【作者】 任莹晖；

【导师】 张璧； 周志雄；

【作者基本信息】 湖南大学 ， 机械制造及其自动化， 2009， 博士

【摘要】 高硬度、高韧性和高耐磨损性的纳米结构硬质合金,在各种对材料有高耐磨损性和高热稳定性要求的工程领域具有广泛的应用前景。纳米结构硬质合金优异的物理机械性能有助于该类材料在工程中的应用,但高硬度、高耐磨损的性能也使得对它的磨削加工变得非常困难。现阶段,对纳米结构硬质合金的研究多集中在如何改善其物理机械性能、保持其质量的稳定性和对其耐磨损性能的评价等材料学研究领域。还未对这类新型材料的机械加工理论(特别是磨削理论)和相关加工工艺展开系统的研究。本文旨在通过理论建模和磨削工艺实验,对纳米结构硬质合金的磨削理论和工艺进行了深入系统的研究:首先,介绍了纳米结构硬质合金的制备技术、应用领域和应用中的问题;回顾了工程陶瓷磨削的主要研究成果,包括硬质合金的磨削机理和磨损机理研究成果;进而提出了本文的研究结构和内容。然后,分析了材料物理机械性能与磨削力和磨削损伤之间的关系;在磨粒与工件相互干涉的运动学和几何学基础上,建立了基于断裂力学理论的磨削力数学模型,并进一步建立了比磨削能数学模型。数学模型说明磨削力和比磨削的大小与磨削工艺参数和材料的物理机械性能相关。接着,选用了四种具有不同WC晶粒度(从微米级到纳米级)的硬质合金进行磨削工艺实验,分析磨削工艺参数和WC晶粒度对纳米结构硬质合金磨削性能的影响规律。实验过程中采用实时测力系统采集三向磨削力;工艺实验后用扫描电子显微镜、能谱仪、原子力显微镜、表面轮廓仪和X射线衍射仪分析磨削后试样的表面形貌、表面粗糙度和材料去除机理;采用端面研磨腐蚀法、直接腐蚀法和表面研磨法制备了亚表面磨削损伤观察的试样,观察了试样亚表面的形貌和磨削损伤;采用X射线衍射法,分析残余应力在亚表面深度方向的变化规律。最后,比较了磨削工艺实验和理论模型预测的结果,揭示了材料的物理机械性能和磨削工艺参数对磨削力、比磨削能、材料去除机理、表面形貌、表面粗糙度和磨削损伤的影响规律。磨削力数学模型的预测结果与工艺实验结果相吻合,证明本文中建立的磨削力数学模型正确。随之,比磨削能数学模型的正确性也被证实。在本文工艺实验条件下,虽然随着单颗磨粒最大未变形切屑厚度增加,纳米结构硬质合金以脆性断裂方式去除的比例增加,但是非弹性变形方式仍然是纳米结构硬质合金磨削主要的材料去除方式。纳米结构硬质合金的表面粗糙度值随单颗磨粒最大未变形切屑厚度增加而单调递增。磨削表面形貌和粗糙度值证实,树脂结合剂金刚石砂轮比陶瓷结合剂金刚石砂轮更适合磨削纳米结构硬质合金。理论计算和工艺实验结果均表明,本文工艺实验条件下在纳米结构硬质合金的磨削表面和亚表面中未发现宏观磨削裂纹。纳米结构硬质合金亚表面中存在明显的磨削变质层,变质层的组织结构和平均厚度与磨削工艺参数和合金的物理机械性能密切相关。在X射线透射深度范围内,磨削表面的残余应力沿表面层深度方向存在明显的梯度变化,其分布和大小与工艺实验参数有关。非弹性变形的材料去除方式是产生表面残余应力的主要原因。本文的数学模型和工艺实验研究成果揭示了纳米结构硬质合金的磨削机理,其研究成果有助于推动该类新型材料在工程中更为广泛的应用。更多还原

【Abstract】 Due to their high hardness, high toughness and high wear resistance, nanostructured tungsten carbide materials have been widely used in the engineering fields where high wear resistance and thermal stability are required. Although the preeminent mechanical properties of nanostructured tungsten carbides facilitate their wide applications, they also create a challenging problem in grinding. Nowadays, most researches on nanostructurd tungsten carbides are focussed on how to improve the material physical-mechanical properties, maintain the material stability, evaluating the material wear resistance, etc. Very few studies are found on precision processing of such materials, especially on grinding mechanisms and related technologies. In this dissertation, the grinding theory and experiment of nanostructured tungsten carbide are investigated deeply through experimental analysis and theoretical modeling. It is expounded as follow:Firstly, it introduces preparation technologies, application fields, and challenging problems for nanostructured tungsten carbides. Then the section reviews major academic research achievements in grinding hard-brittle materials, which includes grinding mechanisms and wear mechsnisms of tungsten carbides. This sections also briefly introduces the general structure and contents of this dissertation.Secondly, the effect of physical-mechanical properties on grinding force and grinding damage are dicussed. A new mathematical model on grinding force is built based on fracture mechanics of brittle solids, with the geometric and kinematics of grinding process considered. Based on the mathematical model, a specific grinding energy mathematic model is then built. These mathematical models show that grinding force and specific grinding energy not only relate to grinding process parameters but also to physical-mechanical properties of materials.Thirdly, in order to analyze the effects of grinding process parameters, diamond wheel characteristics, and the physical-mechanical properties of nanostructued tungsten carbides on its grindability, four different tungsten carbides of various grain sizes (from the nanometer level to the micrometer level) were selected as grinding experimental samples. Grinding forces in three directions are measured with a force dynamometer. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), atomic force microscopy (AFM), profilometery and X-ray diffractometry (XRD) are used to evaluate the ground workpiece surfaces for information on surface integrity and material-removal mechanisms, etc. Three methods are applied to prepareing workpiece samples for observing surface and subsurface damage induced by grinding. One of the methods is to grinding the side plane perpendicular to ground surface, and polish the ground side plan with diamond powders, and then etch the same plane with chloroazotic acid. Another method is to polish and etch the ground surface layer by layer. Surface residual stress is evaluated by the X-ray diffraction method.Lastly, in terms of physical-mechanical properties and grinding process parameters of nanostructued tungsten carbides, both the experimental and theoretical results on grinding force, specific grinding energy, material-removal mechanisms, ground surface topography, surface roughness and grinding damage are compared. Grinding forces predicted based on the mathematical model shows a good agreement with the experimental results, which validates the mathematical model. Whereafter, the mathematic model on specific grinding energy is validated. Qualitative analysis and quantitative calculations of the experimental results demonstrate that brittle fracture gradually becomes more obvious with the increase in the maximum underformed chip thickness. However, inelastic deformation is the main material-removal mechanism when grinding nanostructured tungsten carbides in this study. Surface roughness also increases with the maximum underformed chip thickness. The resin bond diamond wheel is more suitable for grinding nanostructured tungsten carbides than the vitrified bond diamond wheel. Both the theoretical calculations and the experimental observations demonstrate no grinding cracks in the ground surface and subsurface of the nanostructured tungsten carbides. However, grinding-induced surface integrity problems, such as inelastic deformation and residual stress, are found in the subsurface of the ground workpieces, and are associated with grinding process parameters and workpiece material properties. Surface residual stress is found to have a depth gradient in the ground surface. Inelastic deformation is a primary factor causing surface residual stress in the ground nanostructured tungsten carbides. Based the results of theoretical modeling and experimental analysis, the grinding mechanism of nanostructured tungsten carbide is exposited, which contributes to promote a wider application in the engineering field.更多还原