Microstructure And Properties Control Of Porous Si₃N₄ Ceramics Formed By Selective Laser Sintering

Porous Si₃N₄ ceramics have good mechanical properties,high-temperature properties and wave-transmitting properties,as well as anti-ablation,rain-corrosion,and anti-oxidation characteristics.It has good application prospects in the aerospace field and is a potential ceramic radome material.With the development of supersonic spacecraft in the direction of high Mach number,the radome needs to be designed into a complex sandwich structure to meet the requirements of broadband wave transmission performance.However,the traditional ceramic molding process is difficult to prepare Si₃N₄ ceramic parts with complex shapes and structures.In this paper,nano-AlN is used as an inorganic additive,and selective laser sintering（SLS）technology is used to prepare high-porosity Si₃N₄ ceramics.The effect of SLS laser power and AlN content on the microstructure and properties of porous Si₃N₄ ceramics is systematically studied.The main conclusions are as follows:（1）Under different laser power,porous Si₃N₄ ceramics are prepared by SLS technology.As the laser power increases,the bamboo-shaped amorphous fiber clusters and stacked nanosphere structures on the surface of the sphere increase significantly,and the connection between the hollow microspheres inside the ceramic green body becomes closer.After presintering,part of the α-Si₃N₄ in the porous Si₃N₄ ceramic preform was transformed into β-Si₃N₄,and Al0.16Si1.84N1.84O1.16 phase appeared.The Al0.16Si1.84N1.84O1.16 phase in the porous Si₃N₄ ceramic obtained by sintering at 1800°C disappeared,and both α-Si₃N₄ and Al0.16Si1.84N1.84O1.16 were transformed into β-Si₃N₄.As the laser power increased from 7 W to9 W,the porosity of porous Si₃N₄ ceramics is 44.6-52.8%,and the flexural strength is 12.6-24.6 MPa.（2）Under different AlN content,porous Si₃N₄ ceramics were prepared by SLS technology.Compared with the SLS directly formed Si₃N₄ ceramic green bodies,the added AlN will fill the voids between the poly hollow microspheres,which significantly increases the strength of the SLS directly formed porous Si₃N₄ ceramic green bodies and effectively reduces defects in the subsequent sintering process.In the high temperature sintering process,AlN can consume Si O2 in the forming process and synthesize the enhanced phase Si5 Al ON7 with Si₃N₄ and Al2O3.With the increase of AlN content,the Si5 Al ON7 content in porous Si₃N₄ ceramics gradually increases,and the flexural strength first decreases and then increases.When 20 wt%AlN has the highest Si5 Al ON7 content,the sintering is the densest,the flexural strength is 23.9MPa,and the porosity is 37.2%.In the porous Si₃N₄ ceramics prepared by the infiltration treatment,layered crystal grains appeared on the surface of the Si₃N₄ poly hollow microspheres.Compared with the ceramic samples without the infiltration treatment,the flexural strength increased by 50%.By controlling the SLS forming process parameters and the content of AlN in the Si₃N₄ composite powder,the properties control of porous Si₃N₄ ceramics is realized,which lays the foundation for the manufacture of ceramic missile radomes with complex structures.