【作者】 马俊；

【导师】 朱世根；

【作者基本信息】 东华大学 ， 材料科学与工程， 2011， 博士

【摘要】 硬质合金材料被誉为“工业的牙齿”,具有高的硬度、良好的耐磨性、耐热性和断裂韧性,受到国内外学术界、产业界的高度重视,成为新型工具材料和结构材料的研究热点之一。在硬质合金材料中,钨钴类(WC-Co)合金的研究和应用最广泛,特别是超细晶WC-Co硬质合金成为当今重点的研究发展方向。然而,WC-Co硬质合金中的主要粘结剂Co是一种昂贵而稀缺的金属,是一种重要的战略资源,世界储量极其有限,且添加Co虽然可以改善硬质合金的强韧性,但亦会使硬质合金的硬度和耐蚀性受到影响。因此,开发制备兼有高硬度和高韧性、原料易得的新型复合材料,成为WC-Co硬质合金的理想替代物,具有重要的战略和经济意义。本研究从粉末制备和粉末烧结两个方面开展代钴硬质合金的研究工作。以WC-MgO复合材料作为研究对象,采用固相反应合成、高能行星球磨和热压烧结方法,制备了具有纳米结构的WC-MgO复合粉末以及具有高硬度和高韧性的WC-MgO复合材料。在研制过程中,系统地设计了粉末合成工艺以及块体烧结制备工艺,对固相反应合成粉末的热力学、机械力活化粉末作用、高能行星球磨制备复合粉末过程、块体热压烧结制度及其增韧机理、稀土氧化物对烧结过程促进作用、热压烧结致密化成型机理等问题进行了深入的探讨,取得了以下研究成果：1、利用机械力活化粉末的作用,用钨氧化物(W03)和石墨混合粉末通过直接固相碳热还原反应合成了具有纳米结构的WC粉末。根据反应热力学计算结果,预测了合成过程的温度和产物,并结合实验验证分析了机械力活化原始粉末对反应过程的影响。研究结果表明：经过10小时机械力活化,W03-石墨粉末在1215℃真空条件下发生碳热还原反应合成纳米WC粉末。机械力活化过程使原始粉末的比表面积提高、反应活度增加,有助于合成过程进行,而且期间生成的Magneli中间相,也促进了合成制备过程。2、采用高能行星球磨方法制备WC-MgO复合粉末,利用反向传递(BP)神经网络方法建立了球磨工艺参数与粉末形貌特征性能之间的关系模型。选择球磨转速、球料比、磨球直径作为模型输入参量,合成粉末的形貌特征(晶粒尺寸、中值粒径、比表面积)作为模型的输出结果,利用实验结果对模型进行训练与评价,由此还进一步对球磨工艺参数进行优化。研究结果表明：BP神经网络方法可以建立球磨参数与产物粉末形貌特征之间的关系模型,在球料比为10：1时纳米WC-MgO复合粉体制备最佳工艺为球料比：300-350r/min、磨球直径：8-10mm。3、对制备获得的纳米WC-4wt%MgO复合粉末进行了热压烧结实验和性能表征,通过物相分析、显微结构观察、力学性能测定,研究了烧结工艺条件对块体材料性能的影响,分析了第二相MgO颗粒对烧结块体的增韧作用。结果表明：在烧结温度为1650。C、保温时间为90min、烧结压力为39.6MPa的真空条件下进行热压烧结,制备获得致密度为94.56%TD的WC-MgO块体,维氏硬度可达15.43GPa,断裂韧性可达9.58MPa-m1/2,抗弯强度为1065.3MPa。提高烧结温度或者延长保温时间会造成晶粒粗化和异常长大,反之,降低烧结温度或缩短保温时间会使块体难以实现致密化。观察烧结块体的微观形貌可知,第二相MgO颗粒分布于WC基体之中。结合断裂力学模型分析发现：较小的第二相颗粒粒径与均匀的第二相颗粒分布状态使裂纹在WC-MgO复合材料中扩展路径加长,发生偏转,产生一定的增韧效果。4、为了获得第二相颗粒分布匀细致密的WC-MgO烧结块体,在WC-4wt%MgO原始粉末中添加稀土氧化镧(La2O3)作为烧结助剂,分析了La203添加量对烧结块体性能和微观组织的影响和作用。研究结果表明：优化稀土La203添加量(0.1-0.5wt%)可阻碍WC基体烧结过程中的脱碳反应,细化WC基体和第二相MgO颗粒烧结组织,抑制第二相颗粒的团聚、合并、粗化,提高MgO颗粒分散的均匀性以及颗粒/基体界面结合性能。5、根据粉末烧结过程不同阶段的致密化机理,设计了二步热压烧结方法。通过控制烧结温度变化实现烧结样品致密化,并抑制烧结过程晶粒长大。研究结果表明：不添加任何烧结助剂的条件下,最高烧结温度(T1)为1750℃、最后保温温度(乃)为1550℃时,能使烧结块体致密度达到99%TD,基体WC晶粒细小(2.59μm),第二相MgO颗粒分布匀细。相对于传统热压烧结,二步热压烧结方法制备的WC-MgO复合材料具有更好的综合力学性能(维氏硬度18.4GPa,断裂韧性12.95 MPa·m1/2,抗弯强度1283.7MPa),与热压烧结方法制备的WC-Co硬质合金性能相当。本研究首次利用固相反应合成、高能行星球磨和热压烧结技术成功地制备了纳米WC-MgO复合粉末和具有良好力学性能的WC-MgO复合块体,运用XRD、TG-DTA、DLLS、SEM/EDXS、TEM/HRTEM、DIL、SPM等测试手段对纳米粉末和复合块体结构进行了表征,系统地分析了复合粉末制备过程的机理以及复合块体的致密化、增韧机理,对进一步研究WC-MgO复合材料的批量制备与应用奠定了较坚实的基础。更多还原

【Abstract】 Among hard alloys, the ultra-fine WC-Co cemented carbides with superior hardness and toughness find wide industrial applications as tips for cutting tools and wear-resistant parts. The intrinsic resistance to oxidation and corrosion at high temperature also makes them desirable as protective coating for devices at elevated temperatures. Metallic binder (typically Co) is introduced to improve WC interparticle binding and to increase compact toughness. However, Co is expansive and rare and its reserves all over the world are very limited. Moreover, metallic binders result in reduced hardness and corrosion/oxidation resistance, and enhance grain growth, particularly in conventional liquid phase sintering due to rapid diffusion in the liquid phase. Therefore, efforts to obtain harder materials have attempted the preparation of WC with low amounts of Co and WC with no metal binder.A new composite material, WC-MgO is considered as an ideal material for use in industrial applications. Compared with the commercial micron- and submicron-grained structure WC-Co composites, the WC-MgO can achieve superior high value of hardness and toughness combination. In the current work, high energy planetary ball milling and its subsequent hot-pressing sintering were adopted to the synthesis of nanocomposite WC-MgO powders and the bulk material. The formation of nano-sized WC and WC-MgO composite powders were investigated at first. The solid-state reaction mechanism, the influence of its previous mechanical activation and the process of the composite powder synthesis were discussed. These were followed by an understanding on the hot-pressing sintering behavior and its improvements. Rare earth oxide addition and two-step-sintering method were selected for the further developing on the mechanical properties of the consolidated bulks. Some significant results have been achieved.Firstly, solid-state carbothermic reduction of tungsten oxide (WO3) to nano-sized tungsten carbide (WC) particles was obtained by calcining mechanically activated mixtures of WO3 and graphite at 1215℃under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, Magneli phase (WO2.72 and WO2) and metallic tungsten (W), were observed at 741℃, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.Secondly, a series of artificial-neural-network (ANN) models was developed for the analysis and prediction of correlations between processing (high-energy planetary ball milling) parameters and the morphological characteristics of nanocomposite WC-4wt%MgO powders by applying the back-propagation (BP) neural network technique. The input parameters of the BP network were milling speed, milling ball diameter and ball-to-powder weight ratio. The properties of the as-milled powders (specifically crystallite size, specific surface area and median particle size) were the output for three individual BP network models. These models were based on the mathematic statistical approach and seemed suitable for the complicated ball milling process which is difficult to be accurately described by physical models. Well acceptable performances of the neural networks were achieved. The model can be used for the prediction of properties of composite WC-MgO powders at various milling parameters. It can also be used for the optimization of processing and ball milling parameters.Thirdly, the obtained nanocomposite WC-4wt%MgO powders were consolidated into bulk materials via hot-pressing sintering. The influence of sintering regimes on the microstructures and properties of bulk materials was studied. It can be found that sintering temperature and holding time can greatly affect the properties of the as-sintered bulks. At lower sintering temperature and shorter holding time, the dense bulk structure cannot be obtained, while at higher sintering temperature or longer isothermal treatment, grains might coarsen. As a result, the mechanical properties of the as-sintered WC-MgO bulks might become unsatisfactory as well. The optimized sintering temperature can be determined regarding the bulk density and the best combination of hardness and fracture toughness. Hot-pressing sintering at the temperature of 1650℃with applied pressure of 39.6 MPa for 90 min can obtain a relative density of 94.56 %TD and the sintered compacts maintain their unique properties, i.e. superior hardness (HV= 17.78 GPa), toughness (Kc= 12.21 MPa·m1/2), and flexural strength (σ= 1065.3 MPa) combination. The improved toughness of WC-MgO composite can be attributed to the second phase toughening effects. The observations on the indentation cracks on the surface of the WC-MgO indicates that once the crack has reached particulate-matrix interface, the difference in the crack-tip opening displacement between the ductile particle and the brittle matrix would cause crack to be locally blunted, thus produce closure stress bridging the crack along its length. These effects require more external load to force the crack propagate further, thus induce improvement of toughness. In addition, crack deflections that enhance the energy for crack growth were also observed. It can be concluded achieving a high density and a small grain size are very important for the structural ceramic materials because it brings about an improvement of mechanical properties.Fourthly, a detailed investigation was carried out into the influences of the lanthanum oxide (La2O3) addition upon the microstructural characteristics and the mechanical properties of the WC-MgO composite bulk prepared by hot-pressing sintering. The results indicate that due to the unique properties of rare earth element such as high surface activity and large ionic radius, the addition of trace La2O3 can suppress the decarburization, promote the microstructural refinement and improve the particulate dispersion homogeneity and the particulate/matrix interfacial coherence. Consequently, the relative density of the sintered sample with 0.1 wt% La2O3 addition can be increased by 4.2% as compared with the sample without La2O3 addition. This indicates the possibility of preparing high-hardness (18.02 GPa) and flexural fracture strength (1265.9 MPa) WC-MgO composite material with adding the RE oxide (La2O3) using conventional hot-pressing sintering method.Fifthly, two-step hot-pressing sintering (TSS) was applied to consolidate nanocomposite WC-4wt%MgO powders. The first step sintering was employed at a higher temperature to obtain an initial high density, and the second step was held at a lower temperature by isothermal sintering for several hours to increase bulk density without significant grain growth. The experimental results showed the sintering temperature plays an important role in densification and grain growth of WC-MgO compacts. The optimum TSS regime consisted of heating at 1750℃(1st step) and 1550℃(2nd step), resulting in the formation of near full dense microstructure (99%TD) with suppressed grain growth (2.59μm). Accordingly, the improvement on the mechanical properties, including increase in the hardness (from 16.7 to 18.4 GPa), fracture toughness (from 10.2 to 12.95 MPa-m1/2) and flexural strength (from 976.6 to 1283.7 MPa), was also observed due to the grain refining and full dense bulk.In the current work, nanocomposite WC-MgO powders and the composite bulks, which achieve competitive values of hardness and fracture toughness, can be an ideal engineering material as the alternative of WC-Co. This study laid a solid foundation for the understanding of WC-MgO synthesis process and its batch-preparation application.更多还原