Theoretical And Applied Research Of SPSed Liquid Phase Sintering Of Si₃N₄-based Multiphase Ceramic Composites

Silicon nitride（Si₃N₄）-based ceramics are often used to manufacture cutting tools,high-temperature components of engines,and ceramic bearings due to their excellent mechanical,thermal,and chemical properties.Among the many sintering technologies for Si₃N₄-based ceramics,spark plasma sintering（SPS）is considered to be a fast and efficient processing technology,which can realize the sintering of materials at a faster heating rate and lower temperature,and obtain Si₃N₄-based ceramic products with controlled phase composition and microstructure.However,in the existing literature reports,there is still no systematic research on the micro-coupling mechanism of electric field/current and material and its application.Especially the liquid phase sintering process of Si₃N₄-based ceramics involves many complicated phenomena and behaviors such as particle rearrangement,diffusion mass transfer,phase transformation and crystal anisotropy growth.The effects of electric field and current of SPS act on this series of complicated behaviors and thus material phase composition and structure evolution are still unclear.Under this background,this reserach systematically explores the effects and mechanisms of the coupling between the electric field/current and the conductive phase on liquid phase formation,particle rearrangement,diffusion transformation,dissolution-precipitation and Ostwald ripening,by changing the content and particle size of the conductive second phase Ti N and the current pattern of the SPS process.Based on the theoretical research,the SPS fabrication and application research of layered Si₃N₄-based multiphase graded composites with different formulations and combinations are carried out.Research results show that the conductive second phase Ti N can effectively promote the formation of the liquid phase,wetting and particle rearrangement of the Si₃N₄ matrix through the"overshooting"effect and the"electromechanical forces"effect under the synergistic effect of the electric field/current,thereby accelerating the densification process.The relative size scale relationship between the conductive second phase and the matrix phase will produce different coupling sensitivities under the action of SPS electric field/current.When the Ti N particle size is in the micrometer scale,and Si₃N₄-Ti N composites are sintered by continuous DC,the SPS electric field/current will be more inclined to form a continuous current inside the material,thereby generating a lot of Joule heating.But when the Ti N particle size is reduced to the nanometer scale,and the Si₃N₄-Ti N composites sintered by pulsed DC,the SPS electric field/current will be more inclined to partial discharge or plasma phenomenon,which occurs at the particle contact area or grain interface.Additionally,combined with the application analysis in the SPSed fabrication of layered Si₃N₄-based multiphase graded ceramics via SPS,it can be found that the difference in the content and distribution of the conductive phase between the sample layers of the layered graded structure can be combined with the graphite mold to change the current intensity,current propagation path and the spatial distribution of the electric field,thereby further changing the temperature field distribution inside the sample,which ultimately affects the phase transformation and sintering process.Results of this research work provide a much better insight into of the field mechanism of liquid phase sintering process of Si₃N₄-based multiphase ceramics via SPS process.This would allow one to have better knowledge and application of SPS field sintering technology to achieve the comprehensive control and adjustment of the material phase composition,microstructure and product performance,which ultimately realizes the integrated design,green manufacturing and advanced manufacturing of high-performance Si₃N₄-based composite ceramic materials.