超细、纳米晶WC-Co硬质合金烧结技术的研究现状

WC-Co硬质合金因高硬、耐磨而在切削、釆矿和耐磨零件等领域广泛应用。研究发现,当WC晶粒尺寸小于0.5μm时(即超细、纳米晶WC-Co硬质合金),与普通硬质合金相比,材料的硬度和强度显著提高,其韧性也同样会有所提升。因此,晶粒细化有助于改善硬质合金的力学性能,从而延长其使用寿命。长期以来,有关硬质合金性能改善方面的研究多关注于从粉体出发,即通过采用超细纳米粉体和合理烧结工艺来实现超细晶和纳米结构硬质合金的制备。然而,在合金制备过程中其致密性与晶粒长大之间往往存在较为复杂的交互作用,如何保证在烧结过程中致密化的同时抑制WC晶粒长大是提高合金性能以及保证合金质量稳定性的关键技术问题之一。本文主要阐述了高温液相烧结制备超细、纳米晶WC-Co硬质合金过程中有关致密化和晶粒长大机制之间的关联性,从烧结工艺与添加剂两方面介绍了近年来国内外的研究现状。烧结工艺具体分为常规烧结工艺(主要包括氢气烧结、真空烧结和热等静压烧结等)和快速烧结工艺(主要包括微波烧结、放电等离子烧结、高频感应热烧结等),对比了上述烧结工艺之间的不同以及总结了不同烧结工艺的优缺点。在添加剂方面,重点介绍了过渡族碳化物和稀土元素对硬质合金烧结过程中晶粒生长的抑制作用,并在此基础上阐述了超细、纳米晶WC-Co硬质合金烧结技术的未来发展趋势。     

纳米晶WC-Co硬质合金的研究现状

概述了国内外纳米晶硬质合金的发展现状.纳米晶WC-Co硬质合金制备的关键技术主要包括:优质纳米晶WC粉的制备和烧结过程中WC晶粒长大的控制.综述了优质纳米晶WC粉的特点和制备技术,以及目前国内外烧结过程中控制晶粒长大采取的主要措施:添加晶粒长大抑制剂、调整烧结工艺和开发新型烧结方法.列举了合金的实际应用领域,展望了纳米晶硬质合金的发展前景.     

1400℃放电等离子烧结制备MgO-B_(2)O_(3)增韧无金属黏结相WC硬质合金EI北大核心CSCD

为了降低无金属黏结相碳化钨(WC)硬质合金的烧结温度并获得较高的断裂韧度,采用MgO和B_(2)O_(3)协同增韧WC硬质合金。通过放电等离子烧结技术(SPS)在1400℃的较低温度下制备出致密的WC-MgO-B_(2)O_(3)硬质合金块体材料,研究MgO-B_(2)O_(3)对无金属黏结相WC硬质合金的烧结机理、微观组织演变以及力学性能的影响规律。结果表明:MgO-B_(2)O_(3)的添加促进了WC的烧结致密化,显著降低了无金属黏结相WC硬质合金的烧结温度。随着MgO-B_(2)O_(3)添加量的提高,组织中的部分第二相形貌发生显著改变,逐渐由短杆状转变为长杆状,再转变为聚集时的块状。当MgO-B_(2)O_(3)添加量达到8%(质量分数)时,块体材料具有较好的断裂韧度,为(9.45±0.37)MPa·m^(1/2),同时其硬度为(18.16±0.17)GPa。     

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)。     

VC/Cr3C2添加剂与烧结温度对超细WC-12Co硬质合金的影响

为了有效控制烧结过程中WC晶粒的长大,获得高强度高硬度的超细硬质合金,采用扫描电镜、拉伸机和洛氏硬度仪研究了不同质量分数及配比的VC/Cr3C2晶粒长大抑制剂和烧结温度对超细WC-12Co硬质合金的显微组织及力学性能的影响,并结合试验结果分析了超细硬质合金中VC/Cr3C2晶粒长大抑制剂的作用机理.结果表明,添加适量VC/Cr3C2晶粒长大抑制剂的超细硬质合金中WC晶粒尺寸分布集中,不存在明显的组织缺陷,合金具有细而均匀的微观组织及优异的力学性能.当晶粒长大抑制剂(质量分数)为0.2%VC/0.5%Cr3C2,1450℃烧结制备WC-12Co超细硬质合金的抗弯强度为3710MPa,硬度(HRA)为91.5.VC/Cr3C2晶粒长大抑制剂的作用机理为:VC主要与WC反应生成(W,V)C固溶体聚集在WC/Co界面,降低WC/Co界面能,Cr3C2主要固溶在粘结相中,导致WC在粘结相中的溶解度降低,二者的综合作用减缓了粘结相中WC溶解-析出过程,从而抑制烧结过程中WC晶粒的长大.     

碳化物抑制剂对WC-2.5TiC-10Co超细晶硬质合金微观组织及力学性能的影响

采用高能球磨和真空热压烧结相结合的方法制备了WC-2.5TiC-10Co超细晶硬质合金,并利用X射线衍射仪(XRD)、场发射扫描电镜(FESEM)等性能测试手段研究了Cr_3C_2、VC、TaC和NbC的添加对超细晶硬质合金微观组织和力学性能的影响。结果表明:经过球料比10∶1及转速为350r/min行星式高能球磨处理30h后,WC粉末的粒径由0.6mm减小到0.2mm以下;经过1410℃真空热压烧结1h后,XRD检测未发现新的反应物生成。添加0.45%Cr_3C_2、0.3%VC、0.5%TaC或NbC的硬质合金中有少量异常长大的WC晶粒,断口表面疏松且平坦,分析表明较大的WC晶粒在应力集中的作用下发生解理破坏,并成为材料断裂的裂纹源。当抑制剂Cr_3C_2和VC的含量再增加0.1%后,WC晶粒可以控制在0.5mm以下,断口表面致密成台阶状,抗弯强度可提高20%;TaC和NbC对抑制WC晶粒生长的作用并不显著,但添加NbC对提高硬质合金致密度的效果最显著。     

低压烧结温度对一步法制备超细晶WC-Co基硬质合金组织及性能的影响

采用等离子球磨技术制得W-C-10Co-0.9VC-0.3Cr_3C_2纳米复合粉体,并利用单向模压成型法将其压制成生坯,再经低压烧结一步法制备成硬质合金。研究表明,等离子球磨3h所获得的复合粉体呈片层状形貌,并且成分分布均匀。在1 380℃及1 400℃烧结时,由于等离子球磨的特殊作用,VC、Cr_3C_2对WC晶粒长大抑制作用突显。1 380℃烧结制备的硬质合金,致密度为99.2%,WC平均晶粒尺寸为250nm,硬度和横向断裂强度分别为92.3HRA和2 443 MPa,具有最佳的WC晶粒尺寸与致密度配合,以及最佳的综合力学性能。     

超细WC和细WC/Co添加对WC-10Co硬质合金微观结构与力学性能的影响

在粒径为0.5μm的超细碳化钨(WC)粉体表面包覆钴(Co)纳米颗粒获得细WC/Co,将细WC/Co、粗WC和Co粉通过球磨混合均匀，压制成型后在1420℃下真空烧结1 h,得到WC-10Co硬质合金。借助扫描电子显微镜、透射电子显微镜、万能试验机等对比研究细WC/Co和超细WC对WC-10Co硬质合金微观形貌和力学性能的影响。结果表明：相比于超细WC,细WC/Co促进合金的致密化，并形成双晶结构。添加细WC/Co和超细WC制备的硬质合金的平均晶粒度分别为2.18μm和3.57μm。细WC/Co的添加会降低晶粒生长速度并抑制细晶完全溶解，而粗晶通过缺陷辅助生长及溶解-析出生长机制生长为表面阶梯状的缺角三棱柱形；硬质合金的硬度和断裂韧度得到提升，二者分别为1131HV₃₀和22.1 MPa·m^1/2,而在1131HV₃₀同等硬度下，其断裂韧度比线性拟合的断裂韧度高27.7%。机理分析认为，超细WC的添加会导致异常晶粒产生，不利于性能；而细WC/Co的添加能够同时形成双晶结构和均匀的钴相分布结构，降低晶粒缺陷，提升综合力学性...     

烧结保温时间对超粗晶WC-10Co硬质合金微观结构及性能的影响

超粗晶WC-Co硬质合金因耐磨性高和韧性好成为研究的一个热点,而致密度和晶粒的控制是获得优异性能的关键.采用轻度球磨法获得添加超细WC的复合粉末,通过真空烧结制备平均晶粒尺寸为8.3～8.8μm的超粗晶WC-10Co硬质合金,研究烧结保温时间对致密度、WC晶粒及力学性能的影响.结果表明:随着烧结保温时间从30 min增至120 min,致密度先增加后下降,Co在合金表面聚集氧化并使内部孔隙增多,部分WC晶粒聚集形成异常晶粒,这些缺陷结构阻碍了孔隙的消除;超细WC和球磨破碎细WC的先后溶解析出,使WC平均晶粒度先增加后减小,晶粒分布变宽.当烧结保温时间为60 min时,曲面类球状WC部分通过台阶生长机制转变为性能友好型的圆边六棱柱晶粒,抗弯强度和冲击韧性达到最高,分别为1733 MPa和28 kJ·m-2.此外,烧结过程中部分晶粒中原生缺陷难以完全消除,而较长的烧结保温时间下,多种缺陷的增多降低合金性能.     

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。     

Grundlagen,Herstellung und Anwendungsaspekte des Supersolidus Flüssigphasensinterns von hochlegierten Werkzeugstählen und Metallmatrix‐Verbundwerkstoffen

Principles, manufacturing and application aspects of super solidus liquid phase sintering of high‐alloyed tool steels and metal matrix composites Iron‐based metal matrix composites (MMC) are applied for abrasive wear resistant applications. A common production route uses hot isostatic pressing (HIP) of metal and carbide powders, a comparatively cost intensive process. Using high‐alloyed tool steels as matrix materials it is possible to obtain dense materials by liquid phase sintering with an internally formed liquid phase. This contribution describes the basic principles of densification of the matrix materials taking thermodynamic calculations into consideration. It points out a production route for processing particulate reinforced, high wear resistant composite materials by sintering. Beside the sintering behaviour concepts for heat treatment as well as the abrasive wear resistance are discussed.     

硬质合金粉末挤出打印中增材制造工艺及其显微结构

粉末挤出打印(PEP)是基于传统金属注塑成型和3D打印相结合的新型增材制造技术,具有打印材料范围广、打印成本低等巨大优势。以WC-13Co硬质合金的PEP增材制造为核心,以热塑性打印材料为重点研究对象,开发打印原料的材料体系,研究打印原料的均匀性、流变性能、成形性能、黏结剂的脱除工艺以及烧结工艺对打印件显微结构及力学性能的影响机制。独立开发了硬质合金PEP打印专用的有机黏结剂材料体系,通过EDS分析黏结剂在打印坯体中分散均匀性。采用两步法脱脂工艺可以完全脱除打印坯体中的黏结剂,并结合真空烧结,在1450 ℃下保温60 min,成功制备高性能硬质合金打印件。研究结果发现打印件线收缩率为17.8%,WC晶粒尺寸分布均匀,维氏硬度1410HV₃₀。本研究采用PEP增材制造技术制备了高性能、打印件尺寸可控的硬质合金材料,为硬质合金的增材制造探索出一条有效的技术路线。     

Reactive processing of titanium carbide with titanium

The reactive sintering of titanium carbide with titanium metal was studied using mechanical mixtures of fine-grained powders heated in vacuum above the TiC-Ti eutectic temperature. Mixtures with bulk compositions of TiC_0.94 to TiC_0.63 yielded nonstoichiometric carbide with less than 0.5 wt% residual titanium metal after sintering, while residual metal was observed at higher titanium concentrations. The effects of time, temperature, and composition on Mohs hardness, final porosity and final grain-size were determined using a Box-Wilson experimental design. The experimental ranges studied were sintering times of 10 to 100 min, sintering temperatures of 1650 to 1850° C, and compositions from TiC_0.94 to TiC_0.58. Over these experimental ranges, the effects of time and temperature were small compared with those of composition. The Mohs hardness increased approximately linearly from two to nine with increasing percentage of titanium metal in the starting powder. The average grain size ranged from 15 to 70μm, increasing with increasing time and temperature. For bulk compositions TiC_0.94 to TiC_0.70 grain growth was largely due to the conversion of titanium to substoichiometric carbide which grows epitaxially on the carbide grains. Substantial grain growth occurred for higher metal concentrations. The open porosity decreased from 28% to 16% as the amount of titanium metal in the starting powders was increased. Both the grain growth and the densification during reactive sintering of titanium-titanium-carbide mixtures were analysed in terms of a sintering model adapted from Kuczynski. A factor which empirically describes the behaviour of the system over a range of compositions was incorporated into the equations proposed by Kuczynski. Microstructural evidence and the activation energies for grain growth and densification all indicate that the rapid reaction between titanium metal and titanium carbide to form substoichiometric carbide occurs via short-circuit diffusion of carbon out of the carbide grains along Ti₂C platelets. Low sintered densities are attributed to the rapid formation of a solid titanium-carbide skeleton which prevents significant particle rearrangement in the eutectic liquid. Solution-precipitation processes do not appear to contribute significantly to the densification in this system.     

Grain growth during spark plasma and flash sintering of ceramic nanoparticles: a review

Spark plasma and flash sintering process characteristics together with their corresponding sintering and densification mechanisms and field effects were briefly reviewed. The enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms. When the densification is aided by plastic deformation, the kinetics of grain growth depends on the particles’ rotation/sliding rate and is controlled by lattice and pipe diffusion. When the densification is aided by spark, plasma, and the particles’ surface softening, grain growth kinetics is controlled by viscous diffusion and interface reactions. Grain growth in both cases is hierarchical by grain rotation, grain cluster formation and sliding, as long as the plastic deformation proceeds or as long as plasma exists. Densification by diffusion in a solid state via defects leads to normal grain growth, which takes over at the final stage of sintering. Various field effects, as well as the effect of external pressure on the grain growth behaviour were also addressed.     

陶瓷材料烧结致密化过程的元胞自动机模拟

为研究陶瓷材料烧结致密化过程,以晶界能和晶界曲率生长驱动力理论为基础,建立了含有气孔的二相晶粒生长的元胞自动机模型,对陶瓷材料烧结致密化过程进行了模拟,并与制备的Al2O3/TiN陶瓷材料进行对比.结果表明,模型可有效地模拟陶瓷材料烧结时晶粒的生长及气孔的湮灭情况,能较好地再现烧结致密化过程,模拟结果与制备的陶瓷材料微观形貌组织十分接近.     

Three dimensional simulation of microstructure evolution for ceramic tool materials

The observation and scientific quantitative characterization of three dimensional microstructure evolution during sintering process of ceramic tool materials is important to investigate the influence of nano-particles on mechanical properties. The relationship between microstructure and mechanical properties of ceramic tool materials can be established to direct the development of nano-composite ceramic tool materials by the research of the grain growth, grain boundary migration, distribution of nano-particles and microstructure densification at the different sintering temperature and pressure. In this paper, a 3D Monte Carlo model of three-phase nano-composite ceramic tool material is built and applied to simulate the microstructure evolution during sintering process. In this model, the grain boundary energy of each phase and interfacial energy between two phases are taken into consideration as the driving forces for grain growth. The sintering temperature and pressure are successfully coupled into the Monte Carlo simulation model. The microstructure evolution of defect free three-phase nano-composite ceramic tool materials is successfully simulated at different sintering temperature and pressure. The simulation results show that the higher the sintering temperature is, the faster the grain growth. However, the sintering pressure has little effect on the grain growth.     

Mathematic modeling of diffusion processes of the formation of rim-core structure at sintering TiC–Ni alloys with alloying carbide additions

Using mathematical modeling the kinetics of the rim-core structure formation of TiC grains in an isothermal liquid-phase sintering of TiC–Ni hard alloy, doped with a transition metal carbide has been investigated. The algorithm that has been used to study the nature of the concentration distribution of the diffusant in a TiC grain allows for the structural characteristics of the alloy; diffusion coefficients of alloying additives in the metal melt and a grain, boundary solubility of the diffusant in the binder, and its concentration in the alloy as well as the sintering temperature and time.     

Composite materials of diamond−(Co–Cu–Sn) system with improved mechanical characteristics. Part 1. The influence of hot re-pressing on the structure and properties of diamond−(Co–Cu–Sn) composite

We have studied the structure and tribological behavior of composite materials of the diamond–(Co–Cu–Sn) system, which were produced by sintering in molds at 800°C for 1 hour with subsequent hot re-pressing. It is shown that a change of hot re-pressing conditions makes it possible to produce nanocarbide Co₃C in the diamond–metal binder transition zone, instead of particles of structurally free graphite arising during graphitization of the diamond grain surface. The presence of nanocarbide provides an essential improvement of wear resistance of the composites.     

氧化物陶瓷闪烧机理及其应用研究进展

闪烧是近些年广受关注的一种电场辅助烧结技术。本文介绍了闪烧的起源与发展, 并对闪烧的基本特征进行了分析。在闪烧孕育与引发过程的研究方面, 发现了孕育阶段的非线性电导特征和电化学黑化现象, 提出了氧空位主导的缺陷机制; 在闪烧阶段的快速致密化研究方面, 提出了电场作用导致的缺陷产生和运动会在粉体颗粒间产生库仑力, 有利于烧结前期的致密化过程, 同时发现闪烧致密化过程中还伴随着金属阳离子的快速运动; 在闪烧阶段的晶粒生长和微结构演变方面, 发现了试样温度沿电流方向呈非对称分布, 试样中间位置的晶界迁移率明显提高, 提出电化学缺陷对微观结构有重大影响。基于上述研究成果, 本团队利用电场作用下出现的低温快速传质现象, 发展了陶瓷闪焊技术, 实现了同种陶瓷/陶瓷、陶瓷/金属, 甚至异种陶瓷/陶瓷之间的快速连接; 发展了陶瓷闪烧合成技术, 不仅实现了典型氧化物陶瓷的快速合成, 而且实现了高熵陶瓷和具有共晶形貌的氧化物陶瓷的快速合成; 发展了氧化物陶瓷的电塑性成形技术, 初步实现了氧化锆陶瓷低温低应力下的快速拉伸和弯曲变形。本文最后总结了闪烧机理研究面临的挑战, 并从焦耳热效应和非焦耳热效应两方面展望了闪烧的发展方向, 期望对闪烧技术在国内的发展有所裨益。