Preparation And Properties Of SiC_nf/SiC Composites With Three Dimensional Interpenetrating Structure

SiC fiber reinforced silicon carbide ceramic matrix composites（SiC_f/SiC CMC）are widely used in aerospace,nuclear energy,automobile,bearing and other fields due to their low density,high strength,high temperature resistance,neutron irradiation resistance,good chemical stability and external impact resistance.Compared with SiC_f,SiC nanofiber(SiC_nf)is an excellent reinforcing material with high crystallinity,high strength,high elastic modulus and excellent high temperature mechanical properties.Reactive melt infiltration（RMI）has the advantages of simple process,short preparation cycle and low cost.However,the SiC CMC prepared by RMI process is based on Si as the continuous phase,and the SiC grains are dispersed in the continuous Si phase,which has the disadvantage of a sharp decrease in the performance of the composite at high temperature（more than 1410℃）.In this paper,SiC_nf/SiC composites with three-dimensional interpenetrating structure were prepared by precursor infiltration pyrolysis（PIP）and RMI.The BN interface was prepared on the surface of SiC_nf by chemical vapor deposition（CVD）process.Then,SiC_nf/SiC porous ceramics with continuous SiC phase were prepared by PIP process using polycarbosilane（PCS）as precursor,and then filled carbon was obtained by phenolic resin impregnation and pyrolysis.Finally,SiC_nf/SiC composites with three-dimensional interpenetrating structure of continuous Si phase and continuous SiC phase were prepared by RMI process.The main results are as follows:（1）Preparation of BN interface coating on SiC_nf surface.BN interface coating was prepared on the surface of SiC_nf by CVD process using BCl₃and NH₃ as reaction gases and N₂ as dilution and protective gas.The effect of temperature on the microstructure of BN interface coating on the surface of SiC_nf was studied.The results showed that when the deposition temperature was 750℃,850℃and 950℃,the BN interface coating can be prepared on the surface of SiC_nf.With the increase of temperature,the prepared BN interface coating gradually becomes smooth from rough.（2）SiC_nf/SiC porous ceramic preforms with SiC phase as continuous phase were prepared by PIP process using PCS as precursor.The effects of molding pressure,SiC_nfcontent,PCS solution concentration and PIP cycles on the bulk density,porosity and mechanical properties of porous ceramics were studied.The results showed that with the increase of molding pressure and PIP cycles,the bulk density of the sample increases and the porosity decreases.After 5 PIP cycles,the bulk density of the sample with a molding pressure of 120 MPa was 1.97 g/cm³ and the porosity was 38.45%.The flexure strength and fracture toughness of the sample with a molding pressure of 90 MPa reached the maximum values of 52.33 MPa and 1.119 MPa·m^1/2,respectively.With the increase of SiC_nf content,the bulk density of the sample decreased and the porosity increased.After5 PIP cycles,the bulk density and porosity of the sample with 10 wt%SiC_nf content were1.7364 g/cm³ and 46.05%,respectively.The mechanical properties of the samples increased with the increase of SiC_nf content.When the SiC_nf content was 40 wt%,the flexural strength and fracture toughness of the samples were 32 MPa and 0.648 MPa·m^1/2,respectively.With the increase of the times of PIP cycles,the bulk density of the sample increased and the porosity decreased.After 5 PIP cycles,the bulk density and porosity of the sample using 30 wt%PCS solution for PIP cycle were 1.8672 g/cm³ and 41.65%,respectively.With the increase of PCS concentration,the flexural strength and fracture toughness of the samples did not change obviously.When 40 wt%PCS solution was used for PIP cycle,the flexural strength and fracture toughness of the samples reached the maximum,which were 31 MPa and 0.433 MPa·m^1/2,respectively.（3）Pyrolytic carbon（Py C）was obtained by impregnation pyrolysis of SiC_nf/SiC porous ceramics using phenolic resin as carbon source.SiC_nf/SiC composites with three-dimensional interpenetrating structure of SiC phase and Si phase were prepared by RMI process.The effects of PIP cycles and carbon content of precursor solution on the composites were studied.The results showed that the density and mechanical properties of SiC_nf/SiC composites increased with the increase of PCS cycles.After four PIP cycles,the density,flexural strength and fracture toughness of the composite reached the maximum values,which were 2.921 g/cm³,187 MPa and 6.6 MPa·m^1/2,respectively.The composites contained more residual Si,which affected the improvement of the properties of the composites.The mechanical properties of the composites increased with the increase of carbon content.As the carbon content was 19.24 wt%,the flexural strength and fracture toughness of the composites reached the maximum,which were 207 MPa and 8.63 MPa·m^1/2,respectively.This was because with the increase of carbon content,the SiC content of the composites after RMI increased.In addition,crack deflection,fiber bridging and fiber pull-out were observed in the cross-section microstructure of SiC_nf/SiC composites.In conclusion,BN coated SiC_nf reinforced SiC composites were prepared by PIP-RMI process.Firstly,the BN interface was prepared on the surface of SiC_nf by CVD process.Then,the SiC_nf/SiC porous ceramics with continuous SiC phase were prepared by PIP process,and then the porous ceramics were filled with carbon.Finally,the SiC_nf/SiC composites with three-dimensional interpenetrating structure of continuous SiC phase and continuous Si phase were prepared by RMI process.To a certain extent,it avoided the disadvantage of Si as the continuous phase in the composites prepared by the single RMI process,and had certain reference value for improving the operating temperature and strength of the ceramic matrix composites prepared by RMI process.