| Zirconium diboride(ZrB2)is a desirable combination of high melting temperature, resistance to chemical attack,and other physical properties that make it attractive for these applications for aerospace,refractorily,cutting and electrode fields.However, ZrB2 is hard to sintering densification,easily oxidized and bad thermal shock resistantce in the high-temperature conditions to impact high-temperature properties, which makes it difficultly realize for extensive use.How to achieve the high-density ZrB2 ceramic,improve oxidation resistance and thermal shock resistantce of ZrB2 ceramic at high temperature conditions,which already became hot point for researchers of various countries.In this paper,the train of thought that resolving sintering densification and improving oxidation resistance was proposed.Y2O3-Al2O3 composite powders were coated on the surface of ZrB2 particles to form Y2O3-Al2O3/ZrB2 composite powders with coating structure and easy sintering densification,ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics with coating structure were prepared through spark plasma sintering(SPS).First,Al(OH)3-Y(OH)3/ZrB2 composite particles were synthesized by the chemical precipitation method.The dispersibility of ZrB2 particles and the conditions of synthesizing Al(OH)3-Y(OH)3/ZrB2 composite particles were investigated.Through analysis,the optimum conditions of the dispersibility of ZrB2 particles in the ZrB2 suspension are the dispersant(PMAA)content with the 2vol%of ZrB2 suspension,the ultrasonic dispersion for 10 minutes and pH=9,respectively.The conditions of synthesizing Al(OH)3-Y(OH)3/ZrB2 composite particles with the better coating quality are that ZrB2 particles is under dispersion conditons(the dispersant(PMAA) content with the 2vol%of ZrB2 suspension and the ultrasonic dispersion for 10 minutes),pH=9,the appropriate concentration(Al3+=0.017mol/L,Y3+=0.01mol/L)of solution,titration speed for 0.05ml/s and reaction time for 60 minutes,respectively.Second,the sintering densification of ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics were investigated.Through analysis,the optimization sintering technology for preparing ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics with the SPS is sintering temperature for 1700℃,sintering pressure for 20MPa and holding time for 4min. Relative density of ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics are increased with increasing YAG and YAG-Al2O3 content.Relateive density,Young's modulus and fracture toughness of 70wt%ZrB2-YAG ceramics are 98.3%,426GPa and 4.15MPam1/2,respectively.The results show ZrB2 ceramics realizes densification under the lower sintering temperature(1700℃)through adding YAG.Relateive density,Young's modulus and fracture toughness of 70%wtZrB2-YAG-Al2O3 ceramics are 98.8%,435GPa and 4.23MPam1/2,respectively.Relative density and mechainic properities of ZrB2-YAG-Al2O3 ceramics are increased with increasing Al2O3 content.Besides,ZrB2-YAG ceramics and ZrB2-YAG-Al2O3 ceramics were prepared with mixing raw materials through same conditions,the mechainical properities of sintered ceramics with coated raw materials are higher than that of sintered ceramics for mixed raw materials,which shows ceramics with coated structure possess better mechainical properities.Finally,the oxidation resistances of pure ZrB2,ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics from 700℃to 1600℃were studied.Pure ZrB2 ceramics are shown distinct oxidation weigh gain at 900℃,however,ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics are shown distinct oxidation weigh gain at 1200℃,which shows oxidation resistance of ZrB2-YAG and ZrB2-YAG-Al2O3 ceramics are increased with increasing YAG and YAG-Al2O3 content.Oxidation resistance of ZrB2-YAG-Al2O3 ceramics is increased with increasing Al2O3 proportion,especially for higher temperature(above 1300℃), because Al2O3 react with B2O3 to form compacter oxidation resistance layer (Al18B4O33).Through comparative analysis,oxidation resistance of sintered ceramics with coated raw materials is better than that of sintered ceramics with mixed raw materials,which indicates that ceramics with coated structure improves the high temperature oxidation resistance.
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