Fabrication, Microstructure And Properties Of Si₂N₂O Ceramics

Silicon oxynitride （Si₂N₂O） ceramic is a low density, good mechanical propertiesof strong covalent bonds ceramic material, which has excellent dielectric properties,oxidation resistance, thermal shock resistance, and also a promising structure andfunction integration material. The study of preparation and properties andmicrosturcture of silicon oxynitride （Si₂N₂O） for aerospace applications such asaerospace radome, the antenna window wave-transparent materials has importantsignificance.The paper uses silicon nitride （Si₃N₄） and silicon oxide （SiO₂） as raw materials,lithium carbonate （Li₂CO₃） as a sintering aid, with no pressure sintering of siliconoxynitride to prepare （Si₂N₂O） ceramic materials. The study focus on densificationbehavior and how the parameters such as content of aids, sintering temperature,sintering time, nitrogen pressure that influence the phase composition andmicrostructure of the materials with the methods such as XRD, SEM, etc. The paperalso studied the mechanical properties, dielectric properties, oxidation resistance,thermal shock resistance of the material, including researches on how lithium carbonate（Li₂CO₃） content change impact on these properties. Using the same raw material toprepare silicon oxynitride （Si₂N₂O） ceramics by spark plasma sintering （SPS） and thepaper also studied sintering methods impacts on organization and properties of thematerial.Pressureless sintering densification studies have shown that: As the materialsintering additives carbonate （Li₂CO₃） content increases, the sintering process, canprovide more liquid, that is to say, more liquid phase participate in sintering and in thisway the material has a relative higher density. However, when the sintering additivecontent is too high, the new phase decomposed generating the gaseous phase, resultingin decreased density of the material. Research shows that preparing silicon oxynitride（Si₂N₂O） the added lithium carbonate （Li₂CO₃） content should not be more than7.5mol%.Phase analysis showed that：No adding of sintering aids, no Si₂N₂O phaseformation, there are only the αâ†'β Si₃N₄phase transformation. After adding Li₂CO₃,there Si₂N₂O generate. The study of sintering temperature found that when the sinteringtemperature rises, the formation of Si₂N₂O phase easier, In1700℃，and the Li₂CO₃content was2.5mol%, the Si₂N₂O phase can be obtained, the content of Si₂N₂O phasewas12.39wt%, However, with increasing content of Li₂CO₃, the Si₂N₂O phase decrease,so when sintering in1700℃, Li₂CO₃content should not be more than15mol%. Whensintering in1600℃，more Li₂CO₃are needed，the minimum content is10mol%. When the content of Li₂CO₃up to15mol%, the Si₂N₂O is still not decomposed, the content ofSi₂N₂O is88.13wt%, showed that more Si₂N₂O phase obtained in1600℃. The study ofmolding conditions found that when the compression load is200MPa, Li₂CO₃contentof2.5mol%the nearly pure phase is obtained, shown that high load compressionpromotes Si₂N₂O synthesis. The study on sintering time shown that2h is a suitable timefor sintering but can have disadvantage to get purity Si₂N₂O, which will breakdown.When sintering is1h, the material can be made more pure Si₂N₂O ceramics, the Li₂CO₃5mol%can make Si₂N₂O phase content up to97.62wt%. The study of nitrogen pressureshown that, higher nitrogen pressure can suppressing the decomposition of Si₂N₂O andthe phase transition of Si₃N₄. Finally the paper get the proper sintering parameters:200MPa to squeeze forming, sintering at1650℃for1h, and the pressure of N2is0.5MPa.SEM and EDS analysis results showed that, the morphology of Si₂N₂O changes inplate, sheet, slab, prism order. With the increasing of sintering time, sinteringtemperature, molding pressure, Li₂CO₃content, Si₂N₂O grains developed more fully.The research of mechanical properties of materials suggests that:The increase ofthe relative density improves mechanical properties of materials, and when the materialcontains a certain amount of Si₃N₄phase, the material has better mechanical properties.The research of dielectric properties of materials shows that Si₂N₂O ceramics haveexcellent dielectric properties: The permittivity is about4.50⁷.12, dielectric loss is nothigher than0.0151, loss tangent is not higher than0.00336, with the Si₂N₂O phasecontent and porosity increase the dielectric properties became better. The study ofoxidation resistance of the material indicates that Si₂N₂O ceramics have excellentoxidation resistance, as the temperature rises, the content Si₂N₂O increases the oxidationweight becomes larger. at1300℃, the most oxidation weight gain9.97mg/cm². Thestudy of thermal shock resistance of the material found that Si₂N₂O ceramics haveexcellent thermal shock resistance, with the increase of silicon oxynitride phase, criticalthermal shock temperature increase. The maximum temperature is1277℃.Comparison of Si₂N₂O SPS sintering ceramics and pressureless sintering ceramicsprepared Si₂N₂O found that using SPS method only10minutes and at1500℃canobtain89.92wt%purity and dense ceramic Si₂N₂O, which also has excellent mechanicalproperties, but the microstructural study found that grain growth in SPS is not complete.