Fabrication And High Speed Cutting Performance Of Sialon-Si₃N₄Graded Nano-composite Ceramic Tools

Nickel-based super alloys have excellent mechanical properties, which have been widely employed in aerospace and nuclear industry. However, they are some characteristics in high speed cutting Nickel-based super alloys process, such as high cutting force, high cutting temperature and low tool life. Therefore, high performance cutting tool is one of the key technologies for high efficiency and high quality machining of Nickel-based super alloys. In this dissertation, the research methods of functionally graded materials (FGMs) and nano-composite materials were borrowed. The macro-micro design model of the graded nano-composite ceramic tool materials was constructed by the combination of the characteristics of thermal stress relief and surface compressive stress of the FGMs and the strengthening and toughening mechanisms of nano-composites. The preparation process, mechanical properties and microstructure, tribologiccal properties and antifriction mechanism, thermal shock and thermal fatigue resistance of the graded nano-composite ceramic tool materials were investigated in the dessertation. And the cutting performance and failure mechanism of the cutting tools were also analyzed and researched.A non-homogeneous thermal-mechanical-chemical multi-field coupling model in high speed cutting process was established. The design objectives and requirements of the tool composite and structure were identified. The basic design principles of the tool materials have been proposed. In order to adapt and resist the action of the non-homogeneous multi-field, at first, the macro properties of the tool materials were tailored. And then the micro structure and properties of each local were designed. Thus the macro-micro design model of the graded nano-composite ceramic tool materials was constructed.According to the basic design principles, two kinds of graded nano-composite ceramic tool materials were fabricated by using layered powder filling and hot pressing technique. The optimum mechanical properties were taken as the objective to optimize the structural parameters and sintering parameters. GSS1material with optimum mechanical properties was sintered under a pressure of35MPa at1700℃for60min, while the thickness ratio being e=0.3. The optimum mechanical properties are a flexural strength σf=980MPa, a fracture toughness (surface layer) KIC=9.54MPa·m1/2and a Vicker's hardness (surface layer) HV=16.91GPa. GSS2material with optimum mechanical properties was sintered under a pressure of35MPa at1750℃for60min, while the thickness ratio being e=03. The optimum mechanical properties are σf=810MPa, KIC=9.33MPa·m1/2and HV=16.98GPa.Microstructure, crack propagation patterns and fracture mechanisms of Sialon-Si3N4graded nano-composite ceramic tool materials were thoroughly investigated. The results showed that the fracture surfaces were characterized by a mix mode of intergranular and transgranular fracture. A large amount of fracture and pull-out of rod-like β-Si3N4grains and β-Sialon grains were observed on the fracture surface. The difference in Si3N4grain size led to the formation of the interlocked duplex microstructure of β-Si3N4grains and β-Sialon grains. The interlocked duplex microstructure contributed much to the improvement of flexural strength and fracture toughness. Crack deflection and crack bridging were also observed, which can absorb additional amounts of fracture energy. According to the TEM micrographs analysis, a mix of intergranular and transgranular TiC0.7N0.3particles, stress interference stripes and twin structure acted simultaneously in the surface layer of Sialon-Si3N4graded ceramic materials.The tribological properties and antifriction mechanism of Sialon-Si3N4graded nano-composite ceramic tool materials were researched. The results showed that the friction coefficient and the wear rate of the homogeneous reference material were higher than those of the graded material. The wear mechanisms of the graded nano-composites were adhesion, chipping and abrasion. The compressive residual stresses at the surface layer of the graded specimens can counteract partially the tensile stress during sliding wear process, which cause the decrease friction force. Therefore, the graded specimen exhibits improved antifriction performance than that of the homogeneous reference one.The thermal shock and thermal fatigue resistance of Sialon-Si3N4graded nano-composite ceramic materials were also investigated. The results showed that the graded ceramics exhibited higher thermal shock and thermal fatigue resistance. Both the graded ceramics and homogeneous ceramics exhibited the propagation characteristics of short-cracks. The highest critical temperature difference of GSS1was600℃, while the critical temperature difference of GSS2, ST10ans SAAT10was400℃.Tool life, failure patterns and mechanisms of Sialon-Si3N4graded nano-composite ceramic tools were investigated via high speed cutting of Inconel718. The results showed that the highest tool life was obtained at vc=120m/min in turning. GSS1and GSS2showed higher performance than that of ST10and SAAT10respectively. The main failure mechanisms of the graded tools identified in the turning tests involved adhesive wear, abrasive wear, notch wear and chipping. The optimum cutting speed range was700-900m/min in the face milling tests. The graded tool was possessed of a self-sharpening characteristic and showed better cutting performance compared to the homogeneous ones. The resistance to notch wear of GSS1was higher than that of KY4300. The main failure types of the tools in ultra high speed cutting were flaking, chipping and notch wear. The main cause of tool fracture was the combined effect of the mechanical impact stress and thermal stress. Tool wear accompanied with tool fracture took place, with its mechanisms being abrasive wear and adhesive wear.