Effect of interfacial characteristics on toughness of nanocrystalline cemented carbides

Dense nanocrystalline cemented carbide bulks were prepared using a unique in situ synthesized WC–Co composite powder with a super-ultrafine nanostructure. Remarkable enhancement in the fracture toughness (with high hardness being maintained) was obtained in the nanocrystalline cemented carbides. Based on detailed studies on the combination of WC and Co phases, the WC/Co orientation relationship and the atomic correspondence at interfaces, the mechanisms for high toughness in the present nanocrystalline cemented carbides were demonstrated. The study proposed that interfacial characteristics play a significant role in the toughness of the nanocrystalline cemented carbides, and provided an effective approach to achieve superior combination properties of hardness and toughness in cermet materials.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

Mechanical properties of WC–10Co cemented carbides sintered from nanocrystalline spray conversion processed powders

Mechanical properties and microstructures of nanocrystalline WC–10Co cemented carbides were investigated. The nanocrystalline WC–10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375 °C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC–10Co, commercial WC powders in a diameter range of 0.57–4 μm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr₃C₂ and VC of varying amount were added into nanocrystalline WC–10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC–10Co cemented carbides, Co–W–C alloy was fabricated at the temperature of sintering process for the WC–10Co cemented carbides. The hardness of WC–10Co cemented carbides increased with decreasing WC grain size following a Hall–Petch-type relationship. The fracture toughness of WC–10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.     

WC-Co硬质合金特征晶面分布的研究

采用不同温度进行原位还原碳化反应制备纳米粒径的WC-Co复合粉末,进而利用放电等离子烧结技术进行快速致密化,制备得到致密的纳米晶硬质合金块体材料。系统的检测分析表明,低温下反应制备的以缺碳相为主相的复合粉经烧结致密化后块体试样存在WC晶粒高度取向分布的特征,其中,(0001)面在垂直于压力方向上面积分数达到40%,在平行于压力方向上(0001)面的面积分数仅为10.72%。与之对比,在高温反应制备的复合粉末烧结制备的块体试样中,没有出现特征晶面分布的各向异性。     

Synthesis and processing of nanocrystalline tungsten carbide: Towards cemented carbides with optimal mechanical properties

Nanocrystalline tungsten carbide has been obtained by reduction/carburization at low temperature from precursors obtained by freeze-drying of aqueous solutions. Nanocrystalline WC powders with a adequate content of carbon were mixed with submicrometric Cobalt powder (12 wt.%), obtained by same synthesis method, and sintered in vacuum furnace. The cemented carbides fabricated from experimental powders were compared with both commercial ultrafine and nanocrystalline WC-12Co mixtures consolidated by the same route. The synthesised powders were characterized by X-ray powder diffraction, elemental analysis and scanning and high resolution transmission electron microscopy. On the other hand, density, microstructure, hardness and fracture toughness together with X-ray diffraction analysis of the sintered materials were evaluated. The cemented carbides obtained from synthesised powders exhibited a WC platelet-based homogeneous microstructure. This anisotropic growth might be due to the presence of stacking faults parallel to the basal plane in the starting WC powder, which would promote the defect-assisted preferential growth. These materials showed excellent mechanical properties, with a superior hardness/fracture toughness combination compared to materials prepared from commercial mixtures.     

EFFECTS OF SIZE COMBINATION AND CONTACTING STATE OF RAW POWDER PARTICLES ON SPARK PLASMA SINTERED ULTRAFINE CEMENTED CARBIDES

本文采用亚微米WC粉和纳米Co粉、亚微米WC粉和高能球磨后具有纳米晶组织的微米级Co粉这两种具有不同粒径匹配的混合粉末作为原料粉末，利用放电等离子烧结（SPS）技术制备超细晶WC-10Co硬质合金。对不同原料粉末的SPS过程及烧结试样的显微组织和性能进行了系统的对比分析。实验结果表明，以两种混合粉末为原料均获得了平均晶粒尺寸在200nm以下的超细硬质合金材料，其中，采用亚微米WC粉和高能球磨的微米级Co粉利用SPS技术制备的材料相对密度达到98%以上，硬度达到HRA94.5，断裂韧性达到13.50MPa•m1/2，表明具有优良的综合性能。而采用亚微米WC粉和纳米Co粉利用SPS技术制备出的超细晶硬质合金的组织均匀性和性能较差。根据SPS技术的特殊烧结机理，对采用不同粒径匹配和结合状态的WC和Co混合粉末的SPS致密化机制进行了分析。     

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。     

基体梯度结构对TiN涂层硬质合金力学和切削性能的影晌

研究基体梯度结构对TiN涂层硬质合金力学和切削性能的影响;采用阴极弧蒸发涂层工艺分别在均质和梯度硬质合金基体上制备TiN涂层:运用金相观察、扫描电镜分析、三点抗弯强度测试、显微硬度测试和切削性能测试,研究基体梯度结构对TiN涂层硬质合金组织结构、力学性能和切削性能的影响.结果表明:基体结构梯度化后,TiN涂层表面形貌由平整状变为网状结构,显微硬度提高19%,抗弯强度提高6.1%;基体结构梯度化后,涂层硬质合金的结构发生变化、力学性能得到提高,涂层刀片的抗冲击性能和切削性能分别提高10%和15%左右.     

碳化帆颗粒尺寸对纳米晶硬质合金组织和性能的影响

利用原位还原碳化反应制备纳米尺度的WC-Co复合粉体,应用放电等离子烧结(SPS)技术制备出纳米晶WC-Co硬质合金块体材料。分析了晶粒长大抑制剂碳化钒(VC)颗粒尺寸对纳米晶硬质合金的显微组织、晶粒尺寸及分布和力学性能的影响。结果表明:当VC的粒径减小到100 nm以下时,利用快速烧结技术可制备得到平均晶粒尺寸约为70 nm的致密WC-Co硬质合金块体材料,其物相纯净,晶粒尺寸分布均匀,维氏硬度为19.84 GPa,断裂韧性达到12.10 MPa·m1/2。     

含WB烧结硬质合金的组织结构和性能研究

将不同含量的WB粉末添加到传统成分的WC-Co粉末中,利用低压烧结技术制备了系列含WB的WC-Co型硬质合金,并对其物相组成、组织结构和力学性能进行了系统表征分析。研究发现,在低压烧结过程中WB与Co发生反应,生成了具有超高硬度的WCoB相,由此降低了粘结相Co对WC晶粒的隔离,增加了WC晶粒间的接触度,引起合金韧性下降。添加WB制备的硬质合金材料其摩擦系数更低,随WB添加量的增加,硬度和耐磨性明显提高,当WB添加量为30%(质量分数)时,制备的硬质合金材料的硬度达到19 000 MPa,其磨损速率仅为传统WC-Co硬质合金1/10。然而,添加WB的WC-Co合金的断裂韧性约为传统WC-Co硬质合金的83%~91%。     

SPS技术的进展及其在硬质合金制备中的应用

介绍了放电等离子烧结SPS技术的原理、特征、工艺影响因素及发展,并使用放电等离子烧结制备了WC-Co和TiCN系硬质合金,展望了放电等离子烧结在硬质合金制备中的应用。     

Study of characteristics and properties of spark plasma sintered WC with the use of alternative Fe-Ni-Nb binder as Co replacement

Most of all WC-based cemented carbides use cobalt as binder due to the excellent strength and ductility that this combination provides. Motivators to find alternative binders have been related to factors such as the shortage and price oscillations of the cobalt and toxicity of the WC-Co system. In this work, Fe-Ni-Nb was used as alternative binder for WC sintered via spark plasma sintering (SPS) technique. The composites were sintered at different sintering temperatures (1100 °C, 1200 °C and 1300 °C). In addition, WC-Co was sintered at 1200 °C via SPS for comparison purposes. X-ray diffraction and Scanning electron microscopy (SEM) were employed as characterization methods to investigate the crystalline phase's formation, sintering effectiveness, porosity and phase distribution. Mechanical properties such as Vickers hardness, fracture toughness, nanohardness, elastic modulus and thermal properties (thermal expansion coefficient) were evaluated. The results demonstrate Fe-Ni-Nb as a viable alternative binder to cobalt in hardmetal applications.     

放电等离子烧结温度对纳米硬质合金性能的影响

采用放电等离子烧结(SPS)这种新的烧结技术制取92WC-8Co纳米硬质合金。主要就放电等离子烧结92WC-8Co硬质合金的烧结温度进行研究探索,对不同的烧结温度进行对比实验,以找出最佳的硬质合金SPS烧结温度。最终发现:1150℃为放电等离子烧结纳米92WC-8Co硬质合金的最佳烧结温度,在该温度下,硬质合金制品可达到14.88g/cm3的致密度,硬度可达到94.2HRA。     

过渡族金属碳化物在WC-Co硬质合金中的界面偏析与固溶行为

过渡族金属碳化物是最常见的硬质合金微观组织结构与性能调控添加剂,添加剂在硬质合金中赋存状态的研究是WC晶粒生长抑制机理研究的基础,也是硬质合金材料与工艺设计的基础。从基于第一性原理计算的理论研究、基于高分辨透射电镜和原子探针层析法的实验观察分析等2个方面,综述了过渡族金属碳化物在WC/β(钴基固溶体粘结相)相界偏析行为的研究现状。基于过渡族金属碳化物在硬质合金中晶界与相界的偏析行为、在硬质相与粘结相中的固溶行为,讨论了硬质合金中WC晶粒生长机理与晶粒生长抑制机理以及过渡族金属碳化物对硬质合金性能的影响机理,提出了硬质合金材料与工艺设计的建议。     

高科技的发展对硬质合金的影响和冲击

高科技的迅猛发展，对各个领域均有影响和冲击，硬质合金行业也不例外；在这场影响和冲击中，会促进硬质合金自身性能的提高，并导致向多功能、高功能、智能化、微型化发展；在纳米技术和原子材料的推动下，传统硬质合金性能会有新的突破，而涌现出新一代硬质合金。     

WC-Co-(Ni)-(Cr)硬质合金在中性溶液中的腐蚀行为

以WC粉、Co粉、Ni粉及Cr3C2粉为原料,采用粉末冶金方法制备了3组不同粘结相成分的WC-Co-(Ni)-(Cr)硬质合金,通过极化曲线测试和浸泡实验研究了3组合金在中性溶液中的腐蚀行为,并采用扫描电镜、能谱分析、X射线光电子能谱(XPS)和EBSD等手段对其腐蚀机理进行了探讨。结果表明,WC-Co和WC-Co-Cr硬质合金在中性溶液中主要发生粘结相Co的腐蚀,浸泡产生的腐蚀产物主要是Co(OH)2;添加Cr将提高WC-Co硬质合金在中性溶液中的耐腐蚀性能,这可能与Cr的添加明显降低了粘结相中密排六方Co的含量有关;同时添加Ni和Cr可进一步提高WC-Co合金在中性溶液中的耐腐蚀性能,在pH=7的Na2SO4溶液中浸泡480 h后,WC-Co-Ni-Cr合金发生很少量的腐蚀。     

Microstructure and mechanical properties of polycrystalline-Ag/amorphous-CoZrNb multilayers

Ag/CoZrNb multilayers were prepared by direct current magnetron sputtering. Their microstructure, hardness, elastic modulus and plastic deformation were investigated by X-ray diffraction, high-resolution transmission electron microscopy, X-ray absorption near-edge structure, field emission scanning electron microscopy and nanoindentation technique. The results show that the multilayers have well modulated structure. For all the multilayers, Ag layer is polycrystalline structure, while CoZrNb layer is amorphous structure. With the decrease of modulation periodicity the elastic modulus decreases due to the compliant interface. The polycrystalline Ag layer not only makes the hardness enhanced with the decrease of modulation periodicity, but also restricts the propagation of shear bands and promotes the nucleation of new shear bands, which may be of help for the plasticity enhancement of amorphous CoZrNb layer.     

Wear of a new type of diamond composite

The erosion and abrasion resistances of a new type of composite consisting of diamond particles embedded in a silicon carbide matrix have been evaluated. The investigation is a comparative study in which a conventional cemented carbide, a commercially available polycrystalline diamond material and some common engineering ceramics are included as reference materials. The materials are characterised in terms of wear rates and scanning electron microscopy images of the worn surfaces.

It was found that the new diamond–silicon carbide composites were comparable to the commercial diamond composite, and out-competed the other reference materials in erosive and abrasive wear. The results are discussed in terms of the microstructural characteristics of the composites.     

Microstructure and mechanical properties of WC Ni–Si based cemented carbides developed by powder metallurgy

In this paper the influence of the Ni binder metal and silicon as an additional alloying element on the microstructure and mechanical properties of WC-based cemented carbides processed by conventional powder metallurgy was studied. Microstructural examinations of specimens indicated the presence of a very low and even distributed porosity and the presence of islands of metal binder in the microstructure of the cemented carbides. Furthermore, despite the addition of silicon and carbon in the cemented carbides, it was not observed the presence of small fractions of undissolved SiC and free graphite nodules in their microstructure. Vickers hardness and Flexural strength tests indicated that the cemented carbide WC–Ni–Si with 10 wt.% of binder presented bulk hardness similar to the conventional WC–Co cemented carbides and superior flexure strength and fracture toughness.     

VC和碳含量对超细晶硬质合金室温和高温性能的影响

以原位还原碳化技术制备的WC-8Co复合粉和VC粉末为原料,采用低压烧结技术制备出超细晶硬质合金。系统研究了VC添加量和复合粉中碳含量对硬质合金的相组成、显微组织、室温和高温力学性能的影响。结果表明:硬质合金的晶粒尺寸、硬度和断裂韧性主要受VC添加量的影响,且随VC添加量的增加呈单调变化;抗弯强度随VC添加量的变化趋势与碳含量有关;压缩强度随温度的变化呈现先降低后升高的趋势;当WC-Co复合粉的碳含量为5.60%～5.68%(质量分数)、VC添加量不超过0.5%时,可分别制备出室温抗弯强度为4482MPa和600℃下抗压缩强度为4914MPa的高综合性能的超细晶硬质合金。基于微观组织特征的分析,结合弹-塑性有限元模型对应力分布的模拟,对超细晶硬质合金力学性能的变化规律及影响机理进行了分析。