–Co composites. The model incorporates observed microstructural geometries, anisotropic thermal and elastic properties, and a fracture criterion that reproduces the strengths of known specimens. The results show that the greatest potential for increasing the strength occurs when the [001] axes of the carbide grains are orientated perpendicular to the sample loading direction. Furthermore, the strength increases in proportion to the degree of texture, and the texture-derived strength enhancement is greater in microstructures with a larger contiguity.     

Elastic-Plastic Indentation of Hard, Brittle Materials with Spherical Indenters

A method was developed at the U.S. Bureau of Mines to measure the plastic deformation of hard, brittle materials with low values of elastic modulus to hardness ( E/H ). The method involves indenting the hard materials with spherical indenters and measuring the resulting contact diameter and depth. Subsequent analysis separates the plastic deformation from the elastic deformation. The analysis is an extension of a theoretical study of the indentation of ductile materials by spherical indenters. The analysis is based on Meyer's law and the results include the distribution of the contact pressure and the profile of the deformed surface. Measurements were made on alumina and WC–Co.     

Functionality Diagrams for Hybrid Mechanical Seals with Silicon Nitride Rings

Ring-on-ring tribological experiments were performed with hybrid mechanical seals with silicon nitride (Si₃N₄) rings. The K × PV product, where K is the wear coefficient, P the sealing pressure, and V the linear speed, is proposed as a novel parameter to characterize the total working range of a mechanical seal system, with the advantage of directly indicating the thickness reduction for a certain time of service. The K × PV criterion is represented in a map form called "functionality diagram." The rotary Si₃N₄ rings show the better behavior ( K × PV =0.05 μm/h) against WC–Co mating faces.     

Tribological Characterization of WC–Co Plasma Sprayed Coatings

Atmospheric plasma spraying of WC coatings is typically characterized by increased decarburization, with a consequent reduction of their wear resistance. Indeed, high temperature and oxidizing atmosphere promote the appearance of brittle crystalline and amorphous phases. However, by using a high helium flow rate in a process gas mixture, plasma spraying may easily be optimized by increasing the velocity of sprayed particles and by reducing the degree of WC dissolution. To this purpose, a comparative study was performed at different spray conditions. Both WC–Co powder and coating phases were characterized by X-ray difraction. Their microstructure was investigated by scanning electron microscopy. Mechanical, dry sliding friction, and wear tests were also performed. The wear resistance was highly related to both microstructural and mechanical properties. The experimental data confirmed that high-quality cermet coatings could be manufactured by using optimized Ar–He mixtures. Their enhanced hardness, toughness, and wear resistance resulted in coatings comparable to those sprayed by high velocity oxygen-fuel.