Al₂O₃Based Micro-nano-composite Ceramic Tool Material And Its Cutting Performance

High-speed machining is the development direction of future machining technology, and the cutting tool material is the key to the development of high speed machining technology, so the development of ceramic cutting tools is suitable for high speed dry cutting to meet the requirement of green environmental protection today. In this dissertation, the development of high-performance micro-nano-composite ceramic cutting tool material is chosen as the research objective. Through systematically designing the micro-nano-composite ceramic tool material and optimizing the fabrication processing parameters on the micro-nano-composite ceramic tool material, Al2O3/TiC/ZrO2micro-nano-composite ceramic tool material is successfully prepared, then its mechanical properties, microstructure, dynamic constitutive relation, dynamic simulation of cutting process, the cutting performance and failure modes and mechanisms are studied.Based on machining characteristics of ultra-high strength steel, the design principles of micro-nano-composite ceramic tool material are proposed, and with the micron Al2O3as matrix, micron TiC, nano TiC and nano ZrO2as additives, the composition of micro-nano-composite ceramic tool material is determined. The analysis and calculation on the chemical compatibility show that there is no chemical reaction in the process of sintering and machining. Based on the particle residual stress model, the largest volume content of micron TiC and ZrO2are determined as82.8%and21.3%, respectively. According to the physical compatibility analysis and calculation of each constituent, the nano additive phase composition is determined. Based on close-packed arrangement of equal diameter balls and the principle of grain composition of green compact, the theoretical models for nano additive particle size and volume content are established. By considering the aggregation effect of nanoparticles, the particle size and content of the nano additive are determined. The volume fraction of nano TiC with the diameter of80nm is4vol%, and the volume fraction of nano ZrO2with the diameter of40nm is1.5～2.0vol%. Based on wettability of TiC, the Ni and Mo additives and its content are determined.Dispersion process of nano powder was investigated. Results show when the suspension is an alkaline solution of PH9～10and dispersant PEG amount for nano TiC powder is1.5wt%of nanometer TiC powder quality, relative sedimentation of TiC nanoparticles suspension is lowest, and when suspension is an alkaline solution of PH9-10and dispersant PEG amount is about2wt%of nanometer ZrO2powder quality, relative sedimentation of nanometer ZrO2suspension is smallest.The sintering process of micro-nano-composite ceramic tool materials is studied. The sintering temperature and holding time for hot pressing sintering were optimized. The sintering temperature1700℃, holding time10min and sintering pressure30MPa are suitable for micro-nano-composite ceramic tool material. The constituents ratio are optimized on the basis of the performance of the sintered body, then ATZ4micro-nano-composite ceramic tool material with the highest synthetical properties is produced, and its flexural strength, fracture toughness and Vickers’ hardness and relative density are970MPa,5.9MPa·m1/2,20.3GPa and99.3%, respectively.The microstructure and fracture pattern of ATZ4micro-nano-composite ceramic tool material are studied. Results show that the microstructure is mainly of intragranular/intergranular structure, and uniform grain size distribution. Its fracture modes are mixed by transgranular fracture and intergranular fracture.Strenthening and toughening mechanisms for ATZ4micro-nano-composite ceramic tool material are studied. Results show that the main strenthening and toughening mechanisms of ATZ4are grain refining, grain boundary strengthening, dislocation strengthening and toughening, phase transformation toughening, crack deflection and grain bridging.The dynamic constitutive model of ATZ4micro-nano-composite ceramic tool material and the machining process simulation are studied. Based on the sliding crack model and applying the theory of meso-damage mechanics, the damage type dynamic constitutive equation of ATZ4under uniaxial compression is established. The dynamic stress-strain curve of ATZ4is obtained by SHPB (Split Hopkinson Pressure Bar) test equipment, and compared with theoretical stress-strain curve, so that the rationality of the theoretical formula was verified. The finite element model of cutting simulation for ATZ4is established and the tool rake face maximum tensile stress is obtained. According to the maximum tensile stress theory, damage failure of cutting tool is predicted when it cut into the workpiece, providing a guidance for cutting parameter selection.The empirical formula of cutting tool life in turning300M steel is established, and the cutting parameters are optimized. The failure modes and mechanism of cutting tool under the reasonable cutting parameters are analyzed. Cutting performance, failure mode and mechanisms of different cutting tools in machining300M steel are studied. Results show that in low speed turning, the failure mode of the cutting tool is tool wear, and its mechanism is adhesive wear and abrasive wear. In high-speed turning, the failure mode of cutting tool is tool wear and breakage. The wear mechanism is adhesive wear and abrasive wear. The cutting tool is loaded by the coupling stress of the mechanical stress and thermal stress in machining, and when the couple stress is equal to the ultimate strenghth of the tool material, the cutting tool may fracture. Under different cutting speeds, the fracture and wear resistance properties of ATZ4micro-nano-composite ceramic tool are higher than other cutting tools.This work was financially supported by the National Basic Research Program of China(2009CB724402) and the Scientific Research Foundation for the Excellent Middle-Aged and Youth Scientists of Shandong Province of China(BS2011ZZ010).