Research On The Crack Propagation Resistance Of Si₃N₄Composites Reinforced By BN Nanotubes

The turbocharger is a device which can improve the engine power. As the key part of theturbocharger turbine components, the material performance of turbine rotor affects service lifeof it under high temperature and high speed. Development of ceramic turbine supercharger isrevolution of supercharging technology at present. Si₃N₄ceramics is one of the commonly usedhigh temperature and high strength structural ceramics. But as a ceramic material, Si₃N₄ceramics’ nature brittleness limits the development of it. Boron nitride nanotubes （BNNT） hasgood chemical stability, excellent mechanical properties and heat conduction performance, lowthermal expansion performance and excellent antioxidant properties. Especially, BNNT’sspecial tubular morphology could improve crack resistance of Si₃N₄ceramics when BNNT mixwith Si₃N₄ceramics.In this work, the mathematical model of the crack propagation resistance of Si₃N₄ceramics crack shielding region enhanced by BNNT was constructed. The mathematical modelwas based on fiber toughening ceramic materials. And the influence of BNNT for the crackpropagation resistance of Si₃N₄ceramics was analyzed. Bending strength and fracturetoughness of BNNT/Si₃N₄composites were tested by three point bending strength and SENBmethod. The results indicated that: Si₃N₄ceramics added BNNT had clear appreciation of theresistance curve behavior. Bending strength and fracture toughness of Si₃N₄ceramics added5%BNNT improved about10%and29%compared to those of Si₃N₄ceramics. Those illustratethat BNNT can improve crack propagation resistance of Si₃N₄ceramics obviously.In order to further explore BNNT enhancing the mechanism of crack propagationresistance of Si₃N₄ceramics, The crack tip stresses of crack bifurcate, crack bridge, crackpinning and crack deflection in BNNT/Si₃N₄composites were simulated by using ABAUQS inthis paper. It is shown that BNNT can absorb strain energy of crack tip stress in Si₃N₄ceramicsand reduce crack tip stress that effectively prevents larger stress concentration, and strongstress shielding area is formed that increases the crack propagation resistance of Si₃N₄ceramicswhen the crack extension to the BNNT. Crack bridging and crack pinning to increase the role ofcrack resistance is higher than crack branching and crack deflection. Through observing SEMimages of BNNT/Si₃N₄composites, BNNT play bridging and pinning roles for Si₃N₄ceramics. Analysis model of turbine under the high-speed rotating was established in this work. Themaximum stress position of turbine was it’s blade roots, and this is consistent with it’s actualdamage.Based on researching crack propagation resistance of Si₃N₄ceramics enhanced byBNNT, the models for calculating crack propagation resistances of Si₃N₄ceramics andBNNT/Si₃N₄composites were established using the Dugdale-Barenblatt theory for analysingand calculating crack tip. The calculation results show that the failure of Si₃N₄ceramics turbineblade will occur in80000r/min, however BNNT/Si₃N₄ceramics turbine blade still has a highresidual strength at speed of100000r/min. It is show that compared with Si₃N₄ceramicsturbine, BNNT/Si₃N₄ceramics turbine has obvious advantages on resisting crack extension.Three-dimensional solid model of turbine was established using three-dimensionalmodeling software Solidworks. The stress distributions of Si₃N₄ceramics turbine BNNT/Si₃N₄ceramics turbine under high temperature and high speed were simulated, and crack propagationresistance of the turbines were calculated on the basis of that. Results show that BNNT/Si₃N₄ceramics turbine has an obvious advantage to Si₃N₄ceramics turbine on resisting crackpropagation. The error of calculating crack propagation resistance through theory andsimulating actual dynamic load was calculated, and the maximum error is about6.3%. And theaccuracy of the calculation results was verified. Resistance to crack propagation of turbine ispoor due to the smaller crack propagation resistance under high temperature. Resisting crackpropagation of BNNT/Si₃N₄ceramics turbine and is significantly enhanced at high temperaturebecause of joining BNNT enhancing body.