Study On High Temperature Stability Of SiBCN Composite Ceramics Prepared By Precursor Conversion

Owing to the excellent structural stability,oxidation resistance,creep resistance and high temperature stability,SiBCN ceramics have a wide range of application prospects in transportation,aerospace,energy,information,microelectronics,etc.At present,the preparation methods of SiBCN ceramic mainly include organic precursor conversion,magnetron sputtering and mechanical alloying heating with pressure sintering.Through the organic precursor conversion,designed molecular structure can be introduced to the polymer with a uniform distribution of the atom.However,the raw material is expensive,the synthetic route is complex and a large-sized ceramic block can not be prepared easily.Magnetron sputtering is mainly used for ceramic films with photoelectric properties;The mechanical alloying with heat press sintering method has a simple process,non-toxic materials and low cost,and can prepare large-size bulk ceramics with excellent comprehensive mechanical properties,but it needs to meet the high pressure of 40 MPa and the high temperature of 1900℃ at the same time,which limits its widespread use.A method of combining the precursor conversion method and the ball milling was designed in this paper:The desired structure can be introduced by the precursor,and the inorganic powders used are totally industrialized production or industrial recyclables;The sintering temperature is only 1400℃,far lower than the mechanical alloying hot-press sintering method.The preparation method not only utilizes common industrialized materials to prepare SiBCN ceramics with good stability,but also provides a new exploration idea for demonstrating the high temperature stability mechanism of the series of SiBCN ceramics.X-ray diffraction,transmission electron microscopy,scanning electron microscopy,laser particle size analyzer,thermogravimetric analysis,and organic element analysis were used to characterize the structural properties and morphology of SiBCN ceramics and their intermediates.It was found that the powder prepared by ball milling of polysilazane and inorganic powder was a nearly spherical nanoparticle agglomerate,and the average particle size of agglomerates was less than 1μm;Under the same milling process conditions,the atomic ratio of the ball milling powder prepared by this method is Si₂BC_2.5N,which has the best thermal stability.Based on the determination of the atomic ratio,different milling process parameters have a certain influence on the particle size distribution,surface structure and thermal stability of the prepared Si₂BC_2.5N powder.The Si₂BC_2.5N powder prepared by ball milling for 30 h has an ideal amorphous structure and a relatively uniform morphology at a ball-to-material mass ratio of 40:1 and a rotation speed of 1200 rpm.The ceramic powder after high-temperature sintering has no loss in Ar atmosphere of 1400℃.XPS shows that a complex chemical reaction occurs between the polysilazane and the inorganic powder during the sintering process,and BNC structure with high temperature resistance have been formed.The Si₂BC_2.5N powder prepared by this method has good compressibility,and can be pressed and sintered under a certain pressure to prepare a bulk ceramic.The bulk ceramic prepared under 40 MPa has a better thermal shock resistance.The sintered SiBCN ceramic powder has good acid and alkali corrosion resistance,but has poor corrosion resistance to Na₂CO₃.Microscopic morphology observations revealed that after the acid-alkali corrosion,a protective film was formed on the surface of ceramic powder,which effectively played a role in corrosion resistance.After the Na₂CO₃ corrosion,no protective film was formed.