Structural Damage Research Of SiC Fibers During Preparation Of SiC_f/MoSi₂ Via Liquid Silicon Infiltration

Silicon carbide fiber reinforced MoSi2 matrix composite(SiCf/MoSi2)is prepared via liquid silicon infiltration(LSI).The microstructures of the products during the preparation process are analyzed by SEM,TEM and XRD.The corrosion damage behavior of SiC fibers is studied.The main conclusions are summarized as follows:1 SiC fiber preform is first impregnated with phosphomolybdic acid(PMA)in ethyl ethanol.After calcinations,the PMA is converted into molybdenum trioxide(MoO3).Then the MoO3 is converted into metal molybdenum(Mo)by hydrogen reduction and obtain SiCf/Mo composites.Finally,dence SiCf/MoSi2 composites is prepared via liquid silicon infiltration.In the hydrogen reduction process,formation of vapour products lead to the presence of nano-scale pores in Mo,which will accelerate the infiltration of the molten Si.After liquid silicon infiltration,SiCf/MoSi2 composite is dense,and the SiC fibers distribute uniformly in MoSi2 matrix.2.Less number of impregnation-calcination cycles will lead to the low Mo content,resulting in the presence of Si in SiCf/MoSi2 composite.Increasing the Mo content can effectively reduce the residual Si.TEM shows that the MoSi2 is single crystal and particle size is much larger than the predicted value under the mechanism of reaction-diffusion.Therefore,it can be concluded that the MoSi2 is formed via dissolution-precipitationand mechanism.3.Continuous SiC fiber will be corroded during the formation of SiCf/MoSi2 composites if there is no fiber coating.Before silicon melting,it is corroded by Mo matrix and during infiltration it is corroded by melten silicon.In the former case,it forms two intermediate products which are Mo2C and Mo4.8Si3C0.6.The diffusion depth of Mo to SiC is about 1 μm after heating to 1450℃ for 30 min and it will be deeper with the increasing of temperature and holding time.During the liquid silicon infiltration,Si has significant erosion to the SiC fiber,and the degree of corrosion is related to the Mo content.If Mo is absence,the SiC fiber is completely corroded and recrystallizes to form a petal-like structure composed of micron-sized single crystalβ-SiC at the position of the initral fiber area.If Mo is present(the mass fraction in SiCf/Mo is 54%),the SiC fiber will form a sandwich structure,where the core region is unreacted fibers,the intermediate layer is recrystallized SiC containing Al,and the outer layer is recrystallized SiC without Al.With increasing the Mo content(mass fraction of Mo is 78%),it will form a dense MoSi2 coating on the surface of fiber through the dissolution-precipitation mechanism.Then the coating gradually thickens with the rapid progress of the reaction and thus the SiC fibers do not undergo significant corrosion damage anymore.