The Study On Boron Nitride Nanotube Toughened Silicon Nitride And The Corresponding Mechanism

As an important high-temperature structural ceramic,silicon nitride（Si₃N₄）is used in numerous areas because of its superior properties such as high strength and hardness,good oxidation resistance,low coefficient of friction,negligible creep,good thermal shock resistance,good resistance to corrosion,and so on.However,the intrinsic brittleness of Si₃N₄ ceramic highly restricts its applications.Since the discovery of carbon nanotubes（CNTs）in 1991,they have been quickly become a hottest topic in the fields of materials,chemistry and physics because of their unique physical,chemical,electronic and mechanical properties.A large number of studies have been demonstrated that CNTs are very promising for strengthening composite materials.Boron nitride nanotube（BNNT）can be imagined as a rolled up hexagonal BN layer or as a carbon nanotube（CNT）in which alternating B and N atoms entirely substitute for C atoms.BNNTs have excellent elastic modulus and tensile strength,and possess better thermal stability and chemical stability than CNTs.Therefore,BNNTs are more promising as strengthening agency for high-temperature ceramic materials.Si₃N₄ ceramics were fabricated by hot-pressing method in this study.The influences of sintering aid,sintering temperature and BNNTs content on the mechanical properties of Si₃N₄ ceramics were investigated in detail.The results show that Si₃N₄ ceramic sintered with Al₂O₃-Gd₂O₃ additive may have a high density and good mechanical properties.It is found that both flexural strength and fracture toughness of the Si₃N₄ ceramics are improved with the increase of sintering temperature until 1800 ℃.At 1800 ℃ and over,α-Si₃N₄ phase can be completely transformed into β-Si₃N₄ phase.The fracture toughness of the Si₃N₄ ceramics could be dramatically enhanced by the introduction of BNNTs,increasing from 7.2 to 10.4 MPa·m^1/2 when BNNT amounts are increased from 0 to 0.8 wt.%.However,excessive addition of BNNTs causes the reduction of fracture toughness.Meanwhile,the relative density and flexural strength of the ceramics decrease slightly with BNNTs content increasing.Even when 1.2 wt.%BNNTs is added,the fracture toughness and flexural strength can be keep at high levels of 8.8 MPa·m^1/2 and 711 MPa,respectively.The toughening mechanism of BNNTs is also discussed,including the bridging and pull-out of nanotubes and crack deflection.