Preparation And Properties Of Silicon Nitride Ceramics Based On Multi-scale Structure Design

Porous silicon nitride with excellent high temperature resistance,high strength and low dielectric constant,has been an important material choice in the aerospace field.However,limited by poor mechanical and thermal shock resistance,and silicon nitride is prone to mechanical impact fracture and thermal shock fracture,which severely hampers the application in engineering.Therefore,improving the ability of silicon nitride to resist mechanical impact and thermal shock has become the research goal of this paper.The method of improving the thermal performance of porous silicon nitride is studied on the material structure design and control.Biomaterials such as bones,shells and bone needles are rich in multi-scale structures and have different mechanical effects at different scales,which can avoid performance defects at a single scale.This is the problem that engineering ceramic materials meet.The layered ceramics utilize the combination of high and low modulus layers to deflect the cracks along the weak interface during the fracture process,which can achieve the effect of energy dissipation and toughening.Therefore,the multi-scale structure,referencing biomaterials,is designed to obtain the multi-scale structure of silicon nitride for more effective crack deflection,in which the layered structure plays the macroscopic structure,the porous silicon nitride is the microstructure,and the combination of the two layers is controlled.In order to achieve this goal,computer simulation design has been used.Simulating the layered structure,when the ratio R of the high and low modulus is large,the stress concentration of the material is significant,and the overall strength of the material is affected,and the interlayer delamination easily occurs during the fracture.In order to ensure the material strength,the layered structure is designed from the ratio of high and low modulus R<2,and the minimum R value that can effectively deflect the crack is studied.A multi-scale silicon nitride structure was prepared by the tape casting and lamination process.The effects of the type and content of dispersant,the solid content of the powder,the ratio of binder to plasticizer on the rheological properties of the cast paste were studied,and a thin layer of silicon nitride ceramic with a thickness of 100-300 μm was prepared.In the cast molding paste,appropriate materials are added to regulate the modulus and micro morphology of the porous ceramic.A single layer of silicon nitride with a modulus of 35-270 GPa,82-166 GPa,and 70-174 GPa can be prepared by changing the content of the sintering aid,the BN content,and the β-phase seed crystal content.At the same time,the additive content will change the aspect ratio of the silicon nitride,BN will inhibit the growth of rod-like crystals,while the β-phase seed crystal will promote the growth of rod-shaped crystals.The effect of microstructure modulus and morphology on the strength and work of fracture of the multi-scale structure was investigated.The results show that when the ratio R of the high and low modulus of silicon nitride with multi-scale structure BN added is equal to 2.28,and that of the multi-scale structure β-phase seed crystals added is equal to 2.81,the material begins to show obvious crack deflection during the fracture process.Compared with the ordinary structure,the work of fracture can be increased by 40% and 90% respectively at the same density.When the aspect ratio of silicon nitride in the low-modulus layer is large,the work of fracture is greatly affected by the porosity,while the overlap of the rod-like crystals inhibits the influence of the porosity on the fracture properties of the material to some extent.Under the same porosity value,the bending strength of multi-scale silicon nitride with BN added is more than 90% higher than that of tradition bulk structure,and the multi-scale structure of seed added has little difference in strength with tradition bulk structure.The residual strength method to evaluate the thermal shock resistance of multi-scale structure silicon nitride shows that the multi-scale structure can significantly improve the thermal shock resistance of silicon nitride materials,and can increase the material’s thermal shock temperature by 200-600 °C.The multi-scale structure design has basically achieved the goal of effectively improving the work of fracture and thermal shock resistance of silicon nitride materials,which means important engineering application value.