Preparation And Properties Of SiC_f/SiC-MoSi₂ Composites Via Liquid Silicon Infiltration

SiC fiber reinforced ceramic matrix composite(SiCf-CMC)has a series of advantages,such as high strength,high temperature resistance,oxidation resistance,small thermal expansion coefficient,low density,etc.Which has widely used in aerospace,energy and other fields.SiC is the most widely used ceramic matrix in CMC,its preparing technologies mainly contain Chemical Vapor Infiltration(CVI),Precursor Impregnation and Pyrolysis(PIP),and Liquid Silicon Infiltration(LSI).Among them,LSI process has the advantages of low cost,short cycle high density,etc.As for the design of preforms in this paper,Mo powder and phosphomolybdic acid(PMA)were used as Mo source and phenolic resin were used as C source to prepare porous preforms with SiCf.By controlling the content ratio of Mo and C,can obtain different components and pore structure preforms.The preform with Mo powder as Mo source contains C,Mo and Mo2C components.When the Mo addition was 0wt%,the porosity of the carbonized preform was 29%,and the pores were mainly composed of cracks separated by massive dense carbon.As the addition of Mo increased,the porosity of the preform increased,too.When the addition of Mo was 50%,the porosity was 40%,but still has dense carbon in the the carbonized preform.By contrast,the carbonized preform with PMA as Mo source contains C,and Mo2C components.With the increase of PMA,the bulk of dense carbon in the preform disappeared and became a uniform porous structure.Dense SiCf/SiC-MoSi2 composites can be obtained through the preforms with two different Mo source by LSI.In the composite prepared with Mo powder as Mo source,which matrix still contains unreacted residual carbon and with the increase of Mo,the residual carbon will also decrease.However,when Mo increases to 50%,the sample will show local cracking after LSI.In the composite prepared with PMA as Mo source,when the addition of PMA increased to 30%,the residual carbon in the sample disappeared,and the MoSi2 phase was evenly distributed in the matrix.In addition,residual Si was found in all samples after LSI and the volume content of residual Si will decrease with the increase of Mo source,the minimum content of SiCf/SiC-MoSi2 composite prepared by Mo powder was 19.3%and the minimum content of SiCf/SiC-MoSi2 composite prepared by PMA was 14.3%.The SiCf/SiC-MoSi2 composites were tested for mechanical properties and high-temperature oxidation resistance,and the results showed that all the samples were brittle fracture with no fiber pulling out,which indicating that the SiC fibers formed a strong interfacial bond after being eroded by molten silicon.When Mo was added at 30%,the bending strength was the highest at 149.3MPa;When Mo was added at 0%,the bending strength was the lowest at 86.4 MPa.In the study of oxidation resistance at 1200℃ for 20h,it was found that the weight loss rate of sample without Mo was 19.9%,while the weight loss rate of the sample containing 40%Mo was only 0.19%,which indicates that the residual carbon in the matrix seriously affects the oxidation resistance of the composites.However,when PMA was used as Mo source and the PMA content in the sample was 30%,there was no residual carbon in the matrix,and the weight gain after oxidation was 0.09%,which showed a good high temperature oxidation resistance.Thermodynamic and kinetic analysis was carried out on the reaction,and the formation mechanism of SiC and MoSi2 were studied.In this paper,the formation of SiC is determined by diffusion and solution-precipitation,while the formation of MoSi2 mainly follows the solution-precipitation mechanism.