高温高压下碳化钨晶体的结构、力学、电子、光学以及热力学性能的第一性原理计算

本文利用密度泛函理论中的广义梯度近似对碳化钨晶体的三种结构(碳化钨相、闪锌矿相以及纤锌矿相)进行了优化,得到能量最低的稳定构型,并在此基础上计算了它的力学、电子、光学和高温高压下的热力学性质.研究表明:在0～300 GPa压力范围内,碳化钨相具有最高的稳定性.同时,高压下碳化钨相的弹性常数满足Born-Huang准则,且0 GPa和300 GPa下的声子色散没有虚频,证明了高压下碳化钨相的静力学稳定性和动力学稳定性.电子性质表明了碳化钨的金属性.光学性质表明碳化钨在高能区很难吸收光.热力学性质的研究表明:体积比V/V_0对压强的变化更敏感;高温时C_V曲线近似一条直线;给定压强下热膨胀系数α在600 K温度以上增长非常缓慢;压强对德拜温度Θ_D的影响较大;在低压下格林艾森系数γ的变化较大.     

Synthesis of micro- or nano-crystalline diamond films on WC-Co substrates with various pretreatments by hot filament chemical vapor deposition

Diamond films deposited on tungsten carbide can lead to major improvements in the life and performance of cutting tools. However, deposition of diamond onto cemented tungsten carbide (WC-Co) is problematic due to the cobalt binder in the WC. This binder provides additional toughness to the tool but results in poor adhesion and low nucleation density of any diamond film. A two-step chemical etching pretreatment (Murakami reagent and Caro acid, (MC)-pretreatment) and a boronization pretreatment have both been used extensively to improve adhesion of CVD diamond film on WC-Co substrates. Here we discuss the applicability of MC-pretreatment for a range of Co-containing WC-Co substrates, and demonstrate a controlled synthesis process based on liquid boronizing pretreatment for obtaining smooth and dense micro- or nano-crystalline diamond films on high Co-containing WC-Co substrates. Substrate treatments and deposition parameters were found to have major influences on the smoothness, structure and quality of the diamond films. The best quality diamond films were achieved under conditions of relatively high substrate temperature (T_s) and the best adhesion was achieved at T_s = 800 °C.     

Rare earth Ce-modified (Ti,Ce)/a-C:H carbon-based film on WC cemented carbide substrate

WC cemented carbide suffers severe wear in water environments. A novel carbon-based film could be a feasible way to overcome this drawback. In this study, a rare earth Ce-modified(Ti,Ce)/a-C:H carbon-based film is successfully prepared on WC cemented carbide using a DC reactive magnetron sputtering process. The microstructure, mechanical properties,and tribological behavior of the as-prepared carbon-based film are systematically investigated. The results show that the doping Ti forms Ti C nanocrystallites that are uniformly dispersed in the amorphous carbon matrix, whereas the doping Ce forms CeO_2 that exists with the amorphous phase in the co-doped(Ti,Ce)/a-C:H carbon-based film. The mechanical properties of this(Ti,Ce)/a-C:H film exhibit remarkable improvements, which could suggest higher hardness and elastic modulus as well as better adhesive strength compared to solitary Ti-doped Ti/a-C:H film. In particular, the as-prepared(Ti,Ce)/a-C:H film presents a relatively low friction coefficient and wear rate in both ambient air and deionized water,indicating that(Ti,Ce)/a-C:H film could feasibly improve the tribological performance of WC cemented carbide in a water environment.     

激光熔覆硬质合金强化化纤切断刀的研究

在机械压制法预置硬质合金WC/Co粉末的条形55Si2Mn弹簧钢上,用激光熔覆方法制备了化纤切断刀.调整熔覆层粉末配方中Al、TiC的加入量,结果表明加适量的Al粉能有效地抑制气孔,加TiC粉末能提高熔覆硬度.通过优化激光熔覆工艺参数,得到了无气孔缺陷、组织性能良好、硬度达到HV0.21250的激光熔覆层,达到了化纤切断刀的性能要求.     

Study on Laser Heat Treatment of Cemented Carbide

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Study of the Microstructure and Phase-Composition of a WC-Alloy with a Hadfield Steel Binder

The phase composition and structure of a tungsten carbide alloy with a Hadfield steel binder are studied using metallography, x-ray diffraction analysis, and transmission electron microscopy. It is found that tungsten carbide is dissolved in the binder during sintering. This gives rise to a nonuniform distribution of the alloying additions in the -solid solution, which results in precipitation of complex carbides of the (W, Fe) _x C type having different crystal-lattice parameters. The volume fraction of these carbides is as high as 9–10%.     

Phase transformation during surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser

Surface ablation of cobalt-cemented tungsten carbide hard metal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. Surface microphotography and XRD, as well as an electron probe have been used to investigate the transformation of phase and microstructure as a function of the pulse-number of laser shots at a laser fluence of 2.5 J/cm². The experimental results show that the microstructure of cemented tungsten carbide is transformed from the original polygonal grains of size 3 μm to interlaced large, long grains with an increase in the number of laser shots up to 300, and finally to gross grains of size 10 μm with clear grain boundaries after 700 shots of laser irradiation. The crystalline structure of the irradiated area is partly transformed from the original WC to βWC_1-x, then to αW₂C and CW₃, and finally to W crystal. It is suggested that the undulating ‘hill–valley’ morphology may be the result of selective removal of cobalt binder from the surface layer of the hard metal. The formation of non-stoichiometric tungsten carbide may result from the escape of elemental carbon due to accumulated heating of the surface by pulsed laser irradiation. Received: 13 July 2000 / Accepted: 27 October 2000 / Published online: 10 January 2001     

Automated laser fabrication of cemented carbide components

Automated Laser Fabrication (ALFa) is one of the most rapidly growing rapid-manufacturing technologies. It is similar to laser cladding at process level with different end applications. In general, laser cladding technique is used to deposit materials on the substrate either to improve the surface properties or to refurbish the worn-out parts, while ALFa is capable of near net shaping the components by layer-by-layer deposition of the material directly from CAD model. This manufacturing method is very attractive for low volume manufacturing of hard materials, as near net shaping minimizes machining of hard material and subsequently brings significant savings in time and costly material. To date, many researchers have used this technology to fabricate components using various alloy steels, nickel-based alloys and cobalt-based alloys. In the present study, the work is extended to tungsten carbide cobalt (WC–Co) composites. A set of comprehensive experiments was carried out to study the effect of processing parameters during multi-layer fabrication. The process parameters were optimized for the component-level fabrication. Fabricated components were subjected to dye-penetrant testing, three-point flexural testing, hardness measurement, optical and scanning electron microscopy and X-ray diffraction analysis. The test results revealed that the laser-fabricated material was defect free and more ductile in nature. Thus, ALFa technology, not only produced the quality components, but also minimized machining of hard material and brought significant saving of time and costly WC–Co material.     

Applications for ion implantation as a surface treatment process in production engineering

The special advantages offered by ion implantation for the treatment of metal components without inducing distortion or softening due to temperature rise are being examined in a number of industrial trials. Some examples of current evaluations are discussed covering manufacturing tools, dies and similar items. The types of potential application are broadly grouped into categories defined by cutting and drilling, corrosion and extrusion operations. The mechanisms for improved wear performance as a result of high dose gaseous ion implantation into tool steels and cemented tungsten carbide are briefly discussed.     

Mo离子注入对金刚石涂层附着性能的影响

采用Mo离子注入工艺对YG6硬质合金基体表面进行处理,用微波等离子体CVD(MPCVD)法沉积金刚石涂层,研究了Mo离子注入工艺对金刚石涂层附着性能的影响.结果表明,Mo离子注入后,硬质合金基体表面的化学成分发生了明显变化;采用适当剂量的Mo离子注入基体,可使CVD金刚石涂层的附着性能显著提高. 关键词： 金刚石涂层 Mo离子注入 硬质合金基体 附着性能     

Co元素对硬质合金基底金刚石涂层膜基界面结合强度的影响

采用基于密度泛函理论的第一性原理平面波赝势方法, 研究了硬质合金刀具基底黏结相Co元素对金刚石涂层膜基界面结合强度的影响机理. 借助Materials Studio软件建立了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型, 采用CASTEP仿真软件计算了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的最优稳定结构. 通过仿真计算, 获得了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的界面结合能、电荷密度图及Mulliken重叠布居数. 经对比分析后发现, 硬质合金基底中磁性元素Co的存在能转移金刚石涂层膜基界面处W元素及C元素的电荷, 从而使膜基界面处的原子因失电荷而相斥, 这直接导致了金刚石涂层膜基界面间距变大, 使得金刚石涂层膜基界面结合能降低.     

Physical and Mechanical Properties of W-Ni-Fe-Co Metal Foam Modified by Titanium Tungsten Carbide Alloying

Russian Physics Journal - The paper studies physical and mechanical properties of tungsten-nickel-iron-cobalt metal foam alloyed with titanium tungsten carbide. Test specimens are obtained by the...     

Abnormal grain growth of WC with small amount of cobalt

Abnormal grain growth (AGG) often appears in the synthesis of cemented tungsten carbide (WC), especially where ultra-fine powder is used. In this study, it was observed that AGG was strongly affected by the amount of Co. The obvious AGG was found to occur when the amount of Co was between 0.2 and 0.9 wt%. Careful examination of the etched samples found crack-like defects in abnormal grains, which were indications of grain boundaries in these large grains. Therefore, it is suggested that the considerable AGG of WC at lower Co concentrations be attributed to a grain boundary re-entrant edge (GBRE)-assisted two-dimensional nucleation process. Grain coalescence during sintering was the source of the grain boundaries. Due to the selective wetting of (0001) WC planes with the Co liquid, those grain boundaries were parallel to the (0001) planes resulting in platelet-like large grains with high aspect ratio. The amount of Co would affect the number of the wetted (0001) planes and the probability of grain boundary formation, and hence affect the AGG. The correspondence of the calculated weight percentage of Co required surrounding all the planes of WC grains and the experimental upper limit of the range supported this mechanism.     

复合型多晶金刚石末级压砧的制备并标定六面顶压机6-8型压腔压力至35GPa

通过分析二级6-8型大腔体静高压装置八面体压腔的受力状况, 研制了一种使用成本低、尺寸大且易于加工的多晶金刚石-硬质合金复合二级(末级)顶锤(压砧). 采用原位电阻测量观测Zr在高压下相变(α-ω, 7.96 GPa; ω-β, 34.5 GPa)的方法, 标定了由多晶金刚石-硬质合金复合末级压砧构建的5.5/1.5(传压介质边长/二级顶锤锤面边长, 单位: mm)组装的腔体压力. 实验表明, 自行研制的多晶金刚石-硬质合金复合末级压砧可使基于国产六面顶压机构架的二级加压系统的压力产生上限从约20 GPa提高到35 GPa以上, 拓展了国内大腔体静高压技术的压力产生范围. 应用这一技术, 我们期望经过末级压砧材料与压腔设计的进一步优化, 在基于国产六面顶压机的二级6-8 型大腔体静高压装置压腔中产生超过50 GPa的高压. 关键词： 二级6-8型大腔体静高压装置 多晶金刚石-硬质合金复合末级压砧 压力标定     

Study on electrical behaviour of copper and its alloys containing dispersed nanoparticles

Nanoparticles can be added to metals to tune their properties for numerous applications. Recently extensive research has been conducted to measure the mechanical properties of nanoparticle reinforced metals. However, few theories exist to understand how nanoparticles interact with metals to affect their electrical performance, partly due to the difficulty in producing bulk metal samples, containing dispersed nanoparticles. In this work, copper and copper alloys (Cu, Cu-40 wt% Zn, and Cu-60 wt% Ag) containing dispersed tungsten carbide (WC) nanoparticles of more than 20 vol% were successfully fabricated via solidification processing. The experimental results show that copper and its alloys with an increasing volume fraction of nanoparticles, the electrical conductivity of the samples decays exponentially. Therefore, a theoretical model, compatible with the Nordheim's rule was established to predict the electrical behaviour of metals containing dispersed nanoparticles. This new model on the electrical behaviour of copper nanocomposites is experimentally validated by low-temperature resistivity measurements and electronic heat capacity measurements above Debye temperature.     

First principles study of the structural,electronic, mechanical and superconducting properties of WX (X=C,N)

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure.     

Effect of active screen plasma nitriding pretreatment on wear behavior of TiN coating deposited by PACVD technique

Titanium based alloys are used extensively for improving wear properties of different parts due to their high hardness contents. Titanium nitride (TiN) is among these coatings which can be deposited on surface using various techniques such as CVD, PVD and PACVD. Their weak interface with substrate is one major drawback which can increase the total wear in spite of favorite wear behavior of TiN. Disc shaped samples from AISI H13 (DIN 1.2344) steel were prepared in this study. Single TiN coating was deposited on some of them while others have experienced a TiN deposition by active screen plasma nitriding (ASPN). Hardness at the surface and depth of samples was measured through Vickers micro hardness test which revealed 1810 Hv hardness as the maximum values for a dual-layered ASPN–TiN. Pin-on-disc wear test was done in order to study the wear mechanism. In this regard, the wear behavior of samples was investigated against pins from 100Cr6 (Din 1.3505) bearing steel and tungsten carbide–cobalt (WC–Co) steel. It was evidenced that the dual-layer ASPN–TiN coating has shown the least weight loss with the best wearing behavior because of its high hardness values, stable interface and acceptable resistance against peeling during wearing period.     

Numerical Study of Mechanical Properties of Nanoparticles of β-Type Ti-Nb Alloy under Conditions Identical to Laser Sintering. Multilevel Approach

A multilevel approach is used to numerically investigate physical and mechanical properties of titanium-based bcc alloys and their behavior under conditions identical to selective laser sintering. Plastic properties of P-Ti-Nb alloy are calculated within the first principles approach. An algorithm is proposed and tested to optimize the calculations and reduce their number by more than 5 times. A molecular dynamics method is employed to study structural changes of titanium and niobium powder particles during sintering and to calculate adhesion characteristics of nanoparticles of the produced alloy depending on the external action. The simulation results are in good agreement with the known experimental data and can be used as input data both for numerical models of a higher spatial scale and for the optimization of production parameters of titanium alloys by additive technologies.     

粗晶和纳米晶Sm_3Co合金的制备及其性能研究

本文针对Sm3Co粗晶和纳米晶合金材料的制备和基础性能进行了研究.采用磁悬浮熔炼技术多次精炼制备出Sm3Co粗晶合金.以此为母材,利用高能球磨非晶化和放电等离子烧结致密化并同步晶化的技术路线,制备出平均晶粒尺寸为8 nm的超细纳米晶Sm3Co合金块体材料.构建了Sm3Co纳米晶合金的晶体结构模型,并结合其显微组织的表征,分析了Sm3Co纳米晶合金的磁性能和力学性能,并与粗晶合金进行了比较粗晶Sm3Co合金不具有硬磁特性,而同种成分的纳米晶合金则表现出一定的硬磁特性.纳米晶Sm3Co合金的显微硬度和弹性模量分别达到4.87 GPa和63.7 GPa,比粗晶合金增大约8.7%和13.3%.本文研究结果为Sm-Co体系合金的基础性能及其纳米尺度效应提供了系统的参考依据.     

Laser induced breakdown spectroscopy for multielement analysis of powdered materials used in additive technologies

Additive manufacturing technologies utilizing metal parts production with layer by layer printing are of high demand for different fields of science and technology. Analytical chemistry is challenged to provide multielemental quantitative analysis of any metal powders within a few minutes and onsite to fulfil the requirements for high quality metal parts production. Powder materials utilized in additive technologies were quantitatively analyzed by laser induced breakdown spectroscopy for the first time. Laser induced breakdown spectroscopy mapping of loose metal powder attached to the double-sided adhesive tape provided high reproducibility of measurements even for powder mixtures with large difference of particles densities (tungsten carbide particles in nickel alloy powder). Laser induced breakdown spectroscopy analytical capabilities were estimated for tungsten and carbon analysis by calibration curve construction and accuracy estimation by leave-one-out cross-validation procedure. Laser induced breakdown spectroscopy and X-ray fluorescence spectroscopy techniques comparison revealed better results for laser induced breakdown spectroscopy analysis. Improved accuracy of analysis and capability to quantify light elements (carbon, etc.) demonstrated the potential of laser induced breakdown spectroscopy as a promising technique for express onsite multielement analysis of powder materials utilized in additive technologies.