The Preparation Of ZrN Ceramics Formed By The Reaction Of Si₃N₄-ZrO₂-SrO And The Construction Of Phase Diagrams

ZrN as a member of ultra-high temperature ceramics?UHTCs?material has high melting point,high hardness,high wear resistance and good electrical conductivity.It is important that ZrN ceramic is very difficult to sinter because of the strong covalent bond,high melting point and small diffusion coefficient of ZrN.Preparation of the ultra-high temperature ceramics?UHTCs?via reaction-sintering is considered as a low-cost method.ZrN is inevitably produced when sintered Si₃N₄ ceramics using ZrO₂ as a reinforcing agent,but the reaction is accompanied by a weight loss of more than 50%.ZrN+X?+SrZrO₃?composite phase was synthesized via the reaction of Si₃N₄-ZrO₂-SrO ternary system by introducing alkaline earth oxide SrO into Si₃N₄-ZrO₂ system to promote the reciprocal reaction of SiO₂-Si₃N₄-ZrO₂-ZrN quaternary system.The reactions of Si₃N₄-ZrO₂-SrO ternary system at 1500?and 1700?were investigated by pressureless sintering and thermodynamic calculations.The phase compositions of the samples were characterized by X-ray diffraction.The results show that Zr N+SrSi₂O₂N₂ and ZrN+Sr₃SiO₅+SrZrO₃ can be formed in Si₃N₄-ZrO₂-SrO system when the sintering temperature is 1500?.When the sintering temperature increases to 1700?,the coexisted phases of ZrN include SrSi₂O₂N₂ and Sr₃Si O₅+SrZrO₃ as well as Sr₂SiO₄+Sr ZrO₃ and Sr₇ZrSi₆O₂₁+SrZrO₃,in which Sr ZrO₃ is the reaction result of the binary system Zr O₂-SrO.The reaction products are not in the ternary system,but in its neighboring system.In order to determine the phase equilibrium relationship between ZrN and the phase of neighboring system,the phase relationship of the neighboring system were investigated,and the triangular prism phase diagram of the ZrO₂-Zr N-SrO-Sr₃N₂-SiO₂-Si₃N₄?Zr-Sr-Si-O-N?system at 1500?was presented.According to the reactions of Si₃N₄-ZrO₂-SrO ternary system,ZrN multiphase ceramic could be prepared in one step at a lower temperature.These solid-state reactions may provide a new reaction system for reaction sintering of the ZrN ceramic and ZrN-based composites.According to the phase relationship of Si₃N₄-ZrO₂-SrO system,it was found that all the ZrN-generating substitution reactions would release oxygen.Therefore,an appropriate oxygen absorbent is necessary.The ZrN ultrahigh temperature ceramics were prepared by Si₃N₄-ZrO₂-SrO system assisted by refractory metal Zr,Ti and alkali metal Ca as oxygen absorbers.The effects of the X-phase coexisting with ZrN,Si₃N₄ content and ZrN phase content on the properties of ZrN ceramics were studied.The sintering process and phase composition of different composite ceramic were analyzed.The experimental results show that ZrN composite ceramics can be prepared by assisting sintering of three metals.However,the effect of the two refractory metals Zr and Ti is more significant than that of alkali Ca.The Zr N ceramics prepared by Zr or Ti-assisted Si₃N₄-ZrO₂-SrO systems have the highest flexural strength up to 402 MPa,while the Ca assisted is lower than 230MPa.At the same time,The nitrogen-containing silicate?SrSi₂O₂N₂?coexisting with ZrN is more conducive to improving the mechanical properties of ZrN ceramic than silicate?Sr₂SiO₄?.And than,the appropriate excess of Si₃N₄ and the addition of ZrN are beneficial to the mechanical properties of ZrN ceramics prepared by the reaction.