Preparation And Properties Of Dual-scale Mo-12Si-8.5B-ZrO₂（Y₂O₃） Composite

Mo-Si-B three-phase alloy has the advantages of high melting point,high strength,excellent high temperature mechanical properties and high temperature oxidation resistance.It is expected to become a new type of ultra-high temperature structural material to replace the traditional nickel-based superalloy,and has broad applications in the aerospace field.However,the mechanical properties and high temperature oxidation resistance of Mo-Si-B alloys need to be improved.This paper proposes to use nano-ZrO₂（Y₂O₃）to strengthen and toughen Mo-12Si-8.5B alloy,using homemade nano-Mo-ZrO₂（Y₂O₃）powder,micron Mopowder,Sipowder and B powder as raw materials to construct a dual scale of coarse and nanocrystalline With the microstructure of grain distribution,Mo-12Si-8.5B-ZrO₂（Y₂O₃）composite material is prepared by spark plasma sintering technology,and it is expected to achieve the purpose of improving its comprehensive mechanical properties.The main conclusions are as follows:（1）Prepare MoO₃-ZrO₂（Y₂O₃）composite powder by sol-gel method,and obtain better quality precursor powder through optimized process parameters（PH=1,citric acid content 1.2 times the mass of ammonium molybdate）;The best process for calcination of precursor powder is to obtain evenly distributed MoO₃-ZrO₂（Y₂O₃）composite powder after550℃×5h,and then reduce at 750℃for 3h to obtain nano-level and high purity Mo-ZrO₂（Y₂O₃）composite powder.When kept for 3h,the initial temperature of MoO₃reduction to Mois 550℃.（2）Mo-12Si-8.5B-ZrO₂（Y₂O₃）composite materials with single-scale and dual-scale microstructures were successfully prepared using different proportions of micro/nano Mopowder,Sipowder and B powder through SPS technology,The sintered composite material is composed of three phases ofα-Mo,Mo₃Siand Mo₅SiB₂.As the content of nano-molybdenum in the composite increases,the hardness,flexural strength and fracture toughness of the composite material increase first and then decrease.Among them,when the content of nano powder is 30wt%,the composite has the highest hardness（9.75GPa）,bending strength（629.6MPa）and room temperature fracture toughness(10.61MPa·m^1/2),which is attributed to the FGα-Mozone helps to increase the strength,and the CGα-Mozone has good ductility.All Mo-12Si-8.5B-ZrO₂（Y₂O₃）composites after sintering showed a mixed mode of transgranular and intergranular fracture.（3）With the increase of the content of nanometer ZrO₂（Y₂O₃）,the relative density of the Mo-12Si-8.5B-ZrO₂（Y₂O₃）sintered body gradually decreases.When the content of ZrO₂（Y₂O₃）is in the range of 1.5～2.5wt%,the composite has a higher hardness（9.76～9.98GPa）,flexural strength（672～678MPa）and room temperature fracture toughness(12.68～12.82MPa·m^1/2).When oxidized at 800℃,the higher the content of ZrO₂（Y₂O₃）（5～10wt%）,the better the oxidation resistance of the composite;while at high temperatures of 1000℃and 1200℃,the lower the content of ZrO₂（Y₂O₃）（0.5～2.5wt%）MSBZ composites have better oxidation resistance than composites with high ZrO₂（Y₂O₃）content（5%～10wt%）.In the friction and wear experiment,when the ambient temperature is lower than 400℃,the MSBZ2.5 composite has a low wear rate in the air and good wear resistance.The wear mechanism is mainly adhesive wear and oxidative wear;When the ambient temperature is higher than 400℃,the wear rate of the composite in vacuum is low,and the wear resistance is better.The wear mechanism is mainly adhesive wear and plow wear;Compared to the vacuum atmosphere,the composite in the air is due to friction and oxidation form a MoO₃ film with a low friction coefficient,which reduces the minimum friction coefficient to 0.28.