The Fabrication And Properties Research Of Molybdenum Disilicide Bonded Silicon Carbide（MoSi₂-SiC）Composite

Molybdenum disilicide（MoSi₂）has the advantages of high melting point,excellent high temperature oxidation resistance.Recrystallized silicon carbide（R-SiC）has excellent high-temperature mechanical,creep resistance,corrosion resistance,highly thermal conductivity.However,R-SiC which sintering at high-temperature has a high porosity which was easy to oxidation.In this project,MoSi₂-SiC composite was prepared by high-temperature melting MoSi2 which infiltrated into SiC matrix without high-temperature recrystallization sintering.Composites with high density,excellent oxidation resistance and thermal shock resistance were expected to obtain.Since SiC was used as raw material,water-soluble resin as binder,mixing 30min,laid 48h,pressed into 150mm×25mm×25mm green body,dried at 120℃for 3h,embedded with MoSi2 powders into crucible.Then high density MoSi2-SiC composites was prepared by sintering at 1970℃for 3 hours.Effect of the addition of micron powder,particle size distribution,and Si powders on the properties of SiC blanks was studied.1600℃oxidation resistance and 1300℃thermal shock resistance of MoSi₂-SiC,R-SiC and Si₃N₄-SiC materials were comparatively studied.The results were summaried as follows:1.Three optimal particle size compositions were obtained in three kinds of accumulate density design schemes which had the lowest apparent porosity,highest bulk density,and highest mechanical strength,respectively.The best ratio of particle accumulation was 12%（2.5μm SiCaddition）when the particle size is 2.5¹.43mm;when the particle size is 1.43mm⁰.5mm and 0.5⁰mm,the best ratio is 15%,respectively.2.High-density and high-performance MoSi₂-SiC composite was prepared by high-temperature infiltration method.The apparent porosity of composite was significantly reduced and the density was significantly increased.Optimized with a particle size of 1.43⁰.5mm composition design which had lower apparent porosity（3.7%）,higher compactness,and its particle size distribution（less than 5μm pores account for a larger proportion）was conducive to improve the densification of composite.Pore size had a great influence on infiltration.When pore size is less than5μm,infiltration was facilitated.Pores of 5μm³0μm was remained in the sample,which is not conducive to sintering densification.3.Addition of Si powderswas not conducive to the improvement of the material performance.Si addition reacted with the carbon residue formed whisker-likeβ-SiC.The apparent porosity,bulk density and pore size distribution of the green body unchanged,the mechanical properties increased.The introduction of Si powders was not conducive to the oxidation resistance due to the fact thatβ-SiC crystallites formed by the reaction were small and easy to oxidize,resulting in a large weight gain,meanwhile the introduction of Si was not conducive to the improvement of the thermal shock resistance.4.The oxidation resistance of MoSi₂-SiC is significantly better than that of R-SiC and Si₃N₄-SiC.In the air atmosphere of 1600℃,the oxidation weight gain per unit area of MoSi₂-SiC is 2/7 that of R-SiC.The oxidation of Si₃N₄-SiC was active oxidation and cannot be used for long time.5.The thermal shock resistance of MoSi₂-SiC was superior to that of R-SiC and Si₃N₄-SiC.The flexural strength retention of MoSi₂-SiC measured by air-cooling method was 1.33 times that of R-SiC,1.35 times of Si₃N₄-SiC.The retention of flexural strength measured by water cooling was 1.75 times that of R-SiC,2.56 times of Si₃N₄-SiC.The softening of MoSi₂ at high temperature made MoSi₂-SiC plastic,which helped to improve high-temperature mechanical properties and thermal shock resistance.