Hot Pressing Sintering Of SiC Ceramic Matrix Composite Reinforced By SiC Nanofiber With High Volume Fraction

Silicon carbide（SiC）ceramic fiber reinforced SiC ceramic matrix composites have wide application prospects in the field of high temperature structural materials.Compared to micron-scale SiC ceramic fibers,an ideal reinforcement of SiC ceramic matrix composites,SiC nanofibers(SiC_nf)have higher Young’s modulus fracture strength,breaking strength and plastic elongation.At present,SiC_nf is mainly used as a secondary reinforcement by in-situ growth to strengthen and toughen the micro-area matrix and improve the matrix’s anti-crack growth ability.However,the growth of matrix is affected by the size and distribution of pores of SiC_nf.And there are also many problems such as the bridging effect,the difficult of interface preparation and the low fiber content,which makes it difficult to sufficiently show the excellent mechanical properties of SiC_nf.In this study,Al-B-C as sintering assistant system,the effects of sintering temperature,pressure and time on the density,micro-structure and mechanical properties of SiC_nf/SiC composites by hot pressing were investigated.Then the preparation of cracking carbon（PyC）coating on SiC_nf surface by precursor infiltration and pyrolysis method was studied.And then the influence of PyC interface layer on the micro-structure,mechanical properties and fracture behavior of SiC ceramic matrix composites reinforced by SiC nanofibers with high volume fraction were investigated.The main conclusions are as follows:（1）SiC_nf/SiC composites were prepared by hot pressing sintering processes in different conditions using Al-B-C as sintering assistant system.The effects of sintering temperature,the pressure and time on the hot-pressed sintering properties of the composites were studied.The experimental results shows that the fracture surface structure of the composite became denser with the increase of temperature,pressure and time,and the bonding strength between the fiber and the matrix also increased.Furthermore,high sintering temperature and long holding time would lead to grain coarsening in SiC matrix and the decrease of mechanical properties of composites.The optimal comprehensive performance of the composite were achieved when the sintering temperature was 1850℃,the sintering pressure was 30 MPa and the holding time was 60 min,the flexural strength and fracture toughness of the composite are 409.83±19.73 MPa and 13.56±0.66 MPa·m^1/2,respectively.（2）PyC coating on SiC_nf surface was prepared by precursor infiltration and pyrolysis method using phenolic resin solution,and the effect of phenolic resin solution concentration on coating effect of SiC_nf was studied.The results showed that the coating layer of the fiber surface was thinner when the concentration of the precursor solution was too low.The results also showed that the coating thickness does not increase and the fibers tends to adhere to each other when the concentration was too high.Furthermore,the coating layer of PyC was uniform and there was no obvious adhesion between the fibers when the concentration of the precursor solution was 4%.The results of XRD and FT-IR shows that the PyC coating was amorphous with low crystallinity.（3）SiC_nf/SiC CMC were prepared by hot-pressed sintering process.The effects of SiC_nfvolume fraction and PyC interface on the mechanical properties of SiC_nf/SiC composites were studied.The experimental results showed that the flexural strength of SiC_nf/SiC composites containing PyC interfacial phase was increased from 419.82 MPa to 569.72 MPa when the content of SiC_nf was 10 vol%,which was an increase of 35.7%compared with that of the unbounded interfacial phase.The micro-structure morphology of SiC_nf/SiC composites containing PyC interfacial phase was relatively porous,and the flexural strength was low while the content of SiC_nfwas 30 vol%～60 vol%.Meanwhile,the phenomenon of crack deflection,interfacial debonding and fiber pulling out increased in the fracture surface.The composite exhibited typical ductile fracture behavior in the three-point bending test.It was indicated that the introduction of PyC interfacial phase with moderate bond strength in SiC_nf/SiC was beneficial to load transfer and fiber pull-out.In summary,SiC nanofibers as reinforcement,Al-B-C as sintering aid and PyC as interface layer in this work.SiC_nf/SiC composites with high volume fraction of were synthesized by hot pressing sintering process which achieved certain reinforcing and toughening effect.Meanwhile,the typical ductile fracture was showed in the three-point bending test crack.There were mainly problems such as low sintering density and weak interfacial bonding strength while the composites achieved toughening effect.In the future,it is still necessary to systematically and theoretically study SiC_nf/SiC composites in the aspects of sintering aid,sintering process and interfacial design.