Research On The Preparation And Properties Of Si₃N₄ Poly-hollow Microsphere Ceramics With A-typed Sandwich Structure

Porous Si₃N₄ ceramics have become a potential radome materials due to its high strength,high temperature resistance and good wave-transmitting properties.However,the preparation process of the sandwich structure Si₃N₄ ceramics for radome applications is still immature.In this paper,Si₃N₄ poly-hollow microspheres（Si₃N₄PHMs）are used as raw materials,and direct coagulation casting（DCC）composed with infiltration process and selective laser sintering（SLS）technology are used to prepare A-typed sandwich Si₃N₄ PHMs ceramics.The effects of forming parameters on the micromorphology,phase composition,porosity,and mechanical properties of Si₃N₄PHMs ceramics are systematically studied.The mechanism of Si₃N₄ PHMs ceramics prepared by SLS without binder is revealed.The main results are summarized as follows:（1）Porous core layer of Si₃N₄ PHMs ceramics is prepared via DCC based on the Si₃N₄ PHMs powders coated with nano sintering additives by chemical co-precipitation method.As the liquid phase diffuses from the surface of the Si₃N₄ PHMs to the interior of the spherical shell,the phase transition proceeds from the surface of the sphere-shell to the interior,and finally bimodal porous Si₃N₄ PHMs ceramics with the porosity of54%and the flexural strength of 110 MPa are obtained.As the solid content of the slurry increases from 50 wt%to 65 wt%,the mosaic depth between the microspheres gradually increases from 0μm to 8.16μm,and the fracture mode changes from along the spheres to through the spheres.（2）A-typed sandwich Si₃N₄ PHMs ceramics are prepared by DCC composed with infiltration,which has excellent interface bonding performance between dense layer and core layer.Si₃N₄ PHMs ceramics can obtain a higher density and better mechanical properties under negative pressure infiltration.When the solid loading of infiltration slurry is 30 vol%and the infiltration pressure is negative pressure,as the infiltration time increases from 10 min to 30 min,the thickness of the dense layer gradually increases from 44.08μm to 96.33μm,the porosity of Si₃N₄ PHMs ceramics is 38～42%and the flexural strength reaches 95～100 MPa.（3）To improve the freedom of design and manufacturing of Si₃N₄ ceramic,SLS is used to prepare core-layer Si₃N₄ PHMs ceramics.Under the action of a transient laser heat source,the surface of the Si₃N₄ PHMs is oxidized to form a"fluff"structure composed of bamboo-like amorphous fiber clusters and stacked nanospheres.These structures bridge the surface of the Si₃N₄ powder and realize the bonding between the polyhollow microspheres.After sintering at 1800°C for 2 h of green bodies prepared by 9 W-50 mm/s,Si₃N₄ PHMs ceramics obtain the porosity of 55%and the flexural strength of 18 MPa.By controlling the composition of SLS forming materials,forming parameters and structural design,it is expected to achieve near-net-shape forming of A-typed sandwich structure Si₃N₄ PHMs ceramics and promote the preparation and application of radome materials.