Study On The Preparation And Properties Of Ferrosilicon Nitride Ceramic

Silicon nitride(Si₃N₄)ceramic is a typical representative of non-oxide ceramic,which has many excellent properties such as high specific strength,high specific modulus,high hardness,impact resistance,creep resistance,thermal shock resistance,corrosion resistance and wear resistance.The related theory and application of Si₃N₄ ceramic has been well studied,but the high cost limits its application.Ferrosilicon nitride(Fe-Si₃N₄)is a kind of complex phase material mainly composed of Si₃N₄ in the field of high temperature.At present,it is mainly used as a refractory material and rarely reported as a structural material.Due to its low cost and large performance space,Fe-Si₃N₄ is a kind of high-temperature material with good application prospect.In this paper,the preparation process and properties of Fe-Si₃N₄materials were mainly studied.The main research contents and conclusions are as follows:Fe-Si₃N₄ power was prepared taking ferrosilicon powder as raw material,Fe-Si₃N₄ and NH₄Cl as addictive.The process of sintering temperature system was determined by thermal analysis,and it was confirmed that the nitriding reaction of ferrosilicon powder was mainly carried out in two temperature ranges.XRD and micromorphology analysis showed that NH₄Cl is beneficial to the formation of?-Si₃N₄ phase,and the content of?-Si₃N₄ in the product can reach 76.6%?However,the NH₄Cl is forbidden when preparing high strength Fe-Si₃N₄ bulk material because of the fluffy effect.It was found that the diluents can not only promote the growth of Si₃N₄ whiskers in the nitride product but also improve the nitridation rate of the product.In addition,it is founded that the strength of the reaction-sintered Fe-Si₃N₄ ceramic obtained 106 MPa,which indicates that the method has practical significance for the production of Fe-Si₃N₄ ceramics.The formation mechanism of authigenic Si₃N₄ fibers in reaction-sintered Fe-Si₃N₄ ceramics was also studied.Fe-Si₃N₄ ceramics were fabricated by gas pressure sintering(GPS)using micron sized Fe-Si₃N₄ as raw material and Al₂O₃-Y₂O₃ as sintering aids.Phase composition and microstructure of the specimens were analyzed by X-ray diffraction and scanning electronic microscopy,and effect of sintering temperature on the microstructure and mechanical properties of Fe-Si₃N₄ ceramics were investigated.The results showed that?-phase Fe-Si₃N₄ceramic with a high density and good mechanical properties can be obtained by GPS.With the sintering temperature increasing,the density,flexural strength and fracture toughness of the samples increased firstly and then decreased.The highest density,flexural strength and fracture toughness of the samples sintered at 1770?are 3.31 g/cm³,435 MPa and 6.97MPa·m^1/2,respectively.Below 1770?,the main crystal phase was elongated columnar?-Si₃N₄,which was uniform and well-developed.The compact connection between the grains results in the less pores of grain boundaries.Reactions of iron containing phase and Si₃N₄were appeared as sintering temperature further increases,leading increasing of porosity and decreasing of flexural strength and fracture toughness.The non-nitrided iron-rich particle exists in sintered bodies,the removal of which may hopefully increase the mechanical properties of Fe-Si₃N₄ ceramics furthermore.Si₃N₄-Fe₃Si composites were prepared using Fe-Si₃N₄ as the source of Fe₃Si by gas-pressure sintering.By adding different amounts of Fe-Si₃N₄ into the starting powders,Si₃N₄-Fe₃Si composites with various Fe₃Si phase contents were obtained.The microstructure and mechanical properties of the composites were investigated.With the increase of Fe-Si₃N₄contents,the content and particle size of Fe₃Si both increased.When more than 60 wt%Fe-Si₃N₄ were added,the abnormal growth of Fe₃Si particles occurred and oversized Fe₃Si particles appeared,leading to non-uniform microstructures and worse mechanical properties of the composites.It has been found that Fe₃Si particles could toughen the composites through particle pull-out,interface debonding,crack deflection,and particle bridging.Uniform microstructure and improved mechanical properties(flexural strength of 354 MPa and fracture toughness of 8.4 MPa·m^1/2)can be achieved for FSN40.The oxidation behavior of Si₃N₄ ceramics containing iron in air at 1300?1500?was investigated.The results indicate that the presence of iron in the ceramics causes a slight increase in the oxidation rate.The oxidation of Fe-Si₃N₄ obeys the parabolic oxidation law,with an activation energy of 285 KJ/mol.Based on the element mapping analysis,iron diffused to the surface during the oxidation process,causing a decrease in the oxidation resistance.However,the oxidation rate of Fe-Si₃N₄ is still appropriate for applications in air at high temperatures.Fe-Si₃N₄ was prepared by means of a gas-pressure sintering method.The ablation behavior was studied by plasma wind tunnel tests.The data suggested that at low heat flux and stagnation pressure conditions,the ablation process was controlled by the atomic oxidation of Si₃N₄,leading to elimination of Si₃N₄.By contrast,the erosion was mainly produced from the decomposition of Si₃N₄ at high heat flux and stagnation pressure conditions.At these conditions,a fraction of Si phase,formed upon decomposition of Si₃N₄,was volatilized.The remained Si had melted due to high temperatures and scoured away from stagnation point area,generating mushroom-shaped samples.