| In this dissertation, the advantage of nano-composite ceramic and functionally graded ceramic were combined to achieve both the micro toughing-strengthening of nano-composite and improved macro thermal shock resistance of functionally graded composite. A graded nano-composite ceramic tool material with high thermal shock resistance was developed.The macro-micro design, phase contents radio, processing and its optimization, mechanical properties, microstructure, thermal shock resistance of the tool materials, toughening and strengthening mechanisms, cutting performance and wear or failure mechanisms of the new graded nano-composite ceramic tool materials were investigated systematically.The macro-micro design model of graded nano-composite ceramic tool materials was developed, the relationship between thermal shock resistance and macro-micro structure and physical performance parameters was investigated, and the design basic of the graded nano-composite ceramic tool materials with high thermal shock resistance was proposed. The results indicated that for Al2O3 based graded nano-composite ceramic tool materials, the elastic modulus should decrease from outside layer to inside layer, and thermal conductivity, Poisson’s ratio, and thermal expansion coefficient should increase from outside layer to the inside layer. The layer thickness ratio should be small. The micro-scale design method was proposed through cross-scale coupling. It was convinced that for Al2O3-based graded nano-composite ceramic materials, the volume fraction of nano-phase in the surface layer should be controlled at around 4vol%, and the particle size ratio of nano-phase and micro-phase should be controlled at about 0.2.FEM simulation of thermal shock resistance performance for Al2O3 based graded nano-composite ceramic tool materials was carried out using finite element software (ANSYS). The layer thickness radio of Al2O3 based graded nano-composite ceramic materials with three-layer structure should be determined in small value (0.2 will be appropriate), and the different volume fractions of reinforced phase among graded layers exhibit little effect on the thermal shock resistance. According to the design basis of the macro and microcosmic design model in this paper, two kinds of Al2O3 based graded nano-composite ceramic tool materials were fabricated by using hot pressing and layered powder filling techniques. The following conclusions can be drawn in the fabricatioin process:(1) the optimum dispersion effect of 100nm Al2O3 was obtained under the condition with deionized water as dispersion medium, with the content of dispersing agent (sodium hexametaphosphate) equal to lwt% of nanopowder, with PH value of PUDs equal to 9, and with the time of ultrasonic vibration equal to 15 min; (2) Al2O3 based graded nano-composite ceramic tool material with three-layer structure possesses the best mechanical properties when the thickness radio is 0.2. With 4vol% 100nm Al2O3 in the surface layer, the tool material has the best mechanical properties both for three-layered and five-layered structure; (3) the mechanical properties of three-layerd and five-layerd graded materials were better than seven-layerd’s and nine-layerd’s; (4) three-layerd graded materials with optimum mechanical properties were AT34 (-corresponding cutting tool AT3) with a bending strength of 965MPa, a Vicker’s hardness (surface layer) of 19.8GPa, and a fracture toughness of 6.4 MPa-m1/2. Five-layerd graded materials with optimum mechanical properties were AT52 (-corresponding cutting tool AT5) with a flexural strength of 991 MPa, a Vicker’s hardness (surface layer) of 19.4GPa, and a fracture toughness (surface layer) of 7.2 MPa-m1/2; (5) with the increase of sintering temperature and holding time, the flexural strength increases, whilst the Vicker’s hardness and fracture toughness firstly increases and then decreases. The optimum sintering temperature is 1650℃, and the optimum holding time is 20 minutes.The thermal shock resistance of Al2O3 based graded nano-composite ceramic tool materials was investigated.by means of thermal shock experiments using intensity-attenuation and indentation-quenching methods. The thermal shock temperature difference of the material with three-layered symmetrical structure and five-layered symmetrical structure are much higher (increased by 14.3%,28.6%,) than the homogeneous nano-composite ceramic tool material with the same composition in the surface layer of graded nano-composite ceramic tool material. The thermal shock temperature difference of the material with three-layered symmetrical structure and five-layered symmetrical structure are 400℃and 450℃.The toughing and strengthening mechanisms of Al2O3 based graded nano-composite ceramic tool material were analyzed. The indentation crack forms of the material with three-layered symmetrical structure and five-layered symmetrical structure are intergranular fracture, grain bridging, transgranular fracture and fracture bifurcation. The relationship between thermal shock resistance and the toughing and strengthening mechanisms was established. Fracture strength and fracture toughness were improved by grain bridging. The thermal shock resistance was improved by residual stress. The critical formula of nano-particle -grain-size was established.The cutting performances of Al2O3 based graded nano-composite ceramic tools when continuous turning 316L and intermittent turning stainless steels 304 and 316L were investigated. The results indicated that Al2O3 based graded nano-composite ceramic tools with three-layered symmetrical structure were suitable for high speed continuous turning stainless steels 316L, and the wear mechanism was abrasive wear and cohesive wear. Al2O3 based graded nano-composite ceramic tools with five-layered symmetrical structure were suitable for high speed intermittent turning stainless steels 304 and 316L, and the wear mechanism was adhesive wear and thermal shock damage. The FEM simulation of the cutting performances of Al2O3 based graded nano-composite ceramic tools was carried out using the DEFORM-2D software. The simulated results were verified and in good agreement with the intermittent turning experimental results. The graded tools with five-layered symmetrical structure were more suitable for high speed intermittent turning stainless steels than those with three-layered symmetrical structure. |
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