Research On Technical Process And Properties For Reaction Formation And Post-Hot-Pressing Preparation Of Si3N4/MoSi2 Composite

A special procedure which consists of pre-sintering,reaction-bonding and post-hot-pressing was employed to prepare Si₃N₄/MoSi₂ composite,using as starting materials Si,Mo power mixture.The microstructural development during each processing step of the composite was characterized by X-ray diffractometer(XRD), scan electron microscopic analysis(SEM)etc.Mechanical properties of post-hot-pressed specimens as well as reaction-bonded specimens were characterised at room and high-temperatures.The Si₃N₄/MoSi₂ composite showed a pronounced increase in bend strength from room temperature up to 1200℃.Microstructural analysis was performed on the fracture surfaces of specimens with different temperatures.The isothermal oxidation behavior of Si₃N₄/MoSi₂ composites with different MoSi₂ content was studied.Phase composite and microstructure of oxidized specimens were investigated by XRD and SEM.It was found that this composit was oxidized in a parabolic manner for weight gain curves exposure to air at temperature range from 1200℃to 1400℃.The activation energy increased as the MoSi₂ contents increased,the higher the MoSi₂ contents the more the flexure strength increase.The ceramic composites of Si₃N₄/MoSi₂/NbN were prepared using Nb,Mo and Si powders by a three-step sinter procedure,i.e.pre-sintering,nitridation reaction and post-hot-press under N₂ atmosphere.The microstructure and phase compositions of the samples during every steps were characterized by SEM and XRD.The results indicate that the materials obtained from pre-sintering step composed of NbN,MoSi₂ and un-reacted Si phase.Nitridation reaction at elevated temperature promoted the formation ofα-Si₃N₄,and small fraction ofβ-Si₃N₄.The final step of post hot-press process led to the formation of ceramic composites with compositions ofβ-Si₃N₄, MoSi₂ and NbN.The prepared Si₃N₄/MoSi₂/NbN complex material was investigated.The influence of different porosity to oxidation susceptibility was studied.XRD,SEM and EDS was used to determine the oxidation product and surface texture of complex material.The result indicates that test sample at the temperature range from 1150 to 1400℃appears two stages,from 0 to 15h is line shape fast oxidation stage,the oxidation activation energy of A is 98kJ/mol,B is132 kJ/mol,after 15h is parabolic curve slow oxidation stage,the oxidation activation energy of A is 426 kJ/mol,B is 492 kJ/mol;the more altitude the porosity,the more severity the oxidation,the longer the fast oxidation stage last.