Research On The Fabrication And Mechanical Properties Of Si₃N₄-based Nano-ceramic Composites

The Si₃N₄ matrix nano-ceramic composites were fabricated by hot press sintering using nano-Si₃N₄ powders. The microstructures were analyzed by means of XRD, SEM, TEM. EDX and so on. The effect of the second phases such as TiC,TiN,WC particles and CNTs on the mechanical properties of Si₃N₄ nano-ceramics were investigated.First, the microstructure, fabrication methods, mechanical properties, toughening mechanisms and development tendency of Si₃N₄ ceramics as well as the development status of nano-composite ceramics were introduced. Based on the above work, the purposes and significance of this thesis were pointed out. Secend, the fabrication techniques, the testing methods of mechanical properties and the observing methods of microstructure of Si₃N₄ matrix nano-ceramic composites were introduced.The research results show that the dispersion effect of nano-Si₃N₄ powders can be improved after being ultrasonically dispersed. The introduction of proper amount of surfactant can improve the dispersion effect of nano-Si₃N₄ powders. In addition, PH values of dispersing system also affect the dispersion effect. The main phases in Si₃N₄ nano-ceramics areα-Si₃N₄,β-Si₃N₄ and Si₂N₂O, and the SEM micrographs show that the microstructure of sintered materials consists of grains with approximate size of 100 nm. The flexural strength and the fracture toughness increase initially with the increase of the amount ofα-Si₃N₄ starting powders then decrease, and the maximum mechanical properties are obtained when the amount ofα-Si₃N₄ powders is 40wt.%. The hardness values decrease with the increase ofα-Si₃N₄ starting powders amount.The TiC particles that added as a dispersed phase react with Si₃N₄ during the liquid phase sintering, with the formation of TiC_0.7N_0.3. Adding proper amount of TiC particles can increase the flexural strength and the fracture toughness of Si₃N₄ nano-ceramics. The maximum values of the flexural strength and the fracture toughness were obtained when the amount of TiC particles is 10wt.%. The Vickers hardness of Si₃N₄ nano-ceramic composites increases with the increase of TiC amount.There is a good chemical consistency between TiN particles and Si₃N₄ matrix. The addition of proper amount nano-TiN particles can significantly increase the flexural strength and the fracture toughness. The maximum values of the fracture toughness and the flexural strength could be obtained when the amount of TiN particles was 10wt.% and 15wt.% respectively. However, the addition of nano-TiN particles has little effect on the hardness of Si₃N₄ nano-ceramics. The main toughening mechanism existing in Si₃N₄-TiN nano-ceramic composites are thermal expansion mismatch toughening mechanism and crack deflection toughening mechanism.The Vickers hardness of Si₃N₄-WC nano-ceramic composites is lower than that of Si₃N₄ nano-ceramics, while increases with the increase of WC amount. Adding proper amount of WC particles can increase the fracture toughness and the flexural strength of Si₃N₄ nano-ceramics. The maximum values of the fracture toughness and the flexural strength can be obtained when the amount of WC particles is 4wt.% and 8wt.% respectively.The main phases in CNTs-Si₃N₄ nano-ceramic composites areα-Si₃N₄,β-Si₃N₄ and Si₂N₂O. The carbon nanotubes can maintain good thermal stability during the sintering process of Si₃N₄ ceramics. The fracture toughness and the flexural strength initially increase with the increase of carbon nanotubes amount then decrease, and the maximum values of the fracture toughness and the flexural strength can be obtained when the amount of carbon nanotubes is 4wt.% and 2wt.% respectively. The Vickers hardness values has a little increase when the amount of carbon nanotubes is 2wt.%, then gradually decrease with the increase of carbon nanotubes amount. The main toughening mechanism existing in CNTs-Si₃N₄ nano-ceramic composites are pullout of CNTs, bridging of CNTs and crack deflection.The thermal shock behavior of Si₃N₄ nano-ceramics is similar to the mode of Hassalman. The existence ofα-Si₃N₄ starting powders can improve thermal shock resistance. The addition of proper amount TiC particles can improve the thermal shock resistance. When the TiC amount is 10wt.%, the critical temperature difference is highest and the thermal fatigue resistance is the best. It is because that TiC particles can induce crack passivation and deflection, which dissipates more crack strain energy and hinders the extension of thermal shock cracks.