Research On Machined Surface Integrity In Ultrasonic Vibration Mill-grinding Of Silicon Nitride Ceramics

Ceramic materials have exhibited great application value and broad application prospect in high-tech fields due to their superior properties such as high hardness, high strength, wear resistance and corrosion resistance. But the molecular structure determines the brittleness, high hardness and poor machinability, which greatly limits its engineering application. At present the popular machining method is diamond grinding and polishing, but the machining process is difficult to meet the needs of the engineering application because of the low efficiency and high cost. Ultrasonic vibration mill-grinding technology combines ultrasonic vibration and diamond grinding to realize the efficient and precision machining of ceramic materials. Nowdays, the investigation of the technology is not yet mature, further in-depth studies of the technology is significant to promote the development of the precision machining of ceramic materials.The research of machining mechanism is the foundation of ceramic precision machining technology. In this work, the critical load of hot pressed siliconnitride was determined through micro indentation test. Based on the indentation fracture mechanics theory, the critical cutting depth and the effect of ultrasonic vibration on crack system were analyzed and the critical condition of the cracks shielding effect were proposed. Based on the indentation fracture mechanics theory, the mathematical model of grinding force of ultrasonic vibration mill-grinding was established, and the grinding forces were measured using different processing parameters. Comparing the theoretical and experimental results, it was found that they had a similar variation trend. Meanwhile, experimental results also indicated that ultrasonic vibration is benefit to reduce grinding force, so that the ratio of plastic material removal were relatively increased. Experiments were conducted on silicon nitride with different machining parameters in this work and the morphology of machined surface and grinding debris were observed and analyzed, the results indicate that materials were removed by brittle fracture and ductile model.Engineering applications of ceramic materials have high request for the surface roughness, the research of influence factors and rules on surface roughness is the basis of reducing surface roughness. In this work, the predicted model of surface roughness was established based on the simulation of grinding wheel surface topography and the analysis of the motion of abrasive grain, and experiments were carried out to verify the effectiveness of the model. Finally, the orthogonal experiments were carried out to optimize parameters with the aim of improving surface quality, this provide the basis for the choice of parameters in practical machining.Ceramic parts have strict requirements for wear resistance and lubricity in some special engineering applications, the two-dimensional surface roughness sometimes can’t provide enough informations to control the machining process, while three-dimensional surface evaluation technology can provide more rich, more accurate and more comprehensive informations. Therefore, in this works, three-dimensional surface characterization technology were studied, and the influence rules of machining parameters on three-dimensional topography of machined surface were investigated in ultrasonic vibration mill-grinding of ceramics. Combining with the power spectral density analysis, the amplitude feature, spatial feature, functional feature and texture feature of machined surface under different machining conditions were compared and analyzed, which provides theoretical and experimental foundation for choosing machining parameters for different applications.Damages introduced by mechanical machining have great effects on performance and lifetime of ceramic workpiece, how to detect and quantitatively evaluation of machining damage accurately is a big problem for ceramic precision machining. In this works, digital image processing technology was used and four parameters: the ratio of crack area, the ratio of maximum crack area, the total number of crack zone and number of big crack zone were difined to evaluate the damage of machined surface under different machining conditions; Corrosion polishing technology and microscop morphology observation of cross section were used to study the depth and characteristics of subsurface damage; X-ray diffraction method was used to measure residual stress of machined surface, and the effects of machining parameters on residual stress were also investigated. Comparison of subsurface damage machined by ultrasonic vibration mill-grinding of silicon nitride and common mill-grinding, it can be find that ultrasonic vibration mill-grinding can adopt big depth of cut but not increase the damage of subsurface. consequently, ultrasonic vibration mill-grinding is benefit to realize high efficient and precision machining of ceramic materials.