Development Of Si3_N4/（W,Ti）C/Co Graded Nanocomposite Ceramic Tools And Their Cutting Performance

Ceramic cutting tool materials have good application prospects because of their high hardness, heat resistance, wear resistance and good chemical stability. The concept of graded nano-composite materials provides a new effective way to toughen ceramic tool materials. In this paper, material system of the graded nano-composite ceramic tool was determined via analysis on physical-chemical compatibility with iron-based superalloys GH2132 being as workpiece material. The graded structure was optimized based on mechanical properties prediction results and finally, two kinds of graded nano-composite ceramic tool materials with excellent mechanical properties were fabricated.Material system of the graded nano-composite ceramic tool was determined through analysis on physical-chemical compatibility:the matrix is Si3N4; reinforcing phases are (W, Ti)C and Co; sintering aids are Y2O3+AI2O3. The gradient structure was built using the concept of thickness ratio and number of layers. The finite simulation of tensile test and three-point bending test were conducted to predict the flexural strength, fracture toughness and mechanical impact resistance of the graded ceramic tool materials. And the effects of thickness ratio and number of layers on the properties of the graded materials were analyzed. It could be concluded that increasing the number of layers and reducing the thickness ratio are helpful to improve the mechanical properties of the graded ceramic materials.According to the design results, two kinds of graded nano-composite ceramic tool materials GSWT52 and GSWT52G were fabricated by using hot pressing and layered powder filling techniques. Through the composition optimization, the maximum additive amount of reinforcing phases in each layer of the graded structure was determined, that is, the additive amount of (W, Ti)C is no more than 25vol.%, and that of Co is no more than 3vol.%. And the optimal weight ratio of nano-Si3N4 powders to micro-Si3N4 powders is determined to be 1/3. Through structure optimization, it can be concluded that the five-layer graded nano-composites with a thickness ratio of 0.2, had the optimum comprehensive mechanical properties. The effectiveness of the property prediction models were verified by comparing the predicted with experimental results, excluding the error factors during the fabrication process.GSWT52 and GSWT52G composites, sintered under a pressure of 30MPa at 1700℃ in vacuum condition for 45min, had the optimum comprehensive mechanical properties. The flexural strength, hardness (surface layers), and fracture toughness (surface layers) of GSWT52 can reach 1080MPa,17.64GPa and 10.9MPa·m1/2, respectively, while that of GSWT52G can reach 992MPa,17.83GPa and 10.5MPa-m1/2, respectively, which meets the requirements for ceramic cutting tools. The toughening and strengthening mechanisms of the newly developed graded nano-composite ceramic tool materials including:a. Self-toughening mechanisms of rod like β-Si3N4 matrix; b. The "pinning effect" of nano-particles; c. More transgranular fracture mode induced by the reinforcing phases; d. The residual compressive stress in the surface layer induced by the graded structure.High temperature mechanical properties of the graded nano-composite ceramic tool materials were studied. The flexural strength shows a tendency to firstly slowly decrease and then sharply decrease with the increase of testing temperature, and the turning point is 1000℃. The fracture toughness shows a tendency to increase firstly and then decrease sharply, and the critical temperature is 1000℃ as well. The grain boundary sliding caused by softening of glass phase, the oxidation of material compositions and surface microcracks growth under high temperature lead to the reduction of flexural strength, while microcrack toughening effect and plastic deformation before fracture under high temperature result in the increase of fracture toughness in a certain temperature range (less than 1000℃). The high temperature mechanical properties of the graded ceramic tool materials are higher than those of the comparative homogenous materials.Thermal shock resistance of graded ceramic tool materials was investigated by means of strength-decay method. The thermal shock temperature difference of GSWT52 and GSWT5G are 600℃ and 700 ℃, respectively, which are both higher than that of the comparative homogenous materials. Thermal shock effect causes generation and growth of microcracks and thus leads to decline of flexural strength. The thermal fatigue resistance of graded ceramic tool materials was investigated by means of indention-quenching method. Crack propagation length of graded ceramic materials is shorter than that of homogenous materials after the same shock impact cycles. Residual compressive stress indeced by the graded structure contributes to the improved thermal fatigue resistance of the graded composites.The cutting performance of the graded nano-composite ceramic tools when continuous turning and intermittent turning iron based supper-alloy GH2132 was investigated. In the continuous turning tests, tool life shows a tendency to increase firstly and then decrease with the increase of cutting speed. And the optimal cutting speed range is between 100～150m/min. Now, tool lives of the FGM tools exceed that of the corresponding common ceramic tools with the same composition systems and can be improved by one times compared with the specified commercial ceramic cutting tool materials. Under relatively lower cutting speed, flank wear is the predominant failure mode affecting the tool life, while further increasing the cutting speed, notch wear at the depth of cut becomes the determining factor. FGM tools show better notch wear resistance than the the common reference tools. The tool failure mechanisms involve adhesion, abrasive and oxidation wear. Intermittent turning test results indicate that the tools understand more serious mechanical impact at the tool exit instant and fracture and adhesion are the main reasons causing tool failure. FGM tools have better mechanical impact resistance than the the common reference tools.