Reasearch On Sintering Densification Mechanism And High Temperature Properties Of SA Type SiC Fiber

Continuous silicon carbide(SiC)fiber and its ceramic matrix composite materials have important strategic significance because of their broad application prospects in aviation,aerospace,hypersonic vehicles,nuclear industry and other fields.But at present,the sintering process reaction mechanism of domestic third generation polycrystalline nearly-stoichiometric SiC fiber are not clear enough,and the effects of fiber structure and composition to its properties of high temperature resistance and oxidation resistance remain inadequate.In this paper,polycrystalline nearly-stoichiometric SiC fiber was prepared using polyaluminosilane(PACS)synthesized under atmospheric pressure as a precursor for the first time.The decomposition mechanism of SiC_xO_y phase in SiC fiber was further studied by adjusting oxygen content during curing process and the fiber structure model during the sintering process was built.Also,the influences of sintering temperature and time on grains growth and densification of fiber were explored.Finally,Interrelationships of the fiber composition,structure,mechanical properties,high temperature resistance and oxidation resistance were established.Firstly,the precursors synthesized by the atmospheric pressure method were converted into starting inorganic fibers which were rich in oxygen and carbon.Using liquid polysilane(LPS)and aluminum acetylacetonate(Al(Ac Ac)₃)as raw materials,the precursor PACS was synthesized by atmospheric pressure method.After melt spinning,the fibers were cured in air at 150-250°C.Then three different oxygen contents starting fibers S-10%,S-15%and S-25%were prepared at 1300°C.The compositions of starting fibers were rich in oxygen and carbon,and a small amount of?-SiC crystallites were dispersed in the free carbon and SiC_xO_y phase,which were basically in an amorphous state.As the oxygen content increased,the amount of SiC_xO_yphase in the fibers increased while the crystallinity decreased.Overall,starting fibers were dense,without obvious defects,and showed the characteristic of brittle fracture.Secondly,the decomposition reaction mechanism of starting inorganic fibers sintering process was studied by adjusting the oxygen content.Starting fibers were heated at 1500°C to obtain intermediate fibers I-S-10%,I-S-15%and I-S-25%,and sintered at 1800°C to obtain product fibers P-S-10%,P-S-15%and P-S-25%.Affected by the decomposition of SiC_xO_y phase,skin-core structures appeared in intermediate fibers and product fibers when oxygen content of starting fibers were over 15wt%.The skin layer was high-crystalline,relatively dense and mainly composed of nearly-stoichiometric SiC,and the core was rich in carbon and porous.As oxygen content of starting fibers increased,the skin layer became thicker and the core was more carbon-rich and porous.This is because SiC_xO_y phase decomposed more fastly at high temperature with higher oxygen content.While Si O generated inside the fiber diffused to the surface layer,a large number of hole defects and unreacted C were left in the core.The Si O was fully reacted in the surface carbon-rich layer,forming a relatively dense skin layer with high crystallinity.Only when the initial oxygen content is moderate(?10wt%),the Si O and C generated by the decomposition of SiC_xO_y phase can further fully react to form SiC,and the excess O and C are removed in the form of CO.Then,the high-temperature densification process of the fiber was studied by adjusting sintering temperature and time,finally,third generation polycrystalline nearly-stoichiometric SiC fiber was successfully prepared.The decomposition of the residual SiC_xO_y phase and the growth of grains occurred during the stage from intermediate fiber to product fiber.The decomposition of SiC_xO_y phase produced hole defects in the fiber,and the growth of grains were beneficial to eliminate the defects The generation of hole defects and the realization of densification were carried out gradually from the surface layer of the fiber to the core.During the sintering process,it was also found that SiC would decompose at high temperature to generate Si and C.Si evaporated and C remained inside the fiber,making the fiber rich in carbon.This situation was aggravated when sintering at 2000°C.So the sintering temperature should not be too high.When sintering at 1800°C,SiC decomposed slightly,so the fiber composition did not change much during 10min to 2h.Its C/Si atomic ratio was close to1,but the longer time was beneficial to the elimination of residual oxygen.In addition,a small amount of graphite phase in the fiber was distributed at the grain boundary,and it was more enriched as the holding time extended.Meanwhile,the length of the graphite sheet layer increased,which suppressed the growth of grains.Because the thermal expansion coefficients of graphite and SiC grains were inconsistent,defects often occured at the grain boundaries,resulting in decrease of fiber strength.On basis of the above researches,the residual SiC_xO_y phase in intermediate fiber was slowly decomposed by adjusting the sintering temperature and holding time,and the defects were eliminated by grain growth.Finally,polycrystalline nearly-stoichiometric SiC fiber was successfully prepared with tensile strength of 1.88GPa,Young's modulus of 373GPa and fiber composition was SiC_1.04O_0.02Al_0.004.The grain size was uniform along the fiber radial direction with size of 100-200nm.The grain boundary was clear and clean without impurities,but there was a turbulent graphite phase at the boundary of a few crystal grains,which caused the density of SiC(SF)fibers to be lower than the theoretical density of SiC.There was a carbon-rich layer with the thickness of more than 10nm on the surface of fiber.The internal elements were evenly distributed,with only a few nanometer-sized hole defects.Finally,the high temperature resistance and oxidation resistance of the fiber were systematically studied.The sintered third generation fiber SiC(SF)and the second generation fiber SiC(SG)were heated in Ar at 1400-2200°C for 1-10h,and it was found that?-SiC grain growth and decomposition obviously occurred above 1900°C,resulting in defects on the surface and internal of the fiber to reduce its strength.The?-SiC grain size of SiC(SF)fiber did not increase significantly at high temperature,and the bonding force between the grain boundaries was strong.The strength retention rate of SiC(SF)fiber was still about 60%after holding for 5h at 1900°C under Ar.The?-SiC grains of SiC(SG)fiber grew significantly at high temperature,and the bonding strength between the grain boundaries was low.Its strength retention rate was only about 30%after holding for 5h at 1900°C under Ar.Grain decomposition occured gradually from the surface to the core.When temperature reached 2000°C,the decomposition of SiC intensified and both fibers were severely carbonized and almost completely lost strength.To study the property of oxidation resistance,SiC(SF)fiber and SiC(SG)fiber were kept in dry air at 1000-1500°C for 1-10h.It was found that passive oxidation occured on fibers surface,accompanied by oxidation reaction and gas diffusion.In the initial stage of the reaction,oxidation reaction was dominant.As the oxide layer became thicker,gas diffusion became the dominant factor affecting the oxidation rate.Compared to SiC(SF)fiber,the oxide layer of SiC(SG)fiber was thicker and tend to crystallize The growing stress,thermal expansion coefficient mismatch and phase transition produced large-size cracks.On the one hand,it promoted oxygen to penetrate through the crack into the fiber to accelerate oxidation.On the other hand,the crack also sharply reduced the fiber strength.After holding in air at 1300°C for 10h,the strength retention rate of SiC(SF)fiber was about 50%,while that of SiC(SG)fiber was only about 15%.Overall,the oxidation resistance of SiC(SF)fiber was better than SiC(SG)fiber.