真空和渗氮烧结WC-TiC-Co硬质合金的梯度结构形成机理研究

本实验对真空和渗氮烧结的WC-20TiC-0.5VC-0.5Cr_2C_3-12Co硬质合金的微观结构进行了研究。研究表明烧结气氛对WC-TiC-Co硬质合金的梯度结构具有关键性影响:真空烧结能使硬质合金形成厚度不低于20μm的无立方相表层,该表层主要由WC与Co相组成,无明显TiC相特征;而渗氮烧结促使硬质合金形成以Ti(C,N)与TiC为主要物相的富立方相表层。与此同时,研究发现氮气压强对富立方相表层的形成具有显著促进作用,随着氮气压强的提高,富立方相表层厚度明显增加。真空和渗氮烧结的硬质合金芯部微观组织均由WC相、(W,Ti)C相、TiC相与Co相组成。相对于渗氮烧结,真空烧结会导致硬质合金芯部WC的晶粒度增大。     

表层富立方相功能梯度硬质合金的烧结工艺

本文对表层富立方相功能梯度硬质合金的三套烧结工艺进行了分析和对比。结果表明:烧结工艺的主要影响因素包括烧结温度、烧结时间及充气方式。在满足梯度层组织结构要求的前提下,应尽量降低烧结温度、缩短烧结时间,以防止晶粒粗化。气体在液相出现后充入,可加快反应速率且有利于消除孔隙。所制备的样品表面均生成了梯度层,其主要成分为富含Ti元素的立方相组织。根据分析结果,笔者对烧结工艺进行了重新设计。在液相温度进行气氛烧结后,控制降温曲线,在固相阶段进行保温烧结,以进一步增加梯度层厚度,同时对梯度层进行均匀化处理,消除内应力。     

40 Cr表面双辉低温等离子渗铬后的结构与耐蚀性能

金属表面双辉低温等离子渗铬可提高金属耐蚀性,且不损坏其组织和性能等.在650℃对40Cr钢表面进行双辉低温等离子渗铬,采用光学显微镜、扫描电镜、能谱仪、辉光放电剥层成分分析仪、X射线衍射仪及电化学腐蚀性能测试对渗铬试样的形貌、成分、元素分布、相结构和耐蚀性能等进行了研究.结果表明:40Cr表面沉积层厚4～5μm,沉积层与基体间出现少量的脱碳层,基体组织均匀、晶粒细小;渗镀试样内铬元素呈梯度分布;基材渗铬后表面铬含量提高,渗铬试样氮化后表层物相以CrN为主;与基材相比,渗铬试样在1 moL/L NaCl溶液中的抗点蚀能力增强,耐腐蚀性能提高.     

TC4表面激光渗氮研究

以TC4合金为基材,利用连续激光器对表面激光渗氮,生成了金黄色的氮化层。用SEM,EDS,XRD对试样渗氮层的微观组织结构、元素分布以及物质组成进行了分析,结果表明生成了氮化钛的缺位式固溶体。表层由氮化层、热影响区及母材组成。渗氮层与基材之间处于完全冶金结合状态,结合力大,不易剥落。利用显微硬度仪对渗氮层进行硬度分析,表层硬度提高了4倍以上,显微硬度≥1000HV0.3的厚度超过100μm。     

碳纳米管对硬质合金WC晶粒及强度的影响

以碳纳米管为增强相,WC-10Co硬质合金为基体,采用湿法球磨工艺将多壁碳纳米管(Multiwalled carbon nanotubes,MWCNTs)和WC-10Co硬质合金粉料充分混合,通过真空烧结制成CNTs增强的WC-10Co硬质合金试样。采用SEM观测MWCNT在硬质合金基体中的分布和烧结后硬质合金微观结构。SEM显示碳纳米管嵌插在WC晶粒之间,抑制WC晶粒生长,使WC晶粒得到细化。试样横向断裂强度达到1 780 MPa,对比未增强试样提高了7.8%。     

功能梯度Ti(C, N)基金属陶瓷制备技术

通过真空液相烧结制备出Ti(C,N)基金属陶瓷基体,并对基体表面进行双辉等离子渗碳处理。运用扫描电子显微镜(SEM) 、电子探针(EPMA)、X射线衍射(XRD)等分析手段对渗碳前后材料的显微组织形貌、成分分布以及物相组成进行分析。结果表明,双辉等离子渗碳后金属陶瓷表面富Ti、Mo、W、C、N元素,贫Ni。渗碳过程中表层高的碳活度驱使内部的Ti、Mo、W元素向外迁移,从而迫使Ni向内迁移。渗碳后,材料表层富硬质相,近表层富粘结相。渗碳处理使试样表层硬度得到提高,对横向断裂强度影响不大。     

烧结温度和保温时间对Ti(C, N)基金属陶瓷微观组织结构和性能的影响

通过氮气氛烧结制备了Ti(C, N)-WC-TaC-NbC-Co-Ni金属陶瓷,研究了烧结温度和保温时间对Ti(C, N)基金属陶瓷芯部和表面的微观组织结构及性能的影响。结果表明：Ti(C, N)基金属陶瓷表面在渗氮的作用下会形成富无环黑芯和黏结相层;随着烧结温度的升高,样品的平衡氮分解压力增大,渗氮作用减弱,表层富无环黑芯和黏结相层变薄;而随着保温时间的延长,表层富无环黑芯和黏结相层逐渐变厚,次表层缺黑芯越来越明显。随着烧结温度升高和保温时间延长,溶解再析出过程加剧,芯部Ti(C, N)黑芯体积分数减少,环相变厚且无环黑芯数量减少,硬质相晶粒尺寸增加。随着烧结温度的升高,合金硬度、钴磁和矫顽磁力逐渐减小,断裂韧性先增加后减小;随着保温时间的延长,硬度和断裂韧性均有先下降后升高的趋势,而钴磁和矫顽磁力逐渐增大。在1500 ℃保温40 min烧结的样品综合性能最好,维氏硬度达到1544 HV30,断裂韧性为9.0 MPa·m1/2,钴磁和矫顽磁力分别为4.74和9.42 kA/m。     

添加燃烧合成晶种SPS烧结Yb α-SiAlON陶瓷

通过燃烧合成技术制备单相Ybα-SiAlON粉体，然后作为晶种添加到原料中利用SPS技术快速烧结出致密Ybα-SiAlON陶瓷．对SPS烧结过程中试样的致密化、相转变和形貌发育进行了研究，并探讨了添加晶种对烧结产物相组成和微观形貌的影响．实验结果表明，SPS可使试样迅速完成致密化过程，适当的烧结温度和保温时间可以促进相转变和晶粒发育．添加晶种，不仅促进了相转变过程，而且可以在烧结体内获得柱状晶，有助于改善材料的韧性．     

硬质合金表面预处理状态对TiAlCrSiN/TiAlN涂层结构与机械性能的影响

采用电弧离子镀工艺在不同预处理后的硬质合金表面制备TiAlCrSiN/TiAlN涂层,采用扫描电镜、X射线衍射仪、纳米压痕仪、压痕法等研究硬质合金表面形态和涂层结构、力学性能和结合强度。结果表明：抛光处理后的硬质合金表面十分平整,表面粗糙度低至7.7 nm。酸洗处理使硬质合金表层Co相溶解而暴露出更多的硬质相,表面粗糙度增大,TiAlCrSiN/TiAlN涂层在酸洗处理的硬质合金上生长速率增大,晶粒细小,硬度和弹性模量高,但硬模比较低。喷砂处理使Co相塑性变形而包络更多的硬质相,表面粗糙度大幅增加,TiAlCrSiN/TiAlN涂层在喷砂处理的硬质合金上生长速率稍降低,晶粒粗大,硬度和弹性模量小,但硬模比较高。酸洗+喷砂复合处理的硬质合金表面TiAlCrSiN/TiAlN涂层的硬度、弹性模量、硬模比介于两种单独处理方式的之间。     

TiCN基梯度功能金属陶瓷的制备及其切削性能

以TiCN-WC-Mo2C-TaC-Co-Ni为原料,通过成分和气氛控制,获得了功能梯度TiCN基金属陶瓷材料。采用带有能谱分析仪的扫描电子显微镜和X射线衍射仪观察和分析了材料的微观组织和物相组成,通过车削实验考察了所制备的刀具的切削性能。结果表明,WC含量较低时,通过气氛控制可以获得表面富Ti层、中间富W层和基体的3层TiCN基金属陶瓷梯度层,烧结温度的提高有利于梯度层的厚度的增加;WC含量较高时,烧结后WC在表面富集。15%WC含量的TiCN基金属陶瓷在1 460℃氮气气氛中烧结1 h时,富Ti层的厚度约为15~20μm,富W层厚度约为15~20μm。经氮化烧结后材料表面的(Ti,W,Mo)(C,N)衍射峰相对材料内部向右偏移。真空烧结时,当WC含量较低时,Ti和W元素分布比较均匀;随着WC含量的升高,表面出现了贫Ti富W层,且随着WC含量增加,表面富W层增厚,WC含量为24%时,富W层厚度25μm。切削实验表明表面富Ti的金属陶瓷的切削性能优于表面富W的金属陶瓷材料。     

A novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides

Functionally graded or functionally gradient WC–Co cemented carbides with Co and/or hardness gradients can potentially have great practical importance. In this article is described a novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides. This technique includes (1) employing green carbide bodies with low (or high) carbon contents within the two-phase region of the W–Co–C phase diagram; (2) their pre-sintering in the solid state to obtain a certain green density and consequently gas permeability; (3) selective carburisation (or decarburisation) of their surface layer in a carburising (or decarburising) gas atmosphere; and (4) final liquid-phase sintering at tailored sintering conditions to obtain a Co drift (also known as ‘Co migration’) either from the surface towards the core or from the core towards the surface. The kinetics of Co drift between couples of model alloys with very similar WC mean grain sizes but different carbon contents were examined. The microstructure, hardness, Co contents, residual stresses and wear-resistance of the gradient cemented carbides with low-Co surface layers obtained by the selective surface carburisation of carbide green bodies with the original low carbon content were examined. Their surface layers were found to contain significantly less Co than the core resulting in a higher hardness of the surface layer. The surface layer is also characterised by high residual compressive stresses in both the carbide phase and binder phase, which results in an improved combination of hardness and fracture toughness. The microstructure, hardness and Co contents of gradient cemented carbide comprising high-Co surface layers obtained by selective surface decarburisation of carbide bodies with the original high carbon content were also examined. The surface layer of the gradient cemented carbide contains noticeably more Co than the core which is beneficial when using this functionally gradient carbide as a substrate for polycrystalline diamond coatings.     

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.     

板状WC晶粒WC-(Co-Ni)硬质合金的组织和性能

采用真空烧结法制备了板状WC晶粒WC-(Co-Ni)硬质合金,通过XRD、SEM、EDS等手段研究了Ni/(Ni+Co)比对硬质合金组织和性能的影响规律。结果表明:随着Ni/(Ni+Co)比的增大,硬质合金显微组织中板状WC晶粒的比例逐渐减少,硬质相颗粒的尺寸逐渐增大且平均长厚比逐渐减小。当Ni/(Ni+Co)比过大时,硬质合金中硬质相颗粒出现了团聚现象,使其力学性能显著降低。当Ni/(Ni+Co)比为0.3和0.5时,WC-(Co-Ni)硬质合金的综合力学性能较高,这与其硬质相颗粒较细和平均长厚比较大有关。当Ni/(Ni+Co)比为0.5时,WC-(5Co+5Ni)硬质合金具有较优的综合力学性能,其抗弯强度、硬度和断裂韧性分别为2 448 MPa、90.0HRA、21.2 MPa·m~(1/2)。     

Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide

Ultrafine WC–Co cemented carbides, combining high hardness and high toughness, are expected to find broad applications. In this study, WC–10Co–0.4VC–0.4Cr₃C₂ (wt.%) nanocomposite powders, whose average grain size was about 30 nm, were fabricated by spray pyrolysis-continuous reduction and carbonization technology. The as-prepared nanocomposite powders were characterized and analyzed by chemical methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis and atomic force microscopy (AFM). Furthermore, “sinterhip” was used in the sintering process, by which ultrafine WC–10Co cemented carbides with an average grain size of 240 nm were prepared. The material exhibited high Rockwell A hardness of HRA 92.8, Vickers hardness HV₁ 1918, and transverse rapture strength (TRS) of 3780 MPa. The homogeneously dispersed grain growth inhibitors such as VC, Cr₃C₂ in nanocomposite powder and the special nonmetal–metal nanocomposite structure of WC–10Co nanocomposite powder played very important roles in obtaining ultrafine WC–10Co cemented carbide with the desired properties and microstructure. There was an abundance of triple junctions in the ultrafine WC–10Co cemented carbide; these triple junctions endowed the sintered specimen with high mechanical properties.     

A transmission electron microscopy study of WC-10Co cemented carbides with modified hard and binder phases

The alloy design of WC-10Co cemented carbide, modified with addition of a hard carbide phase, TiC, and with Ni and Mo in the binder phase, has been highlighted by the authors in a number of publications. The present article deals with the fine microstructural features of various phases in such cemented carbides. WC grains in all the investigated cemented carbide compositions appear to develop straight facets during sintering because of their anisotropic nature. In contrast, the TiC phase is characterized by its rounded shape. Dislocations are present in both WC and TiC grains, being of lesser density in the latter. The binder phase is always associated with stacking faults. The nature of the hard phase/binder interfaces has been found to be dependent on the binder phase chemistry. The observed changes in microstructures and mechanical properties have been correlated with the wettability and solubility of the hard phases in the binder melt, and with the different strengthening mechanisms in the binder phase.     

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)。     

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.     

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。     

TiC含量对WC-TiC-TaC硬质合金材料微观组织及力学性能的影响

本研究采用真空热压烧结技术, 在1600℃下制备了WC-TiC-TaC硬质合金材料, 研究了TiC含量对其微观组织及力学性能的影响。结果表明, 随着TiC含量的增多, 硬质合金材料的晶粒显著增大。当TiC的含量从10wt% 增加到25wt%时, 硬质合金材料的硬度逐渐增大, 最高可达19.81 GPa, 这是由于TiC的硬度高于基体WC的硬度; 与此同时, 硬质合金材料的抗弯强度和断裂韧度逐渐减小。当TiC的含量为10wt%时, 材料的抗弯强度有最大值, 其值为1147.24 MPa, 这是由于在材料内部形成了均匀、细小的晶粒组织; 在此含量下, 复合材料的增韧机理为细晶增韧、裂纹偏转、裂纹分支、裂纹桥接和韧窝增韧, 其断裂韧度有最大值, 为14.60 MPa·m^1/2。     

Cutting Performance of WC-Co Alloys Modified by Nano-Additives

In this paper,the microstructure of WC-Co alloys with and without nano-additives was characterized by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The hardness and fracture toughness was tested by using a Vickers hardness tester and a universal testing machine.The cutting test was carried out at different feed velocities(250 r/min and 320 r/min),and the contact pairs are cutting tools and 45# steel bars.Results showed that the hardness and fracture toughness of WC-Co cemented carbides with nano-additives are higher than that of WC-Co cemented carbides without nano-additives,and they are increased 10.21% and 19.69%,respectively.The flank worn width and crater width of cutting tools decrease greatly with the addition of nano-additives.For the nano-modified specimen with WC grain size of 7 μm,both the flank worn width and crater width are the minimum after the cutting process.And there are little built-up layers and some pile-up regions on the flank face leading to high cutting performance for the nano-modified cemented carbides.There are some melted regions on the flank face of cutting tools without nano-additives,and the WC grains on the cross section of alloys without nano-additives show severe fragmentation.The wear type of WC-Co is flank wear,and the wear mechanism is abrasive,adhesion and oxidation wear.