| In order to solve the problems of great blindness and large amount of experiments brought from the trial and error method for developing ceramic tool matarials, the multi-scale design theory of ceramic tool materials was established. The theory revealed the quantitative microstructure-property relationship of composite ceramic tool materials. Two kinds of ceramic tool materials were designed and fabricated under the guidance of the proposed design theory. Their sintering technology, microstructure, strengthening and toughening mechanisms, room and high temperature mechanical properties, dynamic fatigue behavior, cutting performance and cutting reliability were deeply investigated.The multi-scale design theory of composite ceramic tool materials was proposed and established. Based on the studies on the physical and mechanical process of microstructure and the distribution of liquid phase in ceramics, the flexural strength prediction model of composite ceramic tool materials, the residual stresses mechanics model, the mathematical model of the optimal content of nano-scale particles and the mathematical model of the optimal content of the metallic phase were built.Two kinds of multi-scale particles reinforced alumina-based ceramic tool materials were designed according to the above proposed models. One was Al2Oμ-TiCμ-TiCn-Co composite ceramic tool material, and the volume content of micro-scale Al2O3, micro-scale TiC, nano-scale TiC and cobalt were56%,35%,6%and3%repectively, the other one was Al203μ-(W,Ti)Cμ-TiCn-Ni/Co composite ceramic tool materials, and the volume content of micro-scale Al2O3, micro-scale TiC, nano-scale TiC and Ni/Co were57%,33%,6%and4%respectively.The influences of nano-particle content and metallic phase content on the room temperature mechanical properties and microstructure were investigated. The results showed that when the content of nano-scale TiC particles was6vol.%, the ratio of intergranular fractured grains to transgranular fractures grains in the materials was suitable and resulted in the better flexural strength and fracture toughness. These results verified the validity of the mathematical model of the optimal content of nano-scale particles. When the content of cabalt was too low or too high, the mechanical properties of the ceramic tool materials would reduce and there was an optimal content of the cabalt. These results verified the validity of the mathematical model of the optimal content of the metallic phase.The sintering technologies of the composite ceramic tool materialswere optimized. It was found that Al2O3-TiCμ-TiCn-Co micro-nano-composite tool material which was sintered under a pressure of32MPaand a temperature of1650℃in vacuum for20min, had optimum mechanical properties. Its flexural strength, fracture toughness and Vicker’s hardness were916MPa,8.3MPa·m1/2and18GPa, respectively. For the Al2O3μ-(W,Ti)Cμ-TiCn-Co composite ceramic tool material, the best flexural strength, fracture toughness and Vicker’s hardness were882MPa,7.2MPa·m1/2and19GPa, which was sintered under a pressure of32MPa and a temperature of1650℃in vacuum for30min.The strengthening and toughening mechanisms of the composite ceramic tool materials were detailedly studied. It was found that the strengthening and toughening mechanisms of micro-scale particles was grain bridge and cracks deflection. The strengthening and toughening mechanisms of nano-scale particles was grain boundary pinning, grain refining, cracks re-deflection and transgranular fracture. The strengthening and toughening mechanisms of metallic phase was the reduction of the residual tensile stress distributed in matrix and the increase of the interfacial bonding strength.The high temperature mechanical properties and dynamic fatigue behavior of composite ceramic tool materials were deeply investigated. The change regulation of the high temperature mechanical properties and dynamic fatigue behavior with the temperature was revealed. The slow propagation mechanism model of crack at high temperature was built. The weakening mechanisms of the mechanical properties at high temperatures were analysed. The results showed that the flexural strength of Al2O3-TiCμ-TiCn-Co composite decreased with an increase in temperature, and the fracture toughness decreased as a function of the temperature up to1000℃but increased at1200℃due to small plastic defomation. The difference between the room temperature flexural strength s and the high temperature flexural strength was smaller for the composite ceramic tool materials without cabal. However, the excess cobalt led to the decreased the high temperature flexural strength, and the more the content of cobalt, the highly the high temperature flexural strength was dropped. The sub-caracks were propagated in ceramic materials at room and high temperature. The fatigue resistance of the materials at high temperature was greatly dropped. Appropriate metallic phase could improve the anti-fatigue property of ceramic tool materials. The weakening mechanisms of the high temperature flexural strength of Al2O3-TiCμ-TiCn-Co was oxidative attack and microstructure damage, grain boundary softening of metallic phase at high temperature, the reduction of interfacial bonding strength, crack intragranular propagation and fracture, the reduction of elastic modulus at high temperature, plastic deformation, and grain boundary defects caused by grain boundary migration. The fatigue failure mechanism of Al2O3-TiCμ-TiCn-Co composite tool material at room temperature and900℃was ascribed to stress-corrosion cracking, but at1200℃the failure mechanism was creep.The cutting performance and cutting reliability of the multi-scale particles reinforced Al2O3-based ceramic cutting tools were researched. The model of the relationship between the dynamic fatigue property of the ceramic tool materials and the tool life by fatigue wear was established. The distribution models of the tool life by wear and the cutting reliability medels were built. The cutting performance and cutting reliability of the composite ceramic tools when continuous cutting austenitic stainless steel (1Cr18Ni9Ti) were studied. The results showed that when wet cutting1Cr18Ni9Ti with the ATTC ceramic cutting tools (b=0.1mm, r=0.1mm) at a cutting speed of80m/min, a feed rate of0.15mm/r and a depth of cut of0.1mm, the metal removal volume was the greatest, and the tool life was13min. The tool life by wear obeyed the Gamma distribution. When the cutting tool reliability was0.5, the reliable tool life by wear was about9min. When the cutting tool reliability increased to0.8～0.9, the reliable tool life by wear was about5.5-6.5min. The best wear resistance of four kinds of cutting tools was ATTC(b=0.1mm, r=0.1mm), followed by ATTC(b=0.3mm, r=0.3mm), LT55and AWTC. The failure patterns of cutting tool were mainly cutting edge brittle fracture and tool material peeling off the tool clearance caused by fatigue breakage, and the main failure mechanisms were abrasive wear and adhesive wear. The cutting performance and cutting reliability of the composite ceramic tools when continuous cutting cold work die steel (Cr12MoV) were studied. When wet cutting Cr12MoV with the ATTC ceramic cutting tools at a cutting speed of60m/min, a feed rate of O.lmm/r and a depth of cut of0.1mm, the surface roughness was the best and the tool life by waer was9min. The tool life by wear obeyed the lognormal distribution. When the cutting tool reliability was0.5, the reliable tool life by wear was about1.5min. When the cutting tool reliability increased to0.8-0.9, the reliable tool life by wear was about4.5-5.3min. For the ATTC cutting tools with different corner radius and chamfer width, the best surface roughness was got when using the ATTC tool with a larger corner radius and chamfer width in wet cutting. When wet cutting Cr12MoV, the failure patterns of cutting tool were mainly groove wear, boundary wear, tool material peeling off the tool clearance and tool tipping, and the main failure mechanisms were abrasive wear and adhesive wear. |
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