Carbon Nanotubes Reinforced Si₃N₄ Ceramics

The structure , property , fabricate method and toughness mechanics of silicon nitride, and also the discovery of carbon nanotubes and its mechanical property have been introduced in this thesis. Now reinforcement and improve toughness is the core topic study of the ceramic materials, and fiber reinforcement is the most obvious method, it can improve not only the toughness but also the hardness of materials.In this thesis adding the pre-process carbon nanotubes into the powder of silicon nitride, with ball milling , drying and sieving, fabricate the silicon nitride ceramics applying the HP sintering method, which in order to improve the toughness of the silicon nitride. The sample's density, hardness , toughness , X-ray diffraction(XRD), scanning electron microscope(SEM) and transmission electron microscope(TEM) analysis have been tested. From the results, we could find that the sample's mechanical properties improved as the improvement of the sintering temperature without carbon nanotubes. At 1700 ℃ ,the relative density, hardness and fracture toughness were 99.7%,15.49GPa, and 7.16 Mpa.m1/2 respectively. At 1600℃,1650℃,1700℃ sintering temperature with carbon nanotubes, the mechanical properties were decreased as the increase content of the carbon nanotubes. At 1750 ℃,0.99wt.% of the carbon nanotubes, the hardness and the fracture toughness were 16GPa, 7.47 Mpa.m1/2 respectively. Comparing the same condition without carbon nanotubes, the hardness improve a little, the fracture toughness improve about 6%. The result of the XRD, SEM and TEM analysis show that the densification process was controlled by the mechanical of liquid phase sintering with the addition of MgO and Ce02 as the sintering aid. The MgO -CeO2 would react with the SiO2 which was on the surface of the silicon nitride particles to from silicate liquid phase to accelerate the densification. There were only a-Si3N4 and p-Si3N4 in the sample, and mainly 3 -Si3N4 when the sintering temperature was more higher than 1650℃.The phase transitionwas mainly between 1600 ℃ ~ 1650℃.This sample was mainly intergranular-fractured. Extraction effect was the main effect of the carbon nanotubes at the fracture of the silicon nitride which could improve the fracture toughness of silicon nitride.