Al₂O₃ Based Nanocomposite Ceramic Cutting Tool Material And Its Cutting Performance

The compution design of nanocomposite ceramic cutting tool materials is one of the important research direction currently throughout of the world.The methods of the design based on Mori-Tanaka theory of nanocomposite ceramic cutting materials have been proposed in this dissertation.The relationships between micro-structure and mechanical properties have been investigated and the strength,fracture toughness and hardness have been predicted.The fabrication processing parameters is optimized according to the results of predication.The crack extension mechanism of cutting tool materials,cutting performance,abrasion and fracture mechanism of nanocomposite ceramic cutting tool materials also have been analyzed and researched.A theoretical frame for the design of nanocomposite ceramic cutting tool materials is established firstly.Composition design,physical compatibility,chemical compatibility and microstructure design of nanocomposite ceramic cutting tool materials are investigated.Granular-metric composition between nano-particle and micro-particle and volume fraction of nano-particle have been calculated by using multi-component double scaling equation of powder particles packing to attain high relative density of ceramic.biscuit.The predication model of rigidity of two phase cell has been established applying Mori-Tanaka method based on self-consistent theory.The effective Young's modulus of Al₂O₃/(W,Ti)C(Abbreviation:AWT) nanocomposite ceramic cutting tool is predicted and the effective modulus of elasticity would increased with the increase in volume fraction of reinforced phase,the particle size of reinforced phase didn't obviously affect the effective modulus of elasticity.By using the Mori-Tanaka method,a group of equations to predict strengthen and fracture toughness of nanocomposite ceramic cutting tool materials which include random distribution micro-crack.Results show that the strength was obviously influenced by reinforced phase particle size of nanocomposite,the Strength increased with decreasing grain size,and the volume fraction of reinforced phase didn't obviously affect the strength.Grain size obviously influenced the fracture toughness of cutting tool materials,the fracture toughness increases with decreasing grain size and the fracture toughness is higher when volume fraction of reinforced phase is 25-50vol%.Intrinsic behavior of hardness of nanocomposite ceramic cutting tool materials has been studied for atom hardness,ion hardness and chemical bond hardness.The prediction equation based on chemical bond hardness theory has been derived.It is shown from the results that hardness of AWT cutting tool material increased with increasing volume fraction of reinforced phase.Processing parameters of hot pressing sintering were optimized according to predication of mechanical properties of AWT.The optimum results show that the best mechanical properties of AWT10 are obtained under the condition of the sintering temperature 1700℃,soaking time 10min and pressure 30MPa.The mechanical properties are:flexural strength 930MPa,Vickers' hardness 23GPa,fracture toughness 7.55MPa.m^1/2 and thermal conductivity 12.6 W/(m.K).Micro-structure,mode of crack propagation and fracture mechanisms of AWT10 nanocomposite ceramic cutting tool material were thoroughly studied.Results show that the microstructure is mainly intragranular/intergranular structure,the fracture characteristics are mixed by intergranular fracture and transgranular fracture.Crack bending,deflection,bridging,and branching et.al consume much more energy in crack extension and so bending strength and fracture toughness of tool materials can be improved.Strengthening and toughening mechanisms in nanocomposite ceramic cutting tool material were researched.Thermal residual stress field has been researched.The analysis model for thermal residual stress of intra/inter structure of AWT nanocomposite ceramic cutting tool material was established and thermal residual stress was calculated.Results reveal that the structural configuration of tension stress section and compressive stress section in nano-composite ceramic cutting tool material closely related to distribution of nano-scale matrix and reinforced phase grain. Crack deflection,branching and bridging easily carried out in thermal residual stress field.Bonding strength of crystal boundary is strengthened and dislocations easily carry out on crystal boundary under residual compressive stress and crack extensions consume much more energy,and therefore the bending strength and fracture toughness can be improved.Moreover nanopartcles refine grain size,and so the strength of materials can be incresased.Results show that the main strengthening and toughening mechanisms of the material include the effect of grain refining and grain boundary strengthening caused by nano-scale particles,thermal residual stress of intra/inter structure and dislocation toughening.Cutting performance of AWT10 nanocomposite ceramic cutting tool material in turning hardened 0.45%C steel and hardened tool steel T10A are studied.Wear and fracture mechanism of AWT10 cutting tool are investigated.The relationship between cutting parameters and cutting force or cutting temperature is discussed.Results show that the tool life of AWT10 is higher than SG-4 and AWT10 cutting tool material is more suitable for high speed finish machining hardened carbon steel.Wear mechanisms are abrasive wear and adhesion abrasive in turning hardened carbon steel. Failure mode of AWT10 is changed from abrasive failure to dilapidation failure with increasing cutting speed in machining hardened tool steel T10A.Cutting performance of AWT10 nanocomposite ceramic cutting tool material in intermittent turning hardened 0.45%C steel is studied.Tool life of AWT10 is higher than that of SG-4 at both higher and lower speed.Tool life of AWT10 is about 1.2 times than that of SG-4.The fracture mechanisms is mainly mechanical impact when intermittent cutting hardened carbon steel at lower speed,wear patterns can be seen obviously from inserts and wear mechanisms are abrasive wear and adhesion abrasive. The fracture mechanisms are interaction of mechanical impact stress and thermal shock stress when intermittent cutting hardened carbon steel at higher speed.This dissertation is supported by the Foundation for the Author of National Excellent Doctoral Dissertation of P.R.China(200231) and National Basic Research Program of China(2009CB724402).