Preparation,Microstructure And Performance Of Si₃N₄ Based Composites With High Toughness And Hardness

Silicon nitride（Si₃N₄）has good physical and chemical properties,such as high hardness,good chemical stability,low coefficient of thermal expansion,self-lubrication,and so on.Silicon nitride products have been widely used in cutting tools,high-temperature structural parts,and other fields.However,the brittleness of silicon nitride is still an important factor limiting its application field.In addition,obtaining the high proportion ofβ-Si₃N₄ is necessary for toughening silicon nitride ceramics to decrease its hardness.In order to toughen Si₃N₄ ceramics while maintaining the advantage of high hardness,in this manuscript,through the spark plasma sintering（SPS）method,different kinds of composites were prepared by high-purity nanoα-Si₃N₄ powder,yttria powder,molybdenum powder,and nano titanium carbide powder.The effects of sintering technology and phase content on the densification,microstructure and mechanical properties of silicon nitride composites were studied.The main conclusions are as follows:（1）In composites sintered with sintering aids Y₂O₃,the products formed after sintering of TiC and Mo affect each other.When the TiC was fixed with the addition of Mo,the C/N ratio in Ti（C,N）prepared by TiC and Si₃N₄ tends to increase.When the Mo was fixed with the addition of TiC,the Mo-Si compounds containing a Si atom concentration gradient have been produced from Mo Si₂ to Mo₅Si₃.（2）In composites sintered with sintering aids Y₂O₃,The introduction of TiC inhibited the phase transition fromα-Si₃N₄ toβ-Si₃N₄ and the grains growth of Si₃N₄ matrix.The introduction of appropriate amount of Mo promoted the phase transition fromα-Si₃N₄ toβ-Si₃N₄,but the Mo-Si compound grains formed after Mo excess hindered the growth of large-size columnar grains ofβ-Si₃N₄ due to the steric hindrance effect.（3）The Si₃N₄-7Y₂O₃-8TiC-4Mo sintered at 1680℃for 10 min obtains the best comprehensive mechanical properties,with Vickers hardness of HV₁₀ 18.8 GPa and fracture toughness of 14.3 MPa·m^1/2.The addition of TiC and Mo realizes the combination of external grain toughening and self-toughening.The fracture toughness of the composite was improved through microcrack toughening,crack deflection,and columnar crystal pulling out,which can obtain high fracture toughness and maintain high hardness at the same time.（4）It is difficult for Si₃N₄ composite prepared by SPS under 1725℃without sintering aids.These composites show obvious weight lossing in the sintering process.The introduction of Mo promoted the decomposition of matrix Si₃N₄ and the Si₃N₄ completed the transition from phaseαtoβ.There are 49 figures,4 tables and 101 references in this manuscript.