【作者】 刘文彬；

【导师】 张久兴； 宋晓艳；

【作者基本信息】 北京工业大学 ， 材料学， 2009， 博士

【摘要】 与传统的粗晶硬质合金相比,超细晶WC-Co硬质合金具有更高的硬度、耐磨性、断裂韧性与横向断裂强度,是当今硬质合金行业的重点发展方向。原料复合粉末的合成和烧结是制备超细晶硬质合金的关键技术。本文首先利用放电等离子烧结（SPS）的特殊优势,研究了以低成本的氧化钨、氧化钴和碳黑在SPS系统内还原-碳化并快速烧结致密化制备块体WC-6wt.%Co硬质合金的一步法工艺。整个制备过程包括预热、还原和碳化、粉末烧结和保温四个阶段,时间总计在30min之内。系统研究了配碳量、反应温度、烧结温度、烧结时间和压力等对块体试样的物相和性能的影响,制备获得了具有高致密度和良好综合力学性能的块体硬质合金材料。首次建立了SPS快速反应机制模型,根据SPS过程中粉末颗粒接触面异常高温造成局部熔化或汽化的现象结合反应热力学揭示了非平衡态下化学反应速率提高、反应物需要量和生成物种类及含量不同于平衡态化学反应的原因,为SPS反应烧结工艺提供了重要的理论依据。随后研究了钨氧化物、钴氧化物和碳黑在真空环境下平衡反应的热力学、动力学和分步反应过程。利用反应热力学模型系统研究了平衡态原位反应过程中各物相生成的温度、稳定性及转化规律。计算结果表明,在8500C以下的反应基本为氧化物的还原反应,WO₃碳还原的顺序依次为WO_2.9、WO₂、WO_2.72和W,其与中间产物CoWO₄及还原产物Co起反应催化剂的作用。高于8500C的反应为钨的碳化和缺碳η相的逐渐消失,当反应温度高于11270C时,因反应自由能的变化为正值,W₂C将无法转化生成WC。系列模型预测结果得到了实验研究的证实。在WC-Co复合粉原位反应合成的动力学研究中,阐释了氧化钨碳还原包括碳黑与氧化钨的直接还原和利用反应气体产物CO与碳黑反应生成的CO₂作为媒介的间接气-固反应的过程;指出提高反应温度和增大生成物的孔隙度有利于加快反应。深入研究表明,氧化钨碳还原颗粒具有明显的层状结构,自外到内钨的化合价递增,还原程度降低;钨的碳化是碳原子替换体心立方钨晶胞的中心钨原子,然后碳原子转换到钨原子点阵间隙再经晶胞常数微调而形成。利用系列实验全面研究了反应物配碳量和各种工艺参数对制备的复合粉物相和粒径的影响。延长反应物球磨时间和原位反应时间、提高反应温度和真空度有利于提高物相的纯度;增加球磨时间,降低反应温度和保温时间有利于细化复合粉的粒径。在反应热力学理论模型和实验研究相结合的基础上,获得了最佳的复合粉制备工艺参数组合,开发出了低温、快速、高纯度及粒径可控的超细复合粉原位合成制备的新工艺。最后,以制备的复合粉为原料,应用SPS技术研究制备超细晶硬质合金块体材料。全面研究了SPS各工艺参数对烧结硬质合金块体显微组织和性能的影响。实验结果表明,超细复合粉的致密化开始于804℃,于1175℃固相致密化结束;烧结温度、烧结压力和保温时间明显影响WC平均晶粒尺寸及尺寸分布进而影响力学性能。在优化工艺参数-烧结温度1325℃、烧结压力50MPa、保温6-8min的条件下,获得WC-6Co硬质合金块体材料的优良综合性能:硬度92.6HRA、断裂韧性12MPa·m^1/2、横向断裂强度为2180MPa。探讨了复合粉中添加晶粒生长抑制剂对烧结块体WC晶粒尺寸和硬度、韧性的影响,并讨论了硬质合金块体材料的断裂机制。更多还原

【Abstract】 Compared to micro-grained alloy, ultrafine-grained and nanocrystalline cemented carbides have much higher hardness, wear resistant, fracture toughness and transverse rupture strength. Therefore, the research on the ultrafine-grained and nanocrystalline WC-Co cemented carbides becomes an important developing direction in the hardmetal industry. The synthesis of WC-Co powder and its sintering are two key steps in preparing the ultrafine structured cemented carbides.Firstly, taking advantage of spark plasma sintering （SPS） technique, we investigated the rapid route for synthesis of WC-6wt.%Co composite powder by in situ reduction and carbonization reactions and immediate consolidating the composite powder into the bulk in SPS system, using low-cost tungsten oxide, cobalt oxide and carbon black as raw materials. The whole process with the duration less than 30min could be divided into four stages, i.e., the heating of the powder mixture, in situ reduction and carbonization reactions, sintering densification and isothermal holding. The effects of milling time on the particle size and size distribution, and the effects of the amount of carbon addition in the mixture powders, the reaction temperature, the sintering temperature, the holding time and the sintering pressure on the phases and properties of the as-sintered bulk, were analyzed systematically. The cemented carbides bulks with high density and good combined mechanical properties were obtained. The mechanism of the rapid in situ preparation of cemented carbides bulks in the SPS system was proposed for the first time. According to the phenomenon of melting or boiling at the sintering neck and the thermodynamics of reactions, the reasons of the increased rate in the non-equilibrium reactions and the differences of the reaction amount and the type and amount of the resultant from equilibrium reactions were discussed. This mechanism provided an important theoretical basis for the in situ reaction synthesis in the SPS system.Secondly, the thermodynamics, the dynamics and the stepwise reaction processes in vacuum at the equilibrium state, using tungsten oxide, cobalt oxide and carbon black as raw powders, were investigated. By the thermodynamic mechanisms of the reactions in the synthesis process of WC-6wt.%Co composite powder, the starting temperature, regularity and stability of the resultants were quantitatively described. It was found that the reactions of carbon reducing tungsten oxide and cobalt oxide occurred below 8500C, and the sequence of products of WO₃ reduced by carbon was WO_2.9, WO₂, WO_2.72 and W. The intermediate product of CoWO₄, the products of Co and tungsten oxides played as the catalytic role. The reactions generated at above 850℃were the carbonization of tungsten, and theηphases disappeared with the reaction temperature increased. W₂C could not be transformed into WC at above 1127℃, as the change of the free energy of the reaction was higher than zero. The results of a series of synthetic experiments verified the mechanism predictions. In the study of the dynamics of in situ reaction synthesis of WC-Co composite powder, the process of tungsten oxide reduced by carbon consisted of the direct reaction of solid carbon with tungsten oxide and the indirect reaction of CO gas reducing tungsten. The fact that increaseing reaction temperature and porosity of the product were favorable to accelerate the reaction was stated. It was shown by further research that the formed tungsten oxide had an obvious layer-like structure, where from outer to inner layer the valence of tungsten increased and the reduction degree decreased. The carbonization of W depended on the replacement of W atom at the center of the body-centered cubic by C atom and the minor adjustment of C atom position and the lattice parameters.The carbon content in the raw powders and the effects of preparing parameters on the phase constitution and WC grain size were studied comprehensively by a serials of experiments. Increasing the milling and reaction time, reaction temperature and vacuum degree, were favorable to improve the purity of the phases. Increasing milling time, decreaseing reaction temperature and holding time, were good for refining the particle size. Based on the combination of the thermodynamics of reaction mechnism and experiments, the optimum processing parameters for synthesis of WC-Co composite powder were obtained, and an innovative route with low reaction temperature, short reaction time, pure phases and controllable WC particle size was developed.At last, the ultrafine cemented carbide bulk with high properties was prepared by spark plasma sintering the composite powder. The effects of SPS parameters on the microstructure and properties of the as-sintered bulk were investigated systematically. The experimental results showed that the densification temperature of the composite powder started at 804℃and finished at 1175℃. The WC grain size, grain size distribution and mechanical properties were influenced by the sintering temperature, the pressure and the holding time. The WC-6Co cemented carbides bulk with good combined mechanical properties of the hardness of 92.6HRA, the fracture toughness of 12MPa·m^1/2 and the transverse rupture strength of 2180MPa were obtained, with the optimal sintering parameters of the sintering temperature of 1325℃, the pressure of 50MPa and the holding time of 6-8min. When the grain growth inhibitors were added into the composite powder before sintering, the WC grain was refined and the hardness was improved. The rupture mechanism of the cemented carbides bulk was discussed at the end.更多还原