Preparation Of SiC Nanofiber Toughened SiC Ceramic Matrix Composites By PIP-MI Process

SiC ceramics have the characteristics of low density,high strength,creep resistance,corrosion resistance and high temperature resistance.Hence,the SiC ceramics have been widely used in the fields of safety protection,thermal protection,machinery and so on.The SiC ceramics have limited applications as the thermal structural material due to its high brittleness and catastrophic damage.Using SiC fiber to reinforce and toughen the SiC ceramic matrix composites is an important way to improve the brittleness of ceramics effectively.Compared with continuous SiC ceramic fiber,silicon carbide nanofiber(SiC_nf)is a single-crystal structure,having high tensile strength and elastic modulus,Therefore,SiC_nf is an ideal reinforcement.The SiC Ceramic Matrix Composites Toughened by SiC_nf and SiC fibers prepared by in-situ growing method have the problems of low content,short length and weak fiber pull-out effect of SiC_nf.In this paper,SiC_nf/SiC composites with high volume fraction of SiC_nf were prepared by compression molding and the combination of precursor infiltration and pyrolysis and reactive melt infiltration(PIP-MI)process.In order to reduce the damage of SiC_nf in the process of high temperature siliconizing and improve the toughening effect of SiC_nf on the composites,the preforms were filled with SiC for different times and the interfacial phases of C/SiC and BN/SiC were coated by PIP process.The main research contents and results are as follows:(1)Ultra-long SiC_nf is used as enhancer.The preforms with different volume fractions of SiC_nfwere prepared by compression molding under different forming pressures.The high density SiC_nf/SiC composites were prepared by the PIP-MI process.The influence of the forming pressure on the structure and properties of SiC_nf/SiC composites was investigated.The results show that the volume fraction of SiC_nf increases with the increasing forming pressure,which is helpful for the transfer of load between the ceramic matrix and the reinforcement.However,there exists the serious fracture of SiC_nf when the forming pressure is very high.When the forming pressure is 30MPa,the volume fraction of SiC_nf in the preform is 19.95%,and the continuity of SiC_nf is good.The bending strength and fracture toughness of the composites is 188 MPa and 9 MPa·m^1/2respectively.When the forming pressure is 40 MPa,the volume fraction of SiC_nf is as high as22.13%,but the SiC_nf is seriously fractured and the length is only 3?5?m.The bending strength and fracture toughness of the composites is 175 MPa and 8.4 MPa·m^1/2,respectively.(2)In order to decrease the damage of SiC_nf during the reactive melting and infiltration processes,the preform was filled in SiC phase by PIP process,and then the composite was densified by MI process.The influence of the number of PIP cycles on the structure and properties of the composites was investigated.The results show that the coating effect of SiC on the fibers is not good when the PIP process cycles are too few,and the pore diameter of the materials is too small when the PIP process cycles are too many,which affects the filling of pyrolytic carbon.When four cyclic SiC phases were coated,suitable carbon content and effective coating made the composite possess certain ductile fracture,and the fracture toughness of the composite reached15.4 MPa·m^1/2,which was 1.9 times of the composite without SiC coating.(3)The effects of C/SiC and BN/SiC interfaces on the structure and properties of SiC_nf/SiC composites were studied.It is found that the mechanical properties of the composites with C/SiC and BN/SiC interfacial phases are further improved.The flexural strength of the composite is increased to 189.6 MPa and 195.3 MPa,respectively.The fracture toughness is increased to 15.3MPa·m^1/2 and 16.1 MPa·m^1/2,respectively.The analysis believes that this is due to that the layered structure of Py C and BN interfacial phases increases the crack deflection pathway,and the SiC interfacial phases can protect the Py C and BN interfacial phases and SiC_nf from the erosion of molten silicon.In summary,the SiC_nf/SiC composites with high volume fraction of SiC_nf were prepared by the compression molding and PIP-MI process.The ductile fracture of the composite was realized to some extent by the interfacial phase coating of SiC,C/SiC and BN/SiC.At the same time,the research also has the problems of low bending strength of composites and no synchronous enhancement of strength and toughness.In the future,further research in prefabrication,sintering process and interfacial phase coating would be needed.