碳辅助氢还原制备纳米钨粉的机理及其在纳米碳化钨粉和纳米晶WC-Co硬质合金中的应用

The preparation difficulty of tungsten carbide nano-powder and the rapid growth of tungsten carbide grain during liquid phase sintering are two problems restricting the development of nanocrystalline WC-Co cemented carbide. The applying project aims to solve these two problems. Because the fundamental reason of the growth of tungsten powder during hydrogen reduction is the volatility of tungsten oxide, the volatility of tungsten oxide is inhibited by reducing the effect of steam on the volatility of tungsten oxide by the reaction between carbon and steam and adding the tungsten oxide volatility inhibitor of chromium oxide and vanadium oxide. The inhibiting mechanism of tungsten powder growth by hydrogen reduction assisted by carbon is understood by studying the reaction requirement between carbon and steam and its effect on the growth style of tungsten powder. The inhibiting mechanism of the volatility of tungsten oxide by tungsten oxide volatility inhibitor is understood by studying the effects of volatility inhibitor on the volatility thermodynamics and dynamics of tungsten oxide. Tungsten nano-powder is prepared by adjusting hydrogen reduction temperature and carbon adding amount and adding appropriate chromium oxide and vanadium oxide. Tungsten nano-powder is treated by high temperature annealing before carbonization. The carbonization mechanism of tungsten nano-powder containing the tungsten oxide volatility inhibitor of chromium oxide and vanadium oxide is understood by studying the effect of high temperature annealing on the distribution of chromium carbide and vanadium carbide. Tungsten carbide nano-powder with chromium carbide and vanadium carbide distributing on its surface is prepared by controlling high temperature annealing technology and carbonization technology. Nanocrystalline WC-Co cemented carbide with high strength and high hardness is prepared by studying the effects of sintering technology and grain growth inhibitor on the grain size of tungsten carbide.

围绕制约纳米晶WC-Co硬质合金发展的两大难题（纳米碳化钨粉制备上的困难和碳化钨在液相烧结时的快速长大）开展研究工作。针对钨粉长大的根本原因是氧化钨的挥发，利用碳与水蒸汽反应降低水蒸汽对挥发的影响和添加氧化钨挥发抑制剂氧化铬和氧化钒达到抑制氧化钨挥发的目的；研究碳与水蒸汽反应的条件及对钨粉长大方式的影响，理解碳辅助氢还原抑制钨粉长大的机理；研究挥发抑制剂对氧化钨挥发热力学和动力学的影响，理解挥发抑制剂抑制氧化钨挥发的机理；调节还原温度和碳添加量及添加适量的氧化铬和氧化钒制备出纳米钨粉。在碳化前对纳米钨粉进行高温退火处理，研究退火工艺对碳化铬和碳化钒分布形态的影响，理解含有微量氧化铬和氧化钒的纳米钨粉的碳化机理，控制退火工艺和碳化工艺制备出碳化铬和碳化钒分布于碳化钨颗粒表面的纳米碳化钨粉。研究烧结工艺和晶粒长大抑制剂对碳化钨晶粒度的影响，制备出兼有高强度和高硬度的纳米晶WC-Co硬质合金。

制备性能优良的纳米W粉和WC粉是生产超细/纳米晶WC-Co硬质合金的基础和关键。针对氧化钨氢还原过程中因挥发-沉积作用而导致的W粉颗粒快速长大和异常长大的现象，采用碳辅助氢还原法制备纳米W粉，然后分别通过阶段碳化法和碳氢协同还原-碳化法制备纳米WC粉，并采用低压烧结制备超细晶WC-Co硬质合金。.研究了还原方式对W粉形貌、粒度和显微结构的影响，通过分析不同还原温度W粉的晶粒长大曲线，提出了碳辅助氢还原法制备纳米W粉的机理。结果表明：还原产生的水蒸气与碳反应生成CO和H2，显著降低体系中p[H2O]/p[H2]，抑制挥发性水合物WO2(OH)2的产生，W粉的主导长大方式也由挥发-沉积转变为原子扩散。还原方式会对W粉的粒径和形貌产生重要影响，碳氢协同还原W粉的还原长大机制以固相局部化学反应为主，所得W粉为均匀细小的球形颗粒，结构疏松、分散性良好；普通氢还原W粉的还原长大机制以挥发-沉积为主，所得W粉颗粒粗大，发育完全，呈现W本征晶体的多面体形貌。.以碳辅助氢还原法制备纳米W粉和碳黑为原料，采用阶段碳化法制备纳米WC粉。结果表明：WC 粉的粒径取决于 W 粉的碳化速率和长大速率，高的碳化速率和低的长大速率有利于降低粒径；低温预碳化能够在 W 粉颗粒表面形成一定厚度的 WC 层，使颗粒间的接触状态由 W/W 接触变为 WC/WC 接触，抑制碳化初期因W粉颗粒烧结合并长大而导致的 WC 粒径增粗。.采用连续碳氢协同还原-碳化法制备纳米WC粉,WC粉化合碳含量随前驱体配碳比升高而逐渐增加，当前驱体配碳比为3.6时，化合碳含量达到理论值 6.12%，游离碳含量为0.06%；当配碳比高于3.6时，游离碳含量迅速升高。还原-碳化过程中由W向WC的转变具有结构遗传性，长大系数在1.4～1.6之间，WC粉的平均粒径随还原温度升高而降低；升高碳化温度会促进WC粉颗粒的晶界迁移，WC粉的平均粒径随碳化温度升高而增大。调节制备工艺可以制备出粒径87.3nm的均匀细小的近球形WC粉。.以制备的纳米WC粉为原料，采用低压烧结技术制备超细晶 WC-Co 硬质合金，结果表明：随着烧结温度升高和保温时间延长，烧结体的致密度增加，平均晶粒尺寸增大，试样的硬度和抗弯强度也会随致密度上升而提高；若烧结温度过高或保温时间过长，则会使烧结体的晶粒发生异常长大，导致致密度降低，合金力学性能下降。