Toughening Mechanisms in Cemented Carbides

Plastic zones much larger than previously expected have been observed around cracks in several cemented carbide materials. A preliminary estimate of the crack tip shielding due to the plastic zone in one material suggests that a substantial fraction of the fracture toughness may be due to the zone. Moreover, reduced zone sizes were observed in regions of rapid crack growth, implying that the dynamic fracture toughness may be smaller than the quasi-static value.     

Hot-Isostatic-Press Joining of Cemented Carbides

Hot isostatic pressing (HIPing) is investigated as a technique for joining the cermet WC-15% Co to itself. Encapsulation of the specimens prior to HIPing was carried out using steel encapsulation, glass encapsulation, and self-encapsulation. The bonds were evaluated using a four-point-bend method. It is shown that the glass and steel encapsulation methods have a number of inherent problems which make them inappropriate for near net shape processing. In contrast the novel self-encapsulation method, described for the first time in this communication, is both simple and effective, producing joined material with bulk strength. The concept of self-encapsulation is potentially widely applicable for joining composite materials.     

Effects of Particle Packing Characteristics on Solid-State Sintering

Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid-state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.     

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.     

Room-Temperature Thermal Conductivity of Cemented Transition-Metal Carbides

Measurements are reported of the room-temperature thermal conductivity of cemented multicarbides (WC-TiC _x -NbC _x -TaC _x /Co) and straight tungsten carbide (WC/Co), which are widely used tool materials. The thermal conductivity of cemented titanium carbide was found to be lower than that of cemented tungsten carbide. The difference is attributed to strong phonon and electron scattering from carbon atom vacancies in the nonstoichiometric cubic carbide TiC _x ; these defects are absent in stoichiometric hexagonal WC. Higher binder contents in tungsten carbide samples lowered the overall thermal conductivity. Scattering of electrons and phonons by C and W atoms in solid solution in the binder phase presumably reduces its thermal conductivity. No dependence on grain size was detected.     

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.     

锥体硬质合金堆焊工艺及应用

针对某项目煤气化装置排渣罐锥体底部堆焊硬质合金后,表面裂纹较多的问题,采取选择合适的堆焊工艺,调整堆焊施焊参数,设计合理的工装,加强堆焊过程控制,使锥体硬质合金堆焊满足产品耐磨损性能的要求。     

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.     

Exaggerated Grain Growth in Cemented Carbides Due to Inhomogeneous Milling

Tungsten carbide powders mixed with cobalt in an attritor mill with low and high amounts of benzene show pronounced differences in particle size distribution. A significant portion of large crystals remains in the thicker slurry which leads to exaggerated grain growth during sintering.     

Effects of the Homogeneity of Particle Coordination on Solid-State Sintering of Transparent Alumina

A quantitative assessment of the homogeneity of particle coordination in green bodies is achieved by addressing pore structures. Avoiding a coarser tail of the pore size distribution is important but insufficient for optimum sintering. Instead, the steepest slope of the main body of the distribution is responsible for a maximum density at lowest temperature, and further progress is enabled by minimizing interparticle spacing (the average pore size). Ranking different technologies with different associated pore size distributions, the same correlation holds for the impact of homogeneity on (i) sintering densification as (ii) for the onset of intense grain growth during the final sintering stage. For all of the investigated processing approaches the pore size distributions are observed to remain constant through the initial and intermediate states of sintering.     

Cermets: III, Modes of Fracture and Slip in Cemented Carbides

The modes of fracture and slip in cemented carbides were observed. Fracture and slip patterns in cemented carbides were found to vary with the microstructures which in turn were determined by the relative surface energies of various phases. The influence of particle size and metal binder content in cemented Tic and WC was also investigated. The fracture and slip in these systems are discussed in terms of the role of the metal film surrounding the carbide grains.     

Numerical Simulation of Solid-State Sintering: I, Sintering of Three Particles

A kinetic, Monte Carlo model, capable of simulating microstructural evolution sintering in a two-dimensional system of three particles, has been presented. The model can simulate several mechanisms simultaneously. It can simulate curvature-driven grain growth, pore migration and coarsening by surface diffusion, and densification by diffusion of vacancies to grain boundaries and annihilation of these vacancies. Morphologic changes and densification kinetics are used to verify the model.     

Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Thermal Conductivity and Electrical Resistivity of Cemented Transition-Metal Carbides at Low Temperatures

The thermal conductivity, K , and electrical resistivity, ρ, of cemented transition-metal carbides—a class of composites of high toughness—were measured for the first time at cryogenic temperatures. The dependence of these quantities on composition, grain size, and binder content was explored. The major qualitative findings are the following: (1) K for WC/Co grades has a gentle maximum ranging from 80 to 160 W/(m · K) around 150 K; (2) WC/Co has higher values of K and lower values of ρ than TiC _x /NiMo, because of vacancy scattering of electrons and phonons in the latter; (3) grades with larger grains have higher values of K ; (4) grades with higher percentages of binder phase have lower values of K ; (5) the lattice and electronic components of K are comparable for TiC _x /NiMo; (6) multicarbide grades have lower values of K than WC/Co; (7) a Callaway analysis of the lattice component of K identifies scattering of phonons by grain boundaries, point defects, electrons, and other phonons as significant for these materials, with defect scattering dominating in the liquid-nitrogen range.     

降低钨钴硬质合金烧结温度的实验

通过减小WC粉体的粒径及添加微量元素的方法，并运用差热分析及金相技术，研究了降低钨钴硬质合金烧结温度的新途径；确定WC粉体平均粒径为1.65 mm时，其烧结温度可降低到960℃. 在保证硬质合金整体机械性能的前提下，可减少对预植入硬质合金中的单晶金刚石的热损伤.     

Particle Rearrangement and Pore Space Coarsening During Solid-State Sintering

Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) pores in the late sintering stages are well established. Porosity is interconnected up to very high densities. Coarsening of the continuous pore space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the interconnected pore space and of closed pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions.     

Solid-State Reactive Sintering of Transparent Polycrystalline Nd:YAG Ceramics

Transparent polycrystalline Nd:YAG ceramics were fabricated by solid-state reactive sintering a mixture of commercial Al₂O₃, Y₂O₃, and Nd₂O₃ powders. The powders were mixed in methanol and doped with 0.5 wt% tetraethoxysilane (TEOS), dried, and pressed. Pressed samples were sintered from 1700° to 1850°C in vacuum without calcination. Transparent fully dense samples with average grain sizes of ∼50 μm were obtained at 1800°C for all Nd₂O₃ levels studied (0, 1, 3, and 5 at.%). The sintering temperature was little affected by Nd concentration, but SiO₂ doping lowered the sintering temperature by ∼100°C. Abnormal grain growth was frequently observed in samples sintered at 1850°C. The Nd concentration was determined by energy-dispersive spectroscopy to be uniform throughout the samples. The in-line transmittance was >80% in the 350–900 nm range regardless of the Nd concentration. The best 1 at.% Nd:YAG ceramics (2 mm thick) achieved 84% transmittance, which is equivalent to 0.9 at.% Nd:YAG single crystals grown by the Czochralski method.     

硬质合金烧结尾气处理方案分析

硬质合金烧结尾气中的废气成分以石蜡分解产生的烷烃类挥发性有机物为主,该类废气排气风量小,浓度较低且不稳定,同时有机成分的沸点范围广,去除效果难以保障。以某企业硬质合金烧结尾气为处理对象,通过设计分析及验证,表明该类废气适宜采用“旋流洗涤+干式过滤+蜂窝分子筛吸附浓缩+催化燃烧”工艺,其运行能耗较低,且处理效率可稳定达90%以上,该工艺可推广到类似有机废气治理领域。     

Cold Compaction and Solid-State Sintering of WC-Co-Based Structures: Experiments and Modeling

Cold compaction (200-1900 MPa) and sintering (1250°-1350°C) of cermets based on WC-Co were experimentally studied using die compaction, cold isostatic pressing, sintering, and creep tests. Two different-sized WC powders were used. The cobalt content varied over a range of 10-30 wt%. Cold-compaction behavior has been described by using a Cam-Clay model. Die-wall friction was measured by using green powder compacts that had different aspect ratios. Friction coefficients were 0.28-0.85, depending on the WC particle size and cobalt content. Simple constitutive equations have been used to model the high-temperature behavior (sintering and creep). The constitutive equations were implemented in a finite-element program to model the compaction, ejection, and sintering of bilayer structures that had different cobalt contents. The model can represent the effect of die-wall friction on the average density, as well as deformation inside the green compact. Density gradients were generated; they were revealed during sintering, because the compact does not deform homogeneously. Simulation also can be used to evaluate deformations that are induced by sintering.     

Considerations of Inhomogeneity Effects in Sintering

Inhomogeneous regions of powder particle size or of particle packing result in nonuniform sintering rates. This nonuniformity induces stresses which may, in turn, create microstructural defects. A method for calculating these stresses is described and some aspects of defect development are discussed.