Microstructures and erosion resistance of plasma-sprayed WCp/NiCrBSi composite coatings

WCp/NiCrBSi composite coatings have been deposited by plasma spraying with the mixed powders of WC-12Ni and NiCrBSi. The coatings consist mainly of WC, γ-Ni, Ni3B, CrB, Cr2B, M7C3, M23C6 and W2C phases. The W2C content increases with increasing WC mass fraction in the powders. The porosity and microhardness of the coatings are related to the coating WC content. The excessive WC results in decreasing the microhardncss due to increasing the porosity. The WCp/ NiCrBSi coating with 35 % WC mass fraction powder has more excellent erosion resistance. With an increase of impact angles from 15°to 90°the erosion rate of the coating increases, the erosion rate at 15°impact angle being approximately two times lower than that at 90°impact angle. Based on the wear morphology of the coatings at different impact angles, the wear mechanisms were discussed.     

Development of Si3Na/Al composite by pressureless melt infiltration

Pressureless infiltration process to synthesize Si3N4/Al composite was investigated. Al-2%Mg alloy was infiltrated into Si3N4 and Si3N4 containing 10% Al2O3 preforms in the atmosphere of nitrogen. It is possible to infiltrate Al-2%Mg alloy in Si3N4 and Si3N4 containing 10% Al2O3 preforms. The growth of the dense composite of useful thickness was facilitated by the presence of magnesium powder at the interface and by flowing nitrogen. During infiltration Si3N4 reacted with aluminium to form Si and AIN, the growth of composite was found to proceed in two ways, depending on the Al2O3 content in the initial preform. Firstly, preform without Al2O3 content gives rise to AIN, Al3.27Si0.47 and Al type phases after infiltration. Secondly, perform with 10% Al2O3 content gives rise to AIN-Al2O3 solid solution phase （AION）, MgAl2O4, Al and Si type phases. AlON phase was only present in composite, containing 10% Al2O3 in the Si3N4 preforms before infiltration.     

Microstructure and properties of Y2O3-doped steel-cemented WC prepared by microwave sintering

Steel-cemented WC was prepared by ball milling,cold compacting and microwave sintering with Fe powder as the matrix,WC as the hard phase and the addition of rare earth Y2O3.The results show that the interface of the WC particles and Fe matrix exhibits excellent wettability and liquidity when the microwave sintering temperature reaches 1,280°C.The density and mechanical properties of the steel bonded WC carbides could be greatly improved,the hard phases become finer and more uniform dispersed owing to the addition of Y2O3.With the increase of the Y2O3contents,the grain becomes uniform and fine first,and then gathers and grows up.The relative density,microhardness and bending strength all rise first,reaching the maximum values of 97.29%,HV1024 and 1,267.60 MPa at 0.5%Y2O3,respectively,and then decrease.Moreover,the relative density and mechanical properties of the steel-cemented WC with nano-Y2O3are higher than that with micron-Y2O3,which indicates that the effect of nano-Y2O3is better than that of the micron-Y2O3.     

Mechanical and friction properties of aluminum and titanium reinforced Ni-based self-lubricating composite

Ni-20Cr powders mixed with tungsten, aluminum, titanium, and different contents of molybdenum disulfides were hot-pressed in graphite mould by powder metallurgy method. Their tribological properties from room temperature to 600℃ and mechanical properties at atmosphere were tested. The results show that the hardness and anti-bending strength of composites increase by more than 20% when containing 6%（mass fraction） MoS2. But when molybdenum disulfide content exceeds 6%, the hardness and anti-bending strength will decrease gradually. The addition of MoS2 is favored to the reduction of friction coefficient of composite. The friction coefficient of composite decreases with the increase of molybdenum disulfide until the percentage of lubricant reaches 12%. In excess of this value, the friction coefficient value starts to ascend. The wear rates of composite with molybdenum disulfide are one order of magnitude lower than the alloy without lubricant. When the addition amount of MoS2 is in the range of 6% and 12%, the wear rates keep at the resemble level.     

Effects of graphite on the microstructure and properties of ultrafine WC-11Co composites by spark plasma sintering

Ultrafine WC-11Co hard metals added with different proportions of graphite were prepared by spark plasma sintering at 40 MPa/1200°C for 5 min,and the influence of graphite as free carbon on the microstructure and mechanical properties were investigated.The XRD analysis showed that decarbonization could be prevented by adding graphite.Compact hard metals composed of finer and more homogeneous WC grains with little flaws can be achieved after 0 wt.% to 1.5 wt.% graphite was added.The hardness and fracture toughness increase initially with increasing graphite content,and with over 1.5 wt.% they descend due to coarse grains and more defects.Therefore,1.5 wt.% graphite is the optimal addition content in view of the hardness and transverse rupture toughness.Furthermore,the coercive force decreases while the saturated magnetic intensity increases with the increase of graphite content.     

Selective laser sintering of polymer-coated Al2O3／ZrO2／TiC ceramic powder

A type of polymer-coated Al2O3/ZrO2/TiC ceramic powder was prepared. The laser sintering mechanism of polymer-coated Al2 O3/ZrO2/TiC powder was investigated by studying the dynamic laser sintering process.It is found that the mechanism is viscous flow when the sintering temperature is between 80 ℃ and 120 ℃, and it is melting/solidification when the temperature is above 120 ℃. The process parameters of selective laser sintering were optimized by using ortho-design method. The results show that the optimal parameters include laser power of 14 W,scanning velocity of 1 400 mm/s, preheating temperature of 50 ℃ and powder depth of 0.15 mm. A two-step posttreatment process is adopted to improve the mechanical properties of laser sintered part, which includes polymer debinding and high temperature sintering. After vacuum sintering for 2 h at 1 650 ℃, the bending strength and fracture toughness of Al2O3/ZrO2/TiC ceramic part reach 358 Mpa and 6.9 Mpa · m1/2 , respectively.     

Synthesis technology of spinel AlON

The high-purity AION ceramics, AlON/Al2O3 and AION/AIN multiphase ceramics were prepared by pressureless sintering under nitrogen-based atmosphere with raw materials of Al2O3, AIN and AI powders. The compositions of the phase were determined by XRD, the fracture appearance was analyzed by SEM. The results indicate： the aluminum oxynitride phase in the AION ceramics and other multiphase ceramics exist in the form of Al5O6N, whose solid-solution zone is 33.4%（mole fraction） AIN belonging to spinel structures. The average grain size of high-purity A1ON is about 3 p.m. The grain boundary is smooth without other phases. The density is 2.82 g/cm^3, the microhardness （10 N） and about bending strength are 1 470 MPa and 94 MPa, respectively.     

Oxygen content of high ferrotitanium prepared by thermite method with different melt separation temperatures

High ferrotitanium prepared directly by the thermite method has a disadvantageously high O content(≥10 wt%)because of the short slag-metal separation time.In this study,CaO and CaF_2 are added to the melt to improve the basicity of the slag and melt separation under heat preservation is performed to strengthen slag-metal separation.The thermodynamics of the step-by-step reduction process of TiO_2 in the Ti-Al-Fe-Si-O system whose composition is close to the alloy after melt separation were calculated.Samples of alloys and slags before and after melt separation were systematically analyzed.The result indicates that the reaction that TiO is reduced by Al to Ti is the limited step in the reduction process of TiO_2.The O content of the alloys slightly decreases with temperature from 1873 to 2023 K,which agrees with the changes in the law of deoxidation limit.It is mainly attributed to the movement of chemical reactions in the alloy melt at different temperatures and slag-metal interfacial reaction.The addition of Al_2 O_3-CaO-CaF_2 slag and high temperature promote the removal of Al_2 O_3 and titanium suboxides.The minimum contents of O and Al in the alloy reach 1.84 wt% and 3.26 wt%,respectively.     

Fabrication of Ni_3Al by hot pressing from element powders

Ni3Al intermetallic was synthesized by hot pressing from element powders of nickel, aluminum, and boron. The influences of parameters on the properties of Ni3Al were investigated. The parameters include the particle size of nickel powder, adding or without boron powder, hot pressing temperature, etc. The properties include the density of hot-pressed samples, resultant ratio of Ni3Al phase, and bending strength. The microstructures of hot-pressed samples were investigated by X-ray diffraction and scan electronic microscopy, and the properties, such as density and bending strength, were also measured. The results show that a higher bending strength was obtained under the same hot pressing conditions by the fine nickel powder than the coarse one, and there is little difference about density. Boron powder added in this process acceler- ates the formation of Ni3Al and markedly increases the hot pressed density. In the temperature range of this study, the den- sity increases along with the hot pressing temperature. Full dense Ni3Al samples were obtained under the condition of 860°C, 10 min, 45 MPa from Ni-22.89Al-0.5B powder.     

Reactive wetting phenomena of MgO-C refractories in contact with CaO-SiO2 slag

Wetting phenomena between MgO C and CaO SiO2 slags were investigated by varying carbon content.A sessile drop technique was adopted to study the wetting phenomena in conjunction with a high speed camera for the observation of intrinsic wetting phenomena.The results show that the high content of SiO2 and the presence of Al2O3 in slags enhance the diffusion of Mg2+,leading to the promotion of reactive wetting.The carbon in MgO C refractory impedes the penetration of slags by repelling the slag and slowing the diffusion of Mg2+.This accounts for the non-wetting behavior of the slag on MgO C refractory with 17% (mass fraction) carbon similar to that of graphite.     

Effects of Atmosphere and Temperature on the Preparation of Tungsten Carbide via a One-Step Method

介绍了一种一步法制备碳化钨(WC)的新方法,以钨酸钠为原料,通过碳纳米管模板和载体制备小颗粒钨酸,制备出钨酸和碳黑的混合物作为反应前驱体,分别以H2和N2作为还原及保护气体合成WC,当合成温度高于800℃,反应前反应管内抽成真空,并在反应管内通入氢气和氮气流时,WO3首先被还原为W,接下来W被还原为W2C,最后,W2C被还原为最终产物WC。实验过程中通过调节气氛和温度2个重要反应条件,探索出了控制WC形貌及大量制备的方法,为WC的低成本及可控制备提供了新的途径。     

Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite

Solid-state carbothermic reduction of tungsten oxide (WO₃) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO₃ and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO_2.72, WO₂ and metallic tungsten (W), were observed at 741 °C, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.     

Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres

Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO₃ powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH₄, or mechanical activation followed by thermal activation of tungsten and carbon precursors.In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H₂, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO₃ in H₂ containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.     

等离子熔覆添加和内生联合WC颗粒增强铁基涂层的组织和性能

采用等离子熔覆技术,以铸造碳化钨、钨铁粉、镍包石墨和铁基合金粉为原材料,在Q235钢基体上制备了外加和内生联合WC颗粒增强铁基复合涂层,通过扫描电镜和能谱分析、X射线衍射、硬度测试和磨料磨损试验对其微观组织、物相组成、硬度和耐磨性能进行了表征。结果表明,在优化的工艺参数下,可以获得与基体冶金结合良好的涂层,硬质相除外加的WC颗粒,还有内生的WC、W₂C、W₃C、Fe₃W₃C和Fe₂W₂C等;随着混合粉末中除外加WC之外的W含量增加,熔池中合金液密度增大,可以减弱外加WC颗粒下沉;当W含量达到15%时,外加WC颗粒均匀分布在涂层中,没有团聚现象发生,且在外加WC颗粒周围有细小的原位WC颗粒生成,涂层的显微硬度和耐磨损性能显著提高,涂层的平均硬度约为1300 HV0.2,耐磨性为Q235钢基体的10倍。     

A facile pathway to prepare ultrafine WC powder via a carbothermic pre-reduction followed by carbonization with CH4-H2 mixed gases

In the present work, ultrafine tungsten carbide (WC) powder with a high purity has been prepared by first roasting yellow tungsten trioxide (WO₃) and carbon black powder under argon atmosphere followed by the further carbonization reaction with CH₄-H₂ mixed gases. The effects of C/WO₃ molar ratio, CH₄ percentage in the CH₄-H₂ mixed gases on the phase composition, morphology and particle size of the products were discussed. The results revealed that when the C/WO₃ molar ratios were 2.5 and 2.6, nano tungsten carbide powder with the average particle sizes of 93 nm and 77 nm could be prepared. Whereas, when the C/WO₃ molar ratio was in the range of 2.7–3.5, the finally prepared WC has the particle size of 446–192 nm, and became smaller with the increase of C/WO₃ molar ratio. The percentage of CH₄ should be <15% to prepare WC with a low free carbon content. From the results of thermodynamic calculation, X-ray diffraction (XRD), FE-SEM, and infrared carbon‑sulfur analyzer, it was concluded that ultrafine tungsten carbide powders with a high purity could be successfully prepared by this method.     

水溶化学法制备纳米WC/Co复合粉工艺探索

本文通过实验探索了水溶化学法制备纳米WC/C0复合粉工艺,研究了影响喷雾转换、锻烧、碳化和调碳的工艺因素,找到了满足纳米WC/Co复合粉制备的工艺参数。在Kear等人的经典合成技术中,碳化钨钴纳米复合材料是由喷雾转化水溶液的化学计量量的水溶性钨源和钴源,然后用流化床通氢将钨钴氧化物还原为金属钨和钴,之后在一个充满CO/CO2的气体环境中将金属钨和钴碳化成纳米WUCo复合粉末。本研究不同于Kear等人的处理方法,涉及的WC/C0使用水溶性溶液钨、钴和碳前躯体加工的纳米复合材料,大量的WGCo纳米复合粉体是将钨、钴和碳在分子级水平上混合制备成一个复杂的前驱体粉末的独特方案,前驱体粉末在煅烧炉充满惰性气体约100撕00cC的温度下转化成一个含有W-Co-C-0的预复合粉粉末,随后在碳化炉低于1000℃的温度下碳化。实验表明,水溶化学法生产的纳米WC/Co复合粉较常规方法,具有晶粒细而均匀、流动性好等特点,更适于高性能硬质合金的生产。     

Effect of Helmholtz Oscillation on Auto-shroud for APS Tungsten Carbide Coating

The atmospheric-pressure plasma spray (APS) of tungsten coating was performed using tungsten carbide (WC) powder by means of DC plasma torch equipped with a stepped anode nozzle as a potential method of W coating on graphite plasma-facing component of fusion reactors. This nozzle configuration allows Helmholtz oscillation mode dominating in APS arc fluctuation, and the variation of auto-shroud effect with Helmholtz oscillation characteristics can be investigated. Tungsten coating made from WC powder has lower porosity and higher tungsten purity than that made from pure tungsten powder. The porosity and chemical composition of coatings were investigated by mercury intrusion porosimetry and x-ray photoelectron spectroscopy, respectively. The purity of tungsten coating layer is increased with the increasing frequency of Helmholtz oscillation and the increasing arc current. The modulation of Helmholtz oscillation frequency and magnitude may enhance the decarburization of WC to deposit tungsten coating without W-C and W-O bond from WC powder.     

Reactive spark plasma sintering of binderless WC ceramics at 1500 °C

Binderless WC ceramics were prepared by reactive spark plasma sintering, using tungsten trioxide, tungsten and carbon black as the starting materials. Phase assemblages and microstructure of the as-sintered ceramics were investigated. It was found that graphite existed as an impurity phase due to the volatilization of WO₃, and W could compensate for the WO₃ loss to form WC with a single phase. Benefiting from the enhanced sinterability, WC ceramics with high relative density and good hardness could be obtained at temperature as low as 1500 °C.     

水热合成钨前驱体-碳热还原-碳化制备超细纳米级碳化钨

提出了一种以水热合成的PbWO₄为原料,然后通过碳热还原-碳化获得超细WC的方法。以PbWO₄为钨中间产品,避免了氨氮试剂的引入;采用碳还原的方式可避免水蒸气的产生,抑制了钨粉的长大。结果表明：在初始pH为7.0、反应温度为160 ℃,反应时间为4.5 h的条件下,Na₂WO₄溶液中99.9%（质量分数）以上的W以PbWO₄的形式回收。然后采用低温碳还原PbWO₄,在C:W摩尔比为5、950 ℃的条件下还原3 h,获得了W和C的混合物,该混合物中预加富余的C有助于抑制钨粉的团聚。然后将W和C混合物高温碳化,在1200 ℃下反应6 h,获得了粒径约为60 nm的WC粉末。     

激光送粉熔覆WC／Co硬质合金的气孔问题

研究了激光送粉熔覆WC Co硬质合金的气孔问题。结果表明 ,激光熔覆WC Co形成气孔的倾向较大 ,细粉熔覆层的气孔率比粗粉高 ,多道叠加的气孔率比单道高。熔覆层组织为未熔化的WC颗粒、鱼骨状枝晶和Co固溶体基体。熔覆层含有WC、Co、FeW3C、Fe2 W ,CW3,Co3O4 等相。激光熔覆WC Co出现气孔的主要原因归结于高温熔池中的氧与碳反应形成CO或CO2 气体。在WC Co中添加一定量的金属还原剂Ti后 ,Ti能还原熔池中的氧化物 ,阻止O与C的化合 ,抑制CO或CO2 气体的生成 ,从而减少气孔率甚至消除气孔。加Ti后 ,熔覆层中的Co3O4 相消失而出现了Ti2 O和Co6 W6 C相。