Porous Silicon Nitride Ceramics And Their Coating Materials

With the development of aerospace technology and the needs of modern warfare, becauseaircraft flight Mach number, the scope of combat and guidance precision continues to improve,aircraft require wave-transparent materials with higher performance. Silicon nitride ceramichaving excellent electrical, thermal and mechanical properties becomes the focus of nationalresearches, and is recognized as the most promising new generation of wave-transparentmaterials. Combining the advantages of silicon nitride and porous ceramics, porous siliconnitride ceramic gets more and more attention.The presence of the higher porosity of theporous silicon nitride ceramic effectively reduces the dielectric constant of the ceramic material,but the porous structure causes the lower mechanical properties, while high porosity easy toproduce a moisture absorption phenomenon, whice causes unstable electrical performance,therefore it is necessary to solve these problems through preparing one dense coating of thesurface of the porous structure.In this paper, basing on excellent dielectric and mechanical properties silicon nitrideceramic takes the use of pressureless burying sintering with Yb₂O₃and Y2O3as sintering aids,therefore we have successfully prepared the porous silicon nitride ceramic material, while thestudy analyzed the influence of the sintering aids and temperature on the material properties.The results showed that: at1690°C sintering temperature, when the additive amount of Yb₂O₃increased from3wt%to9wt%, the density, the bending strength, the fracture toughness and thedielectric constant of the ceramic material increased; adding9wt%Yb₂O₃, the density was2.17g/cm³, at the same time the bending strength was223.1MPa, and the fracture toughnesswas3.51MPa·m^1/2, at last the dielectric constant was4.17; when the additive amount of Yb₂O₃increased from9wt%to12wt%, the density, bending strength and dielectric dielectric constanthas decreased slightly, but the fracture toughness increased to3.82MPa·m^1/2.With the increase of the sintering temperature, the density, flexural strength, fracturetoughness of material was increasing continuously. At1780°C sintering temperature, whenthe content of Yb₂O₃was9wt%, the bending strength of the material reached the maximum,365.6MPa; when the content of Yb₂O₃was12wt%, the fracture toughness of the material reached5.86MPa·m^1/2; adding the same content（9wt%） of the two additives, the morphologyof the material was formed a three-dimensional space through staggered overlap and relativelyuniform rod-shaped grains; but the materials which add Yb₂O₃as sintering aids have higherdensity and better mechanical properties, Yb₂O₃promoted phase transition of silicon nitrideceramics more effectively, generated β-Si₃N₄grains had greater aspect ratio.On the surface of porous silicon nitride moistureproof-enhancing coating was preparedthrough polymer derived and spraying method. By the design of the coating system, selectingsilicon nitride and Li₂O-Al₂O₃-SiO₂（LAS） micro powder as the component, polysilazane asbinder, dense ceramic coating was prepared at1300℃. Using scanning electron microscopyanalysed the morphology of the surface and cross section of the ceramic coating, the ceramiccoating was dense, uniform thickness, and combined preferably with substrate. The bendingstrength of the porous silicon nitride ceramic material sprayed ceramic coating was increasedby more than18%, the surface hardness increased by1.7times, and apparent porosity reducedby an average of97.7%, water absorption was below0.5%, illustrating that the preparedcoating reached the preferably densified.surface of the substrate.