Theoretical And Experimental Investigations On Grinding Nanostructured Tungsten Carbides

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.