Preparation of WC and W2C by self-propagating high-temperature synthesis using a mixture of tungsten,titanium, and carbon black powders

Tungsten carbide and titanium carbide powders have been prepared from a mixture of tungsten, titanium, and carbon black by self-propagating high-temperature synthesis after mechanochemical processing. Conditions for the synthesis of the WC and W₂C carbides have been found, and the yield of the tungsten carbides has been shown to depend significantly on the composition of the tungsten + titanium + carbon black mixture and milling time.     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

感应等离子体技术制备球形铸造碳化钨粉体及其性能表征

球形碳化钨增强金属基复合涂层具有高硬度、高韧性和优异的耐磨、耐蚀性等特点,可以对材料表面起到有效保护作用。传统铸造碳化钨粉体多呈不规则的片状或多角状,流动性差且硬度低,难以满足高性能涂层材料的要求。本文以多角状铸造碳化钨粉体为原料,采用感应等离子体技术制备球形碳化钨粉体,研究感应等离子体技术工艺参数对碳化钨粉体球化效果的影响规律。采用扫描电子显微镜、X射线衍射仪、霍尔流速计、激光粒度分析仪等对球化处理前后碳化钨粉体的形貌、物相、松装密度、粒度分布进行表征。结果表明:送粉率为110 g/min、载气流量为5.0 L/min时,采用感应等离子体技术可制备颗粒饱满、表面光滑、分散性良好,球化率高达99%以上,且球形度较好的球形碳化钨粉体。球化后碳化钨粉体无孔洞等缺陷,内部组织为典型的细针状WC和W₂C的共晶,组织结构均匀细密。球化后碳化钨粉体的硬度高达3 258HV,提高了408HV;球化后碳化钨粉体的松装密度由8.01 g/cm³提高到9.75 g/cm³,霍尔流速由10.30 s/50 g降低到6.80 s/50 g,粉体的流动性提高。     

Effect of Carbon Addition on Microstructure and Properties of WC-Co Cemented Carbides

Based on a unique method to synthesize WC-Co composite powder by insitu reactions of metal oxides and carbon, the effects of the carbon addition in the initial powders on the phase constitution, microstructure and mechanical properties of the cemented carbides were investigated. It is found that with a suitable carbon addition the pure phase constitution can be obtained in the sintered bulk from the composite powder. The mechanical properties of the cemented carbides depend on the phase constitution and the WC grain structure. To obtain the excellent properties of the WC-Co bulk, it is important to obtain the pure phase constitution from the appropriate carbon addition in the initial powders and a suitable grain size.     

Simple preparation of tungsten carbide supported on carbon for use as a catalyst support in a methanol electro-oxidation

Two types of supported tungsten carbides were prepared via the impregnation of tungsten precursors on carbon support followed by heat treatment. Depending on whether ammonium metatungstate (AMT) or tungsten chloride (WCl₆) was used as the precursor, this process resulted in samples that are referred to as either WC-A or WC-W, respectively. Both WC-A and WC-W showed tungsten subcarbide (W₂C) as the major crystalline phase, with tungsten monocarbide (WC) as a minor phase. More amount of tungsten carbide being formed when WCl₆ was used as the precursor. This increased formation has occurred because the thermodynamically favorable properties of WCl₆ caused the contact area between the tungsten precursor and the carbon support to promote formation of tungsten carbide. The prepared tungsten carbides were used as a catalyst support of the Pt catalyst in a methanol electro-oxidation. The metal dispersion and the catalytic performance were increased as follows: Pt/C<Pt/WC-A<Pt/WC-W. It is believed that the tungsten carbides supported on the carbon support improved the dispersion of Pt and the activation of water for removal of intermediate CO, which enhanced the catalytic performance during the methanol electro-oxidation.     

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.     

The formation of tungsten carbide in cobalt-base wear-resistant coating alloys

Tungsten carbide-cobalt alloys cannot be produced by melting because of a peritectic reaction in the W-C system; they are produced by sintering. Tungsten carbide-cobalt powders (mixed, agglomerated, sintered) can be plasma sprayed or deposited by other techniques but they cannot be fused afterwards without decomposition of the tungsten monocarbide that provides the best mechanical properties for many applications.Wear-resistant cobalt alloys were developed 60 years ago and are based on cobalt-chromium-tungsten-carbon. During studies of the CoCrWC system with various carbon concentrations and at various temperatures we identified MC, M₂C, M₇C₃, M₂₃C₆, M₆C, M₁₂C and M₂₈C carbides. The solidifying M₆C carbide is unstable over a certain concentration range of chromium and decomposes to form tungsten carbide (WC). On heat treatment the tungsten-containing M₆C forms WC in a cobalt-chromium matrix if the chromium content is less than 5 wt.%. It is therefore possible to produce a sprayed and fused or welded layer of WC-cobalt alloy. The rate of WC formation depends on temperature and time. Time-temperature-decomposition diagrams have been established for four alloys. The structures of the heat-treated alloys are similar to those of sintered tungsten carbide-cobalt alloys.     

无金属粘结剂WC硬质合金增强增韧的研究进展与展望

无金属粘结剂WC硬质合金(Binderless tungsten carbide, BTC)硬度高，具有良好的耐磨性、耐腐蚀性，被广泛应用于刀具、耐磨零件等领域，成为近年来硬质合金领域的研究热点。然而，由于没有添加金属粘结剂，其在烧结过程中易出现晶粒长大，致密化难度加大，对烧结方法烧结工艺的要求较高，韧性难与金属粘结剂WC硬质合金相媲美。因此，一些研究人员通过添加非金属粘结剂及调整烧结工艺等方法抑制晶粒长大、促进其致密化，有效改善了BTC材料的性能。本文对于应用金属氧化物、金属碳化物、碳材料及复合增强增韧来提高BTC性能的研究进行综述，介绍了添加剂的种类、增强增韧机制及可以改善材料性能的烧结方法及烧结工艺。      

Effect of sintering temperature on the phase composition and microhardness of WC-8 wt % Co cemented carbide

The effect of sintering temperature (800–1600°C) on the phase composition, density, and microhardness of WC-8 wt % Co cemented carbide has been studied using x-ray diffraction, scanning electron microscopy, optical microscopy, and density measurements. The results indicate that, during sintering of the starting powder mixture, containing not only WC and Co but also the lower carbide W₂C and free carbon, W₂C reacts with cobalt metal to form Co₃W. At sintering temperatures from 900 to 1200°C, the reaction intermediate is the ternary carbide phase Co₆W₆C. During sintering at 1300°C, this phase reacts with carbon to form Co₃W₃C. Sintering at 1000°C and higher temperatures is accompanied by the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co(WC). The density and microhardness of the sintered samples have been measured as functions of sintering temperature, and the optimal sintering temperature has been determined.     

Nanoparticle dispersion-strengthened coatings and electrode materials for electrospark deposition

Advanced electrode compositions were developed using self-propagating high-temperature synthesis (SHS). Electrospark deposition (ESD) was applied to produce tribological coatings which were disperse-strengthened by incorporation of nanosized particles. Nanostructured electrodes of cemented carbides were produced using powder metallurgy technologies. They allow increasing the coatings density, thickness, hardness, Young's modulus and wear resistance. Positive effects of the nanostructure of electrodes on the deposition process and structure and properties of the coatings are discussed. In that case the tungsten carbide phases become predominant in the coatings. A mechanism of the dissolution reaction of WC with Ni at the contact surface of electrode was proposed. It was shown that the formation of the coating structure starts on the electrode and is accomplished on the substrate.     

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     

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)。     

A versatile route for the synthesis of Pt-loaded mesoporous CNT heterostructure

Abstract

A simple and versatile route for the synthesis of mesoporous carbon nanotube (CNT)-supported platinum/tungsten carbide (Pt/WC) nanoparticles (NPs) was set up via vapor deposition process. Amorphous CNT (α-CNT) was used to immobilize Pt/WC NPs. Platinum 2,4-pentanedionate, hexacarbonyltungsten and α-CNTs were selected as raw materials. Large density of W₂C/Pt NPs was uniformly decorated on the wall of α-CNTs and W₂C NPs were amorphous structured. The diameters of Pt NPs ranged from 3 to 6?nm, with mean diameter of about 4?nm. This approach provides an efficient method to fabricate carbides or other non-carbides metal NPs on CNT for the in situ synergistic fabrication of heterostructures.     

Enhancement in the microhardness of arc plasma melted tungsten carbide

Microhardness is found to increase significantly in arc plasma melted tungsten carbide. To understand the mechanism for such increase, tungsten carbide powder was mixed with tungsten metal powder to prepare mixtures of seven different compositions. The mixtures with varying WC/W ratio were pelletized and melted in an arc plasma followed by cooling in the furnace. It is observed that microhardness value enhances in the product when WC/W₂C ratio becomes high. Based on our microstructural finding of <100> WC hard faces and lamellar/acicular structures (due to martensite transformation) carried out by XRD, optical microscope and SEM, an attempt has been made to understand the reason behind the enhancement in microhardness.     

Reactive deposition of tungsten and titanium carbides by induction plasma

A study is reported on the use of induction plasma technology for the preparation of dense free-standing deposits of tungsten carbide and titanium carbide from metallic powders and methane. Phase analysis by X-ray diffraction indicates that primary carburization of the particles takes place in-flight giving rise to the formation of W₂C and TiC_1–x . Secondary carburization occurs in the deposits resulting in the formation of tungsten and titanium carbides. Microstructures revealed by optical and scanning electron microscopy show uniform small grains of the carbides. The reactive plasma spray-formed tungsten carbide shows transgranular fracture, while pure tungsten deposits show intergranular fracture.     

Mechanical alloying and sintering of nanostructured tungsten carbide-reinforced copper composite and its characterization

Elemental powders of copper (Cu), tungsten (W) and graphite (C) were mechanically alloyed in a planetary ball mill with different milling durations (0–60 h), compacted and sintered in order to precipitate hard tungsten carbide particles into a copper matrix. Both powder and sintered composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and assessed for hardness and electrical conductivity to investigate the effects of milling time on formation of nanostructured Cu–WC composite and its properties. No carbide peak was detected in the powder mixtures after milling. Carbide WC and W₂C phases were precipitated only in the sintered composite. The formation of WC began with longer milling times, after W₂C formation. Prolonged milling time decreased the crystallite size as well as the internal strain of Cu. Hardness of the composite was enhanced but electrical conductivity reduced with increasing milling time.     

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.     

Fabrication of Functionally Gradient Cemented Carbide with Ultrafine Grains

At present, the functionally gradient cemented carbide (FGCC) substrate with enrich cobalt on surface is mainly formed from medium grained WC grains. In order to further improve the properties of gradient cemented carbides, the ultrafine powder was chosen in this study and the functionally gradient cemented carbide with ultrafine grains was prepared by a two-step process, where the cemented carbide is first lower pressure pre-sintered and then subjected to a gradient sintering. The results show that it is possible to form gradient layer with enriched cobalt on surface by this method and also the grain growth can be inhibited by low pressure pre-sintering. Ultrafine grain gradient cemented carbide was fabricated after the gradient sintering, the thickness of gradient layer was about 43μm and the average grain size of WC is about 0.42μm. The formational mechanism of the functionally gradient cemented carbide with ultrafine grains are discussed through analyzing the influence of ultrafine microstructure, which was obtain by lower pressure pre-sintering, on atomic diffusion and grain growth during gradient sintering process.     

超音速火焰喷涂制备纳米和微米结构WC-η涂层的耐熔锌腐蚀性能

以钨氧化物、钴氧化物和炭黑为原料, 通过原位还原碳化反应制备纳米WC-η(η为Co₃W₃C、Co₆W₆C等缺碳相)复合粉, 粉末平均粒径为155 nm。该复合粉经团聚造粒制备得到具有高致密性和良好流动性的热喷涂粉末。以此纳米结构和商业化的微米结构低碳WC-12Co粉末作为喂料, 通过超音速火焰喷涂制备硬质合金涂层。结果表明, 纳米结构涂层中生成了一定量等轴状的W₂C相, 裂纹主要沿晶界或相界面扩展, 而微米结构涂层中除W₂C外还含有较多的W相, 主要包裹在WC颗粒表面, 穿晶断裂比例较高, 裂纹扩展路径较平滑。由于纳米结构涂层组织致密、晶粒细小、界面积大, 因此比微米结构涂层具有更高的硬度和断裂韧性。两种涂层在熔融锌液中浸泡200 h后, 微米结构涂层中产生了较多的横向和纵向裂纹, 导致材料的大面积剥落和基材腐蚀; 纳米结构涂层中没有发生锌的浸蚀, 在局部产生了少量纵向裂纹, 裂纹间隙被钨钴氧化物所填充, 反而抑制了熔锌对涂层的腐蚀, 因此纳米结构涂层表现出更高的耐熔锌腐蚀性能。     

抑制剂对WC-Co硬质合金涂层性能的影响

利用原位还原碳化反应合成的超细WC-12Co复合粉末作为原料, 分别添加1.0wt%晶粒长大抑制剂即VC、Cr₃C₂和NbC, 经团聚造粒和超音速火焰(HVOF)喷涂制备了超细结构的硬质合金涂层。研究了不同晶粒长大抑制剂对涂层的显微组织结构、物相、硬度、耐磨性能和耐蚀性能的影响。结果表明, 与未添加晶粒长大抑制剂涂层相比, 添加1.0wt% VC或Cr₃C₂制备的硬质合金涂层中WC颗粒的平均尺寸降低了约49%, 涂层硬度明显提高, 磨损速率降低了约52%~55%。添加1.0wt% NbC对制备涂层中WC颗粒尺寸的抑制作用不明显, Co粘结相中由于形成了(W, Nb)C化合物, 其耐蚀性获得显著提高, 但该化合物脆性大, 导致涂层耐磨性不及添加VC和Cr₃C₂制备的涂层。