Research On Precursor Derived SiC Ceramics And Ceramic Matrix Composites With Different C/Si Ratio

Precursor derived ceramics is an important method for the preparation of SiC ceramic materials.SiC ceramic materials are often characterized by a high C/Si ratio,which was prepared by polycarbosilane used as the precursor.The C/Si ratio has an important effect on the microscopic composition,structure and properties of SiC ceramic materials,but few research of it was reported at present.Therefore,in this paper,SiC ceramic materials with different C/Si ratios are prepared by new precursors.The new precursor could be obtained through heating and crosslinking of the mixture,which was gained by blending different proportions of silicon-rich polymethylsilane(PMS)with PCS.Systematic study on the microscopic crystal structure,high temperature evolution,oxidation performance and sintering characteristics of ceramics was carried out.In addition,the compatibility of SiC ceramic matrix with different C/Si ratios and reinforcing fibers,as well as the mechanical properties of the prepared composite materials was discussed.The main conclusions are as follows:(1)The preparation and composition of SiC ceramics with different C/Si ratios were studied.The precursors with high ceramic yield above 80%could be obtained by heating and crosslinking the mixture of PMS and PCS.The C/Si ratio of SiC ceramics prepared at 1200? has a linear relationship to the mass ratio of PMS in raw materials.When the mass content of PMS is 70%,SiC ceramics with C/Si close to 1 can be obtained.SiC ceramics are mainly composed ofb-SiC phase and amorphous SiC_xO_yphase.With the increasing of C/Si ratio,the content of C in the SiC ceramics increased while the content of Si decreased,and PCS-SiC(C/Si>1)contains elemental C,PMS-SiC(C/Si<1)contains elemental Si,PMC-70-SiC(C/Si close to 1)does not contain elemental Si,mainly composed of SiC microcrystalline surrounded by amorphous phase.(2)The composition of SiC ceramics with different C/Si ratios after heating treatment at 1600? and 1800? was analyzed,and the high temperature evolution of SiC ceramicswas studied.With the increasing of heating treatment temperature,the amorphous SiC_xO_y phase in SiC ceramics gradually decomposes,SiC grains and the degree of ordering in the free carbon phases increased.The SiC ceramics have extremely low content of SiC_xO_y phase when treated at 1800?.The free C phase in C-rich SiC ceramics would separate out from SiC network and be enriched at the surface and grain boundaries which could inhibit the growth of SiC grains.After heating at 1800?,the SiC grain size is only 39.2 nm.PMC-70-SiC would convert to near-stoichiometric SiC.The amorphous phase disappears,and the SiC grains grow sharply to above 100 nm after heating at 1800?.At the same time,there are obvious lattice defects such as twin grain boundaries and stacking faults.The Si phase in PMS-SiC ceramics is easily lost after heating above 1600?,and SiC ceramics transform to near-stoichiometric SiC with the growth of SiC grains more obvious.(3)To study the oxidation behavior of SiC ceramics with different C/Si ratios,isothermal oxidation of SiC ceramics in air at 800-1600? was carried out.SiC ceramics undergo oxidation and weight gain due to the oxidation of SiC.The oxide film formed by oxidation has a good protective effect on SiC,but volume shrinkage and cracking of the Si O₂ film would occured duing to the phase change,which would form an oxygen diffusion channel and making the degree of oxidation of SiC ceramics is obviously increased at 1600?.After heating at 1600?,free C phase separated out from SiC ceramics would oxidize at 800?,and 5%weight loss of PCS-SiC occurred,resulting in a large number of oxygen diffusion channels formed inside the material and the oxidation degree was more significant.Owing to the improvement of the crystallinity of SiC,the oxidation degree of PMC-70-SiC reduced significantly,and the weight gain ratio after oxidized at 1600? is reduced from 19.0%to 9.8%.(4)The sintering characteristics of SiC ceramics with different C/Si ratios at low temperature and high temperaturewere studied by hot pressing sintering process.It is found that the presence of Si phase is beneficial to the sintering of SiC at low temperature,so PMS-SiC can be sintered at 1400? to some extent.Relatively dense SiC monolithic ceramics was obtained by PCS-SiC for its free C phase could be used as a sintering aid at 2100?,but its hardness and elastic modulus are relatively low.The elastic modulus of PMC-70-SiC and PMS-SiC monolithic ceramics is about 400 MPa,equal to pure SiC monolithic ceramics.Furthermore,PMS-SiC and pure SiC monolithic ceramics have similar properties such as open porosity,density,phase structure and hardness.(5)The compatibility of different precursors with C fibers was studied by the preparation of C/SiC single bundle composites.PCS-SiC has little damage to C fiber;while PMC-70-SiC and PMS-SiC have corrosion of Si-to-fiber,and when C/SiC single bundle composites were prepared at 1600?,more serious corrosion and damage happened.Py C coating prepared on fiber surface can effectively alleviate the corrosion of Si on C fibers in SiC matrix,and the maximum tensile load of C/PMC-70-SiC and C/PMS-SiC would be improved greatly,which is equivalent to C/PCS-SiC.(6)The mini and macro C/SiC composites were prepared at 1600? by PIP process,then the effects of different matrices on the properties of the materials were investigated.High modulus of SiC matrix is benefical to the the mechanical properties of composites.The tensile strength and tensile modulus of C/PMC-70-SiC mini composites is 474.71 MPa and 133.60 GPa.While the tensile strength and tensile modulus of C/PMS-SiC mini compositesis is lower than others about 391.52 MPa and88.60 GPa,for the weaker degree of corrosion of Si to C/Py C fibers with the fabrication cycles increased.In C/PMC-70-SiC and C/PMS-SiC mini composites,the SiC matrixfilled inside the composites with the form of particle packing.The porosity of C/SiC macro-composites prepared by 15 cycles is still above 15%duing to the low infiltration efficiency of the precursor.The flexural strength of C/PMC-70-SiC is202.32 MPa with ductile fracture,which is 1.65 times and 2.59 times than C/PCS-SiC and C/PMS-SiC,and the fracture toughness is 9.21 MPa·m^1/2.In C/PMS-SiC,the consumption of Py C coating and the corrosion of fiber are more serious with the increasing of preparation cycles,and the composites exhibit poor performance and low fracture toughness of 5.79 MPa·m^1/2 in the mean time.