Preparation And Properties Of Ultrahigh-temperature SiC-Based Ceramic Fiber

Silicon carbide fiber has been used as the reinforcement fiber for high temperature ceramic matrix composites(CMCs)because of its high strength,high modulus,high temperature resistance and oxidation resistance.The CMCs could be used in harsh or high temperature environment.For example,SiCf/SiC has been used to construct hot-part in gas turbines.Generally,the performance of SiCf/SiC is mainly depended on is reinforced silicon carbide fiber(especially at high temperature).Therefore,to improve the property of SiC fiber is very interesting,in particular,to improve their working temperature.It could be realized by introducing hetero-atoms into their structures or their preceramic polymers.The hetero-atoms could play as sintering additives to densify the obtained silicon carbide fiber or to prevent the grain growth of ?-SiC and pertain its strength at high temperature.As one of examples to introduce hetero-atoms,Wang had prepared a new Ti-and B-containing polycarbosilane,which was used to produce a silicon carbide fiber containing titanium and boron(TB-1 SiC fiber).The oxidative behavior of TB-I SiC fiber that exposed to air flowing at 1100-1300? for 2-300 min was investigated in this work.The result showed that the strength of SiC-based fibers slightly decreased when it was exposed to air for 2 min at 1000-1300?.When the exposure time was prolonged to 300 min or the exposure temperature was increased from 1000? to 1300?,the strength of TB-1 SiC fiber did not change significantly.The morphology studies by SEM revealed that defects on the fiber surface were generated at the beginning of oxidation,while they were healed by the growing of smooth oxide layer during further oxidation.This study showed that the amorphous Ti-and B-containing Si-C-O phase displayed a better oxidation resistance property than the well-known amorphous Si-C-O phase.The decomposition of TB-? SiC fiber at 1800? promotes us to develop a new Ti-and B-containing SiC fiber(TB-? SiC fiber).To this end,the new TB-II polymer was synthesized from the reaction of o-carborane derivatives,polytitanocarbosilane,and polycarbosilane.The obtained TB-? polymer has similar structure to polytitanocarbosilane,except that it has higher molecular weight.To produce TB-? SiC fiber,the TB-? polymer was melt spinning to a polymer fiber and then it was air pre-curing(200?/2 h)follow by heating-curing(440?/5 h).Then the cured TB-? polymer fiber was pyrolyzed at 1000?1000? to give amorphous TB-? SiC fiber.The sintering of the cured TB-? polymer fiber at 1800? was also studied.The results showed that the tensile strength of TB-II SiC fibers was up to 624 MPa(Ti%=2.9%),suggesting that the incorporation of o-carborane can serve as the sintering aids to densify the SiC fiber.The expensive o-carborane may limit its industrial development in the future.To this end,we attempt to explore cheaper phenyl boric acid as boron source in this work.The results showed that the obtained boron.modified polytitanocarbosilane(TB-III polymer)was hard to melting-spin.It may due to the higher spinning temperature,which may cause the further reaction of the TB-III polymer.