Research On Pressureless Rapid Sintering Mechanism And 3D Manufacturing Application Of Silicon Nitride Ceramics

High temperature sintering is an important process in the manufacturing process of advanced ceramic materials,which directly determines the microstructure and physical and chemical properties of the products.However,most of the ceramic sintering process is time-consuming and energy-consuming.How to reduce the sintering temperature and shorten the sintering time while improving the product performance is an important development opportunity and technical challenge for the ceramic manufacturing industry brought by the"carbon peaking and carbon neutrality goal".Especially in today’s rapid development of additive manufacturing technology,it is especially necessary to actively develop rapid sintering technology for complex macroscopic 3D conformal ceramic products.In this context,a new method of pressureless rapid sintering for green manufacturing of advanced ceramics has been carried out using non-oxide ceramic Si₃N₄ as a model material.By systematically analyzing the effects and mechanisms of microwave sintering（MWS）and ultrafast high-temperature sintering（UHS）on the densification,diffusion phase transition,microstructure evolution,structural homogeneity,and mechanical properties of Si₃N₄ ceramics,a deeper understanding of the rapid fabrication of complex 3D structured Si₃N₄ is obtained from the basic and application levels.In this study,two sintering methods,microwave sintering（MWS）and conventional pressureless atmosphere sintering（CS）,were firstly used to sinter Si₃N₄ ceramics with different liquid phase sintering aid introduction methods（mechanical ball milling,chemical mixing,co-precipitation coating）.It was found that the thermal and non-thermal effects of MWS can effectively promote the processes of Si₃N₄ liquid phase formation and particle rearrangement to accelerate the densification of Si₃N₄ compared to the CS sintering method.At the same time,the microstructure and the Si₃N₄ ceramics with better properties can be obtained by MWS that are different from CS,but the promotion of diffusion phase transition will differ depending on the spatial distribution state of the liquid-phase sintering aid relative to Si₃N₄.Therefore,in the MWS process,the microwave electromagnetic field can promote the heating and temperature rise of Si₃N₄ ceramics to shorten the sintering time,but the coupling between the microwave electromagnetic field and the material will appear"selective"and is not universally applicable.Secondly,in order to further realize the rapid sintering and green manufacturing of Si₃N₄ceramics,the high-temperature preparation by UHS technology was switched to focus on the analysis of the effect and influence mechanism of ultrafast temperature rise and ultra-short high-temperature treatment time on the liquid-phase sintering of Si₃N₄ ceramics.The results show that the density of Si₃N₄ obtained by UHS increases with the increase of liquid-phase sintering aid content,reaching a maximum value（91.62%）at 20 wt%liquid-phase sintering aid content.The degree of Si₃N₄ densification and phase transformation also increased with the increase of the holding time,and the microstructure of Si₃N₄ with the UHS holding time of 30-120 s indicated that Si₃N₄ samples with complete macrostructure and uniform microstructure could be obtained by the UHS technique.Finally,at an average heating rate of875℃/min,the UHS technique was able to obtain bulk Si₃N₄ ceramics with a relative density greater than 96%,α-βconversion greater than 80%,and a microstructure different from conventional low-speed sintering microstructures in a sintering time of only 300 s.Therefore,in addition to significantly reducing the processing time,UHS is also an effective method for designing Si₃N₄ ceramics with interlocked bimodal peaks consisting of long rod-shapedβ-Si₃N₄ microstructure.Finally,this topic combines additive manufacturing with rapid sintering technology for the pressureless rapid sintering and preparation of 3D printed Si₃N₄ ceramics at different scales from the perspective of practical applications.The results show that UHS can successfully produce complex-shaped 3D printed Si₃N₄ ceramics with a diameter of 10 mm and high densities and structural integrity in a very short time（a few minutes）,but the structure effect leads to a lower degree of densification in complex structured Si₃N₄ ceramics than in solid structured samples.The size effect leads to deformation of 3D printed Si₃N₄ ceramics at ultra-fast heating rates,and the degree of deformation increases with size,with the highest degree of bending deformation at 20 mm diameter.In addition,the UHS preparation process also affects the sintering process of complex 3D-structured Si₃N₄,where the initial density reduction of the 3D molding method increases the sintering difficulty,and the high ramp-up temperature and high carbon sintering environment are considered to be the direct causes of the bimodal distribution microstructure and carbon diffusion phenomenon in 3D-printed Si₃N₄ceramics.Therefore,the study of pressureless rapid sintering of 3D printed samples with complex configurations at multiple scales will provide a new theoretical basis for the realization of integrated structural-functional rapid manufacturing of ceramic materials.This project innovatively uses green and efficient external field assisted heating technology to combine ceramic additive manufacturing with pressureless rapid high temperature sintering,which is based on the frontier of advanced ceramic 3D manufacturing technology and focuses on the strategic vision of"carbon peak and carbon neutral"in manufacturing industry.The development of this project will provide new design ideas and realization methods for"advanced manufacturing and green manufacturing"of ceramic materials,and promote the innovative development and green upgrading of additive manufacturing and ceramic industry.