| BNNTs/Si3N4 composite ceramic turbine rotor and samples were prepared in this paper, its mechanical properties and thermal coupling performances were also analysised.(1) Based on the thermal shock theory of ceramic materials, thermal shock theoretical model of the BNNTs/Si3N4 composite ceramic was established, its mechanical properties and thermal shock resistance changing with porosity and temperature were obtained. With the higher porosity and temperature, the elastic modulus, bending strength and thermal shock resistance of the composite ceramic are decreased.(2) BNNTs was synthesized in homemade stainless steel autoclave using Na BH4 as boron source, NH4 Cl as nitrogen source and single-walled CNTs as templates. Analysis showed that BNNTs have high purity and good crystallization properties, a nanotube diameter about 25 nm, a length about 0.5-2?m, smooth surface characterization, obviously hollow tubular structure, long length, evenly distributed thin wall thickness and good covering effect. Analysis of preparing mechanism shows that BNNTs have excellently inherited the tubular structure of CNTs. The method for preparing BNNTs has a simple process, low synthesis temperature, high efficient and yields. It is possible to meet the needs of the material prepared.(3) BNNTs/Si3N4 composite ceramic samples and turbine rotor were prepared by vacuum hot pressing method. Tests and analyses of its mechanical properties showed that, BNNTs/Si3N4 composite ceramic has a high relative density and bending strength, BNNTs can refine the matrix grain and makes its fracture mode from intergranular and transgranular coexistence of pure Si3N4 ceramics to mainly transgranular, which significantly improves the mechanical properties of Si3N4 ceramics. When the amount of BNNTs less than 2wt.%, 1.5% BNNTs/Si3N4 composite ceramic samples had the best mechanical properties, its fracture toughness increased 46% than pure Si3N4 ceramics.(4) Thermal shock resistance of BNNTs/Si3N4 composite ceramic samples and turbine rotor were tested. Analyses showed that pure Si3N4 ceramics has a critical thermal shock temperature difference of 600?, and the thermal shock resistance of Si3N4 can significantly improved when BNNTs was added into Si3N4. 1.5% BNNTs/Si3N4 composite ceramic sample has a critical thermal shock difference reached 700?.(5) The toughening mechanism of BNNTs was studied after thermal shock resistance and mechanical properties of BNNTs/Si3N4 composite ceramic was analysed. The results show that the cooperation mechanism of bridging?deflection?bifurcation and pinning in the expansion process of internal cracks is its toughening manner, thereby increased the resistance to crack growth and improved the fracture energy, ultimately improved the properties of composite ceramics.(6) Thermal and mechanical coupling of BNNTs/Si3N4 composite ceramic turbine rotor were simulated and analysed. According to the simulation results, the empirical formula of the maximum stress and strain generated at different ambient temperatures on turbine changing with its speed were futted, and the empirical formula of the maximum stress and strain generated at different speed of turbine changing with its ambient temperatures were futted. The results show that the zone where the maximum stress occured are on the turbine blade root, but the biggest strain is generated at the edge of the blade both at the pure force field or thermal coupling field. When the temperature of the turbine were 500?, 600?, 700? and 800?, the critical speed result its failure were 130000r/min, 115000r/min, 100000r/min and 90000r/min. The critical conditions to guarantee the performance are the turbine speed of 100000r/min and temperature of 700?. For ordinary car and small load truck, BNNTs/Si3N4 composite ceramic turbine rotors can fully meet the requirements for use. |
PDF Full Text Request