氧化钨氢还原过程时砷致钨粉细化机理及在超细晶WC-Co硬质合金中的应用

发布时间：2018-05-31 21:08

本文选题：As元素 + 纳米W粉　；参考：《南昌大学》2016年硕士论文

【摘要】：超细晶WC-Co硬质合金因其同时具备高硬度和高强度的特性,而被广泛应用于切削刀具、冲击工具和耐磨蚀零部件等领域。在硬质合金制备过程中,WC粉的特性直接遗传于W粉,因此制备分散性好、粒度均匀的纳米W粉是超细晶WC-Co硬质合金制备的关键技术。本文采用“固-液”混合法,在仲钨酸铵原料中添加一定量的As元素,通过煅烧、氢还原、碳化、液相烧结等工艺,制备出粒度均匀的纳米W粉、超细WC粉和性能优异的超细晶WC-Co硬质合金。研究过程中,采用XRD测定不同阶段样品的物相组成;SEM观察粉末或硬质合金样品的颗粒或晶粒大小、形貌,并结合EDS分析As元素在样品中的分布情况;TEM鉴定氧化钨粉、钨粉及碳化钨粉样品中As的存在状态及演变规律;利用ICP分析技术对仲钨酸铵、氧化钨粉、钨粉及碳化钨粉中的As含量进行定量分析;通过图像统计法测定粒径或晶粒大小分布;采用排水法、显微维氏硬度计和维氏压痕裂纹法分别测定硬质合金的致密度、硬度和断裂韧性。研究获得的主要结论如下:1.APT-As前驱体复合粉末煅烧过程中,一部分As元素会以气态As2O3的形式从前驱体中逸出,造成三氧化钨团聚体结构更加疏松多孔,这种多孔结构将有利于氧化钨氢还原过程中,氢气的进入及水蒸气的排出,从而降低炉内的水蒸气分压,抑制W颗粒通过“挥发-沉积”而长大。另一部分As元素会与W、O元素结合形成W2O3(AsO4)2三元化合物,最终As将以As2O3和W2O3(AsO4)2的形式存在于三氧化钨粉中。2.W-As复合粉末的粒径,随着As添加量的增加而逐渐减小。其中0.1 wt%As的添加量为W粉细化的临界值,当As添加量达到1.0 wt%时,可制备出平均粒径为80 nm的W-As复合粉末。在WO3-As复合粉末氢还原过程中,生成的WAs2第二相为W形核提供形核核心,增加W的形核率,并形成“核(WAs2)—壳(W)”结构的W-As复合纳米粒子;附着于W颗粒表面的WAs2粒子,将阻碍W颗粒通过晶界迁移而长大;此外,挥发性的As2O3和As将降低炉内水蒸气分压,抑制W颗粒通过“挥发-沉积”而长大。3.WAs2比较稳定,W粉碳化后,As仍以WAs2的形式存在于WC粉中,制得的WC-1%As复合粉末分散性良好、粒径均匀,其平均粒径为0.22μm。在碳化过程中,作为W形核核心的WAs2颗粒将裸露出来,分布于WC颗粒之间,阻碍了WC颗粒通过晶界迁移而长大,并减弱了细小WC颗粒之间的团聚作用。4.在WC-Co硬质合金液相烧结过程中,溶解于Co相内的WAs2将阻碍WC在Co相内的溶解及析出,进而抑制WC颗粒长大。YG6-1%As合金晶粒均匀,其平均晶粒为0.47μm,为超细晶WC-Co硬质合金。YG6-1%As合金的致密度、硬度及断裂韧性分别为97.3%、1239.8 Hv、10.86 Mpa·m1/2,与YG6合金相比,均有较大幅度地提高。
[Abstract]:Ultrafine grained WC-Co cemented carbide is widely used in cutting tools, impact tools and wear resistant parts because of its high hardness and high strength. During the preparation of cemented carbide, the properties of WC powder are inherited directly from W powder, so the preparation of nano-W powder with good dispersion and uniform particle size is the key technology in the preparation of ultrafine WC-Co cemented carbide. In this paper, nanometer W powder with uniform particle size was prepared by "solid-liquid" mixing method, adding a certain amount of as element into the raw material of ammonium paratungstate, through calcination, hydrogen reduction, carbonization, liquid phase sintering and so on. Ultrafine WC powder and ultrafine WC-Co cemented carbide with excellent properties. During the study, XRD was used to determine the phase composition of samples at different stages. The particle or grain size and morphology of the powders or cemented carbide samples were observed. Tungsten oxide powder was identified by EDS analysis of the distribution of as elements in the samples. The existing state and evolution rule of as in tungsten powder and tungsten carbide powder samples were analyzed quantitatively by ICP technique in the content of as in ammonium paratungstate, tungsten oxide powder, tungsten powder and tungsten carbide powder. The particle size or grain size distribution was measured by image statistics, the density, hardness and fracture toughness of cemented carbides were measured by drainage method, Vickers hardness tester and Vickers indentation crack method, respectively. The main conclusions are as follows: 1. During the calcination of the precursor powder, some of as elements will escape from the precursor in the form of gaseous As2O3, resulting in a more porous structure of tungsten trioxide aggregates. This porous structure will be beneficial to the entry of hydrogen and the discharge of water vapor during the reduction of tungsten oxide, thus reducing the partial pressure of water vapor in the furnace and restraining the W particles from growing through "volatilization and deposition". Another part of as will combine with WO to form W2O3(AsO4)2 ternary compound. Finally, as will exist in the form of As2O3 and W2O3(AsO4)2 in the form of W _ 2O _ 3. The particle size of W _ (2) as composite powder decreases with the increase of as content. The addition amount of 0.1 wt%As is the critical value of W powder refinement. When as content reaches 1.0 wt%, the W-As composite powder with an average particle size of 80 nm can be prepared. In the process of hydrogen reduction of WO3-As composite powder, the second phase of WAs2 can provide nucleation for W nucleation, increase the nucleation rate of W, and form W-As composite nanoparticles with the structure of "nucleated WAs2-shell W), and the WAs2 particles attached to the surface of W particles," In addition, volatile As2O3 and as will reduce the partial pressure of water vapor in the furnace. After carbonization of WAs2, the W particles still exist in the form of WAs2 in WC powder. The prepared WC-1%As composite powder has good dispersion and uniform particle size, and its average particle size is 0.22 渭 m. During carbonation, WAs2 particles, which are the core of W-shaped nucleus, will be exposed and distributed among WC particles, which prevents WC particles from growing through grain boundaries and weakens the agglomeration of fine WC particles. During liquid phase sintering of WC-Co cemented carbides, WAs2 dissolved in Co phase will hinder the dissolution and precipitation of WC in Co phase, and then inhibit the growth of WC particles. YG6-1As alloy grain is uniform. The average grain size is 0.47 渭 m, which is the density of ultrafine grained WC-Co cemented carbide. YG6-1As alloy, its hardness and fracture toughness are 97.39.8Hvn 10.86 Mpa M1 / 2, respectively. Compared with YG6 alloy, the average grain size is much higher than that of YG6 alloy.
【学位授予单位】：南昌大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG135.5

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