Effect Of Pyrolysis Temperatures On Microstructure And Properties Of SiC_f/SiC Composites

Continuous silicon carbide fiber-reinforced silicon matrix?SiC_f/SiC?composite is a good candidate for aero-engine components and nuclear fusion reactor.However,the pyrolysis temperature of precursor infiltration and pyrolysis?PIP?method is limited by the high oxygen content and poor temperature resistance of first-generation SiC_fibers.The traditional precursor of SiC matrix has low infiltration efficiency and low pyrolysis yield,resulting in a longer PIP preparation time.Therefore,PIP-SiC_f/SiC composite has many defects,such as high porosit y and low matrix crystallinit y.To improve the crystallizat ion of SiC matrix and reduce the preparation time of PIP method,3D-four-direct ional prefabricated piece of KD-II SiC_fibers was used as the reinforcement,and the pyrolysis products of a new type of liquid precursor,liquid vinly-containing polycarbosilane with low mo lecular weight?LPVCS?,was used as the matrix.By increasing the pyrolysis temperature of PIP method,SiC_f/SiC composites that appropriate for aerospace and fusion reactor applicat ions were obtained.To increase the pyrolysis temperature of PIP method,KD-II SiC_fibers with good temperature resistance and low oxygen content were used to prepare SiC_f/SiC composite.The effect of high-temperature treatment and oxidation treatment on the microstructure and mechanical propert y of SiC_fiber were investigated.The results show that the single filament tensile strength of SiC_fiber was decreased with an increase in treatment temperature,due to the grain size of the ?-SiC grains in the SiC_fiber increased with an increase in treat ment temperature,which indicates an increased degree of crystallization.Beacuse the preparation temperature of the SiC_fiber was 1350 °C,the SiC_fiber showed good anti-oxidant abilit y before 1400 °C,and the Si O2 phase was scarcely found,while the mechanical propert y of the SiC_fiber decreased greatly after the oxidat ion treatment increased above 1400 °C.The composit ion,crystallization and mechanical properties of the SiC_fiber barely changed by prolonging the oxidat ion preservat ion time.To reduce the preparation time and increase the pyrolysis temperature of PIP method,the pyrolysis products of new LPVCS liquid precursor are selected as the matrix.The crosslinking-pyrolysis behaviors of LPVCS liquid precursor were studied.The effects of pyrolysis temperature on the microstructure and properties of SiC matrix were invest igated.The results show that the crosslinking yield of the LPVCS liquid precursor was the highest when the mass rat io of the tetramethyl cyclosiloxane?V4?to the low mo lecular weight polycarbosilane?LPCS?was 1.0 at 300 °C,overall considering the viscosity of the LPVCS liquid precursor,the degree of decarburization of oxygen element in the V4 and the crystallizat ion of ?-SiC.The oxidation resistance ability of the SiC matrix was inferior to that of SiC_fiber,because the crystallizat ion of the SiC matrix was lower than the SiC_fiber,and the larger content of Si-C-O phase in the SiC matrix.With an increase in pyrolysis temperature,the crystallization of SiC increased,the grain size grew,and the densit y and open porosit y increased,causing the improvement of the heatconducting propert ies of the SiC matrix.To obtain SiC_f/SiC composite meeting the application requirements,the effects of pyrolysis temperature on the mechanical,heat-conducting,and ant iirradiation propert ies of the PIP-SiC_f/SiC composites were studied.The results show that the lower pyrolysis temperatures led to the lower crystallizat ion of the SiC_fiber and the SiC matrix,and the higher temperature led to the damage to the SiC_fiber,which had adverse effects on the mechanical propert ies of the SiC_f/SiC composites.The SiC_f/SiC composites without interface had a britt le fracture mode,whereas those with pyrolyt ic carbon?Py C?interface had a pseudo ductile fracture mode.The SiC_f/SiC composites with Py C interface pyrolysised at 1300 °C showed the moderate interfacial shear strength.So the SiC_f/SiC composites with Py C interface prepared at 1300 °C showed the best mechanical propert ies with a flexural strength of 535.6 MPa and a fracture toughness of 17.3 MPa·m^1/2.Affected by the crystallization degree of the SiC_fiber and the SiC matrix,the thermal conductivit y of the SiC_f/SiC composites increased with an increase in pyrolysis temperature,and the thermal diffusivit y decreased with an increase in the test temperature.The SiC_f/SiC composites with Py C interface pyrolysised at 1600 °C had the highest thermal conductivity of 6.17 W·m^-1·K^-1.Si ion irradiation caused certain off normal damage at the fiber/matrix interface of the SiC_f/SiC composites,destroyed the continuity of the SiC_f/SiC composites.After Si ion irradiation,microcracks were generated in the Py C interface at the juncture of the fiber and matrix,and the crystallinit y of SiC was degraded,the grain size decreased and finally resulted in a phase transformation from an ordered C phase to a noncrystalline phase with an amor phous structure,which was the main factor result ing in the decrease of the interfacial shear stress of the SiC_f/SiC composites.