Silicon oxynitride (Si2N2O) exhibits a series of merits, such as low theoretic density, high hardness, excellent oxidation resistance, high strength, and high thermodynamic stability, which renders a great potential for application as a high temperature structural ceramic material. Recently, ab initio calculations showed that the dielectric constant and loss tangent of Si2N2O are very low. Therefore, Si2N2O ceramic may possess a combination of unique mechanical and dielectric properties and can be used as a high temperature radome material. However, it is difficult to sinter dense Si2N2O ceramic because of its strongly covalent bonds and low diffusion coefficient. The thermal, dielectric, friction and wear properties of Si2N2O material have not been widely investigated.
Based on the above consideration, a new method was developed to fabricate dense and pure Si2N2O ceramic at relatively low temperature in this dissertation and the thermal expansion, mechanical, thermal shock resistance, dielectric, friction and wear properties of the as-sintered Si2N2O were systematically investigated. In order to improve the dielectric and other performances of Si2N2O, Si2N2O/β-cristobalite and Si2N2O/h-BN composites were also fabricated and their properties were studied. The following conclusions were drawn:
1.Dense and pure Si2N2O material was fabricated by the in situ hot pressing/liquid reaction synthesis process from SiO2 and Si3N4 using Li2CO3 as a sintering additive. In comparison to other sintering additives, Li2CO3 was more effective in synthesizing Si2N2O. The synthesis/sintering temperature of Si2N2O was greatly lowered and the firing time was shortened. Dense Si2N2O with Li2CO3 additive exhibits similar mechanical properties to those using other additives, as well as low dielectric constant and loss tangent.
2.Si2N2O showed very low friction coefficient and wear rate due to the formation of ultra-smooth surface and the boundary lubrication of water and H4SiO4 when sliding against Si3N4 ball in water
3.Si2N2O/β-cristobalite composites with good mechanical and dielectric performance were synthesized. β → α-cristobalite transformation in the as-sintered composite was suppressed.
4.Si2N2O/BN nanocomposite was fabricated by the hot pressing method from Si3N4, SiO2, and BN using Li2CO3 as a sintering additive. The modulus and hardness of the composite almost linearly decreased with the increase of BN content. However, the flexural strength does not show a dramatic degradation due to the fact that the grain growth of Si2N2O was inhibited by the homogeneously dispersed BN particles. The critical thermal shock temperature, machinability and dielectric properties of Si2N2O were significantly enhanced by the introduction of BN.
5.Si2N2O/BN composite exhibited a lower degradation rate of high temperature flexural strength in comparison with monolithic Si2N2O due to the inhibition of grain boundary sliding by homogeneously dispersed nano-sized BN particles and the formation of self-healing oxide film on the surface.
6.Si2N2O/30 vol.％ BN composite showed very low friction coefficient and wear rate due to the formation of smooth surface when sliding against Si3N4 in water. Its friction coefficient and wear rate was relatively sensitive to normal load.
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