A novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides

Functionally graded or functionally gradient WC–Co cemented carbides with Co and/or hardness gradients can potentially have great practical importance. In this article is described a novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides. This technique includes (1) employing green carbide bodies with low (or high) carbon contents within the two-phase region of the W–Co–C phase diagram; (2) their pre-sintering in the solid state to obtain a certain green density and consequently gas permeability; (3) selective carburisation (or decarburisation) of their surface layer in a carburising (or decarburising) gas atmosphere; and (4) final liquid-phase sintering at tailored sintering conditions to obtain a Co drift (also known as ‘Co migration’) either from the surface towards the core or from the core towards the surface. The kinetics of Co drift between couples of model alloys with very similar WC mean grain sizes but different carbon contents were examined. The microstructure, hardness, Co contents, residual stresses and wear-resistance of the gradient cemented carbides with low-Co surface layers obtained by the selective surface carburisation of carbide green bodies with the original low carbon content were examined. Their surface layers were found to contain significantly less Co than the core resulting in a higher hardness of the surface layer. The surface layer is also characterised by high residual compressive stresses in both the carbide phase and binder phase, which results in an improved combination of hardness and fracture toughness. The microstructure, hardness and Co contents of gradient cemented carbide comprising high-Co surface layers obtained by selective surface decarburisation of carbide bodies with the original high carbon content were also examined. The surface layer of the gradient cemented carbide contains noticeably more Co than the core which is beneficial when using this functionally gradient carbide as a substrate for polycrystalline diamond coatings.     

纳米碳化钒对超细WC基硬质合金的影响

研究了在放电等离子烧结(SPS)条件下,纳米碳化钒(V8C7)对超细WC基硬质合金的相组成、微观组织及性能的影响。结果表明:超细WC基硬质合金主要由WC和Co3C两相组成,相对于未烧结的硬质合金材料,WC的衍射峰向小角度方向偏移;纳米碳化钒可以有效抑制超细WC基硬质合金中WC晶粒的长大,并且随着纳米碳化钒比表面积的增大而增强,添加比表面积为63.36m2/g的纳米V8C7后,硬质合金中大部分WC的晶粒尺寸0.5μm;纳米碳化钒对超细WC基硬质合金的性能具有重要影响,并且随着纳米碳化钒比表面积的增大而增加,添加比表面积为63.36m2/g的纳米V8C7后,超细WC基硬质合金具有较高的性能(相对密度99.7%,洛氏硬度93.4,断裂韧性12.7MPa.m1/2)。     

Cu部分代Co超细硬质合金研究

基于Cu与Co相同的晶型结构和相似的原子结构,采用共沉淀方法,制备Cu部分代Co的WC—10Co硬质合金,研究Cu对材料的组织和力学性能的影响。实验结果表明,通过Cu—Co共沉淀方式将cu加入粘接相中,形成Co（Cu）固溶体,在液相烧结过程中Cu均匀地分布在Co中,可以降低WC在粘接相中的溶解度,有效阻碍WC颗粒的溶解...     

Mechanical properties of WC/Co cemented carbide with larger WC grain size

The mechanical properties of WC/Co cemented carbide with WC grain size of up to 30 μm are investigated through compressive and transverse rupture tests, because it is now to produce WC/Co cemented carbide of which grain sizes are from 20 to 30 μm. From testing specimens with a WC grain size of 3–30 μm and Co content of 5–20 wt.%, it is found that WC/Co cemented carbide with larger WC grains (20–30 μm) exhibit ductility, whereas smaller-grained materials are characteristically brittle.     

Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions

The grain-size dependence of wear resistance of WC-Co cemented carbides(with mean WC grain sizes of 2.2 μm,1.6 μm,0.8 μm and 0.4 μm,respectively) was investigated under different tribological conditions.The results showed that the grain size had opposite effects on wear resistance of the cemented carbides in dry sliding wear and microabrasion tests.In the former condition,with decrease of WC grain size hence the increase of hardness,plastic deformation,fracture,fragmentation and oxidation were all mitigated,leading to a drastic decrease in the wear rate.In the latter condition,pull-out of WC grains after Co removal dominated the wear,so that the hardness of cemented carbide was not a core factor.As a result,the wear resistance of the cemented carbide generally showed a decreasing trend with decrease of the grain size,except for a slight increase in the ultrafine-grained cemented carbide.Single-pass scratching of the cemented carbides under various loads indicated the same failure mechanism as that in the sliding wear tests.Furthermore,the reasons for severe surface oxidation of the coarse-grained cemented carbides were disclosed.     

中国超细晶硬质合金及原料制备技术进展

超细晶硬质合金是WC晶粒度≤0.5μm的硬质合金,这类合金具有高强度和高硬度的优异性能。目前由超细晶硬质合金制备的高效刀具已经广泛用于航空航天、核能、汽车、发电设备、新能源和电子通讯等现代制造业。主要对中国超细晶硬质合金原料(例如超细碳化钨粉、钴粉、复合粉)和超细晶硬质合金制备技术、性能及表征方法作了系统的阐述。最后对超细晶硬质合金制备技术进行了展望。     

Development of the microstructure during gradient sintering of a cemented carbide

A gradient sintered WC–Ti(C,N)–Co-based cemented carbide has been studied. The material was sintered in a nitrogen-free atmosphere, resulting in an outward diffusion of nitrogen. Due to the strong thermodynamic coupling between nitrogen and titanium, an inward diffusion of titanium is created. As a result of the diffusion processes the material develops a cubic carbide depleted and binder phase enriched surface zone, and inside this zone a region enriched in cubic carbide. The structure of the gradient surface zone has been studied, where variations in volume fractions of the different phases present have been investigated by scanning electron microscopy (SEM) and image analysis, and elemental variations have been studied with electron probe microanalysis (EPMA). Computer simulations of the gradient formation, based on diffusion and thermodynamic properties, show good agreement with the experimental results, and the gradient formation can be reasonably well predicted. Also studies of grain composition have been performed. In the rim of the cubic carbide grains, variations in composition are found which can be related to the sintering process. The composition of cubic carbide grains has been studied with SEM and analytical electron microscopy.     

烧结工艺对WC-1.0TiC-3.1TaC-4.5Co硬质合金性能及微观组织的影响

采用传统粉末冶金法,分别用真空烧结和低压烧结工艺制备出一系列WC-1.0TiC-3.1TaC-4.5Co硬质合金样品。利用光学显微镜、扫描电镜与能谱仪对合金微观组织结构特征进行观察与分析。结果表明:提高真空工艺烧结温度或采用低压烧结工艺,能使合金内部的显微孔隙、钴池减少;低压烧结制备的合金WC晶粒度小于真空烧结制备的合金WC晶粒度,合金中易出现WC晶粒异常长大现象。     

Hot isostatic pressing of ultrafine tungsten carbide-cobalt hardmetals

Hot Isostatic Pressing (HIP) has been successfully used to consolidate tungsten carbide-10 weight% cobalt (WC-10 wt% Co) powders mixtures with WC powder particle sizes in the range of 100 nanometers. Fully dense specimens of this composition have been obtained by HIP at 1000°C, a temperature well below those usually required for reaching the closed porosity stage in the WC-Co system. Conventional processing by vacuum sintering has also been carried out to study the individual effects of high isostatic pressure and vanadium carbide additions on densification and WC grain growth control of these hardmetals. The finest WC mean grain size after sintering has been obtained for the combined action of applied isostatic pressure and vanadium carbide (VC) additions. These results show that VC additions are effective in controlling WC grain growth even at temperatures as low as 1000°C.     

金刚石薄膜涂层硬质合金刀具的界面表征

采用SEM对金刚石薄膜涂层硬质合金刀具的金刚石薄膜表面、背面及金刚石薄膜剥落后的硬质合金刀片表面的典型形貌进行了观察,并采用TEM对金刚石薄膜／硬质合金刀片横截面的微观组织进行了研究,还采用FT—Raman光谱法对金刚石薄膜表面及金刚石薄膜剥落后的硬质合金刀片表面的微观结构进行了表征．结果表明：经适当的化学侵蚀脱钻和等离子体刻蚀脱碳预处理后,金刚石薄膜涂层硬质合金刀具的界面通常存在薄的（数十nm）石墨碳层;局部区域见到金刚石粒子直接生长在WC颗粒上,金刚石膜／基横截面的典型组织层次为：金刚石薄膜／薄的石墨碳层／细小的WC层／残留的脱碳层（η相＋W相）／原始的硬质合金基体．     

高性能超细、纳米硬质合金及其制备过程中的关键技术问题

介绍了超细、纳米硬质合金的国内外研究、生产以及市场情况,主要应用领域,硬度、抗弯强度、断裂韧性、热硬度、热导率等力学、热物理性能和使用性能特点,湿磨过程中的研磨与分散、混合料制备过程中的控氧、粉末成形、烧结过程中WC晶粒长大的控制和粉尘控制等合金制备过程中存在的关键技术问题以及解决这些问题的主要途径.     

Effect of boron on the microstructure of spark plasma sintered Ultrafine WC

The microstructure of sintered carbide compacts generally contain facetted and abnormally grown grains. In the present work we show that the addition of a small quantity of boron to tungsten carbide powders can induce isotropic coarsening without any abnormal grain growth. The ability of boron to reduce faceting is brought about by the oxidation of boron at low temperatures which leads to isotropic wetting and roughening of particle boundaries during sinter-coarsening at elevated temperatures. Increase in boron content leads to enhanced grain growth and a limiting value to the boron concentration is suggested. Increase in the ambient pressure during sintering increases oxidation of boron and also the sintering temperature leading to a change in both grain shape and size. The isotropic coarsening behavior of WC in the presence of boron conforms to Jackson’s theory of crystal growth based on the energetics of a rough liquid–solid interface.     

Quantitative microanalysis of carbide/carbide interfaces in WC–Co-base cemented carbides

Abstract

Carbide/carbide boundaries in WC–Co-base cemented carbides containing 6–20 wt-%Co were studied with two high resolution microanalytical techniques: atom probe field ion microscopy and analytical transmission electron microscopy. All boundaries studied, i.e. WC/WC boundaries and, in materials containing cubic carbides (γ-phase), WC/γ and γ/γ boundaries, were found to contain about half a monolayer of cobalt, localized to a zone of monolayer thickness. The carbide/carbide boundaries may thus be described as grain (phase) boundaries to which cobalt has segregated. The carbide skeleton model for WC–Co is thereby confirmed. In WC–Co materials which contain Cr₃C₂ as a grain growth inhibitor, chromium segregates to WC grain boundaries.

MST/354     

WCCo/cBN composites produced by pulse plasma sintering method

WCCo/cBN composites have been considered as a next-generation material for use in cutting-tool edges, being characterized by an optimal combination of hardness and toughness. They can be used instead of WCCo/diamond composites in machining of iron-based materials. The major challenge in sintering these composites is to produce a well-bonded interface between the WCCo matrix and cBN particles. In this study, WCCo/cBN composites were fabricated by the pulse plasma sintering technique. The aim of this work is to obtain sintered parts with density near the theoretical value and with very good contact between the cBN particles and WCCo matrix. cBN/cemented carbide containing 30 vol.% of cBN particles was produced using a mixture of 6 and 12 wt.% Co-added WC powder, with WC grain size of 0.4 μm and cBN powder with grain size ranging from 4 to 40 μm. Scanning electron microscopy (SEM) observations of the microstructure and diffraction phase examinations did not show the presence of hBN phase. The specific heating conditions used to consolidate the material using high-current pulses hamper the transformation of cBN into hBN and ensure a strong bond between the cBN particles and the cemented carbide matrix. Fractures through the WCCo/cBN composite showed that only few cBN particles were torn out from the cemented carbide matrix, with most of them having been cleaved along the fracture plane. This provides evidence that the bond at the WCCo/cBN interface is mechanically strong. Composites sintered at temperature of 1,200 °C under pressure of 100 MPa for 5 min had density near the theoretical value. Increase of the sintering temperature to 1,200 °C resulted in an increase of the hardness to 2,330 HK1 for the WC6Co/cBN(1/3) composite and to 2,160 HK1 for the WC6Co/cBN(37/44) composite.     

Research on Ultrafine Tungsten Carbide-Cobalt (WC-Co) Cemented Carbide Rods of Miniature Drills for Highly Integrated Printed Circuit Board (PCB)

Nanocrystalline tungsten carbide-cobalt (WC-Co) composite powders produced through spray thermal decomposition-continuous reduction and carburization technology were used to prepare φ3.25 mm×38 mm ultrafine tungsten carbide-cobalt (WC-Co) cemented carbide rods through vacuum sintering plus sinterhip technology. The microstructure, Vickers hardness, density and Rockwell A hardness (HRA), transverse rupture strength (TRS), saturated magnetization and coercivity force were tested. The results show that the average grain size of the sintering body prepared through vacuum sintering plus sinterhip technology was 430 nm; transverse rupture strength (TRS) was 3850 MPa; Vickers hardness was 1890 and Rockwell A hardness of sintering body was 93. High strength and high hardness ultrafine WC-Co cemented carbide rods used to manufacture printed circuit board (PCB) drills were obtained.     

一种特殊烧结技术对梯度结构硬质合金性能影响的研究

研究了分段烧结制备梯度结构硬质合金技术以及粘结剂含量对梯度硬质合金的力学性能、梯度结构的影响.测试了梯度硬质合金刀片的切削性能.结果表明,分段烧结技术能成功地制备梯度结构硬质合金,且随着钴含量增多,合金梯度层厚度增厚,梯度结构更明显;合金的强度与磁饱和提高.切削试验表明:具有梯度结构涂层硬质合金刀片的切削性能比无梯度结构涂层硬质合金刀片的切削性能更为优良.达到同一磨损高度VB=0.15mm时,前者的切削寿命较后者提高了近一倍.     

A transmission electron microscopy study of WC-10Co cemented carbides with modified hard and binder phases

The alloy design of WC-10Co cemented carbide, modified with addition of a hard carbide phase, TiC, and with Ni and Mo in the binder phase, has been highlighted by the authors in a number of publications. The present article deals with the fine microstructural features of various phases in such cemented carbides. WC grains in all the investigated cemented carbide compositions appear to develop straight facets during sintering because of their anisotropic nature. In contrast, the TiC phase is characterized by its rounded shape. Dislocations are present in both WC and TiC grains, being of lesser density in the latter. The binder phase is always associated with stacking faults. The nature of the hard phase/binder interfaces has been found to be dependent on the binder phase chemistry. The observed changes in microstructures and mechanical properties have been correlated with the wettability and solubility of the hard phases in the binder melt, and with the different strengthening mechanisms in the binder phase.     

Complex carbide powders for plasma spraying

Various carbide-containing powders are used for plasma spraying. Most of these consist of tungsten carbide. Very frequently the tungsten carbide is mixed with cobalt to produce coatings similar to cemented carbides. The powders can be made by agglomeration of the carbides and the metal matrix powders or by coating the carbides with the matrix metal. As in cemented carbides, cobalt and nickel form the metal binder of the coatings. The coating metals can be increased to 20% of the total weight. Further metal matrix powders can be added also. The W₂C, WC, W₂C-WC eutectic phases and mixtures of them are used as tungsten carbide. The carbon content is very important and can be controlled better in coated particles than in an agglomerated powder. In the case of WCCo the carbon content has to correspond to MC to avoid the embrittling η phase and to achieve a strength of the coating comparable with that of cemented carbides. In addition to the composition, shape and grain size distribution of the powders, the spraying conditions are very important for the properties of the coating. Coated carbide powders are less sensitive to spraying conditions. When it is possible to control the carbon content in carbides better during spraying it will be feasible to use complex carbides also.Titanium carbide forms a solid solution with WC over a wide range of composition and forms mixed crystals with tantalum carbide and niobium carbide. These binary and ternary mixed carbide crystals, sometimes containing additional tungsten carbide, are used in cemented carbides to increase the wear resistance. By spraying these cobalt- or nickel-coated complex carbide powders similar properties of the coatings can be achieved. Spraying conditions and the shape, grain size and grain size distribution of the powders are important. Results will be given.     

Spark plasma sintering on nanometer scale WC–Co powder

Nanometer scale WC–11Co powder was sintered by spark plasma sintering (SPS) process in order to improve the properties of the cemented carbides. Properties such as density and hardness were measured. The microstructures of sintered WC–11Co cemented carbides were observed. The grain size of WC in alloys was also obtained. The results showed that spark plasma sintering could lower the sintering temperature, increased the density and circumscribed the growth of grain size of WC. Besides, the hardness of the sintered cemented alloys that was dependent on the grain size and densification could also be improved by SPS. SPS was an effective method to get WC–11Co cemented carbides with fine grain size and good properties.     

The structure and properties of a hard alloy coating deposited by high-velocity pulsed plasma jet onto a copper substrate

Using a plasmatron operating in specially calculated regimes, tungsten carbide (WC) based coatings were deposited onto a copper crystallizer plate. It was found that a local hardness of the WC-Co coating may reach up to 1.3×10₄ N/mm² and the coating adhesion to substrate may be as high as 270 MPa. The elemental and phase compositions of coatings were studied by Rutherford backscattering spectroscopy, X-ray diffraction, and transmission electron microscopy with electron diffraction. The surface morphology and depth-composition profiles of the coatings were studied by optical and scanning electron microscopy. The coating is composed of WC crystal grains with hexagonal close packed (hcp) lattice, α-and β-Co grains, and cubic WC grains. The average size of the hcp WC grains is 0.15 μm and that of the cobalt particles is about 25 nm. In addition, the grain boundaries contain W₃Co₃C particles with an average size of 15 nm.