Basic Research On The Fabrication Of Polymer-derived Continuous SiBN Ceramic Fibers

Silicon boronitride (SiBN) ceramic fibers, with high mechanical strength, excellenthigh-temperature resistance, good stability against oxidation, and good dielectricproperties, are promising candidates as reinforcements in ceramic matrix composites forhigh temperature microwave-transparent applications. Hence, present work focuses onthe basic research of continuous SiBN ceramic fibers via the polymer-derived ceramics(PDCs) method.The synthesis of polyborosilazane (PBSZ) usually gives preceramic polymers withlow softening point and single composition, which are not suitable to be used asprecursors for continuous SiBN ceramic fibers as they can readily get crosslinked.Therefore, the mechanism of the synthesis of PBSZ using BCl3, HMeSiCl2and HMDZwas investigated, and polyborosilazanes of high softening point and variouscompositions were prepared. The properties of PBSZ with various compositions werestudied so as to synthesize the most suitable precursor for continuous SiBN ceramicfibers. The preparation process of continuous SiBN ceramic fibers was also studied.The reaction of BCl3, HMeSiCl2and HMDZ was investigated by GC-MS based onthe analysis of the reactions between BCl3and HMDZ, HMeSiCl2and HMDZ,respectively. The results suggested that the process of the condensation polymerizationof BCl3, HMeSiCl2and HMDZ could be divided into two stages. At the first stage(RT-150℃), the main reaction was the condensation reaction of Si-Cl and B-Cl withHMDZ, and the products were composed of the products of reaction between BCl3andHMDZ at150℃, the products of reaction of HMeSiCl2and HMDZ at150℃,and someBNSi structures. At the second stage (150-240℃), the mixture formed at the first stagewas condensed to form ring structures with loss of HMDZ or SiMe3Cl.Based on the understanding of the reaction of BCl3, HMeSiCl2and HMDZ, therelationship between the molar ratio of BCl3, HMeSiCl2and HMDZ and the gelationpoint was conformed. The reason why the crosslinking of PBSZ happened was studied.The softening point of PBSZ could be enhanced by increasing the reaction temperatureon the condition that the suitable molar ratio of BCl3, HMeSiCl2and HMDZ was used tomake the average function number of the reactants less than two. The results showedthat the softening point of PBSZ was enhanced to above100℃by increasing thereaction temperature to300℃with the molar ratio of BCl3, HMeSiCl2and HMDZ as1:0:6,1:0.5:6,1:1:6and0.5:2:6, respectively.The composites and structures of preceramic polymer PBSZ-1:0:6, PBSZ-1:0.5:6,PBSZ-1:1:6and PBSZ-0.5:2:6were studied by EA, FT-IR,1H-NMR,11B-NMR and29Si-NMR. The results showed that with the increase of the HMeSiCl2molar ratio, thecontent of B decreased and the contents of Si and N increased. When the HMeSiCl2 molar ratio was less than1, with the decrease of the HMeSiCl2molar ratio, the BN ringswere the main structure in PBSZ and the content of SiN and BNSi structures decreased.While the HMeSiCl2molar ratio was more than1, the SiN rings were the main structureand the content of BN and BNSi structures decreased.The properties of PBSZ precursors with various compositions were studied.(1)The melt-spinning ability of PBSZ-1:0.5:6and PBSZ-1:1:6was better than PBSZ-1:0:6.The melt-spinning ability of PBSZ-0.5:2:6was the worst among the four kinds of PBSZprecursors. Four kinds of PBSZ fibers were fabricated by melt-spinning of PBSZ-1:0:6,PBSZ-1:0.5:6, PBSZ-1:1:6and PBSZ-0.5:2:6.(2) Precursor fibers of PBSZ-1:0:6,PBSZ-1:0.5:6, PBSZ-1:1:6and PBSZ-0.5:2:6hydrolyzed0.6%,1.9%,3.0%and5.2%respectively after being exposed for18h under ambient conditions with the relativehumidity of75%. The hydrolytic experiments of PBSZ-1:0:6, PBSZ-1:0.5:6,PBSZ-1:1:6and PBSZ-0.5:2:6fibers indicated that the PBSZ polymers only with BNrings had better hydrolytic stability than that of preceramic polymers with the mixtureof BN and Si-N-Si groups as the Si-N-Si groups hydrolyzed faster than the BN groupsin polyborosilazane.(3) The ceramic yield of PBSZ-1:0:6, PBSZ-1:0.5:6andPBSZ-1:1:6in N2atmospheres under1000℃is35wt%,47wt%and60wt%,respectively. The ceramic yield of PBSZ was enhanced by increasing Si-N-Si structuresin the PBSZ polymers.(4) The pyrolysis of PBSZ-1:0:6started to form BN crystals at1400℃and that of PBSZ-0.5:2:6started to produce SiC/Si3N4crystals at1600℃,while there had no crystals appeared during the pyrolysis of PBSZ-1:0.5:6andPBSZ-1:1:6at1600℃because of the BNSi structures in the PBSZ.The tensile strength and Young’s modulus of Si0.3BN1.4hollow fiber derived fromPBSZ-1:0:6is1.0GPa and103GPa, respectively. The tensile strength and Young’smodulus of Si0.55BN1.8dense fiber derived from PBSZ-1:0.5:6is1.2GPa and125GPa,respectively. The tensile strength and Young’s modulus of Si0.95BN2.2dense fiberderived from PBSZ-1:1:6is1.5GPa and150GPa, respectively. The results indicated thatthe increase of Si-N-Si structures in the PBSZ was beneficial to enhance the mechanicalstrength of the obtained SiBN ceramic fibers.The Hildebrand solubility parameter of PBSZ-1:1:6was9.0-9.8cal0.5/cm1.5, whichwas calculated by Materials Studio. Based on the Hildebrand solubility parameter, theselected bad solvents for PBSZ-1:1:6were polar solvents such as water, alcohols,imides and cyanides. The Hansen solubility parameter (HSP) of PBSZ-1:1:6wasδD=19.09MPa1/2, δP=19.47MPa1/2, δH=0.69MPa1/2and the radius of HSP sphere is19.9MPa1/2. Based on the Hansen solubility parameter, the selected bad solvents with boilingpoint between25℃and100℃for PBSZ-1:1:6were water, alcohols, alkyl fluoridesand cyanides. Especially, the acetonitrile didn’t dissolve the PBSZ-1:1:6fibers, whichcould be used as sizing finishes for PBSZ-1:1:6fibers.The multifilamental continuous melt-spinning of PBSZ-1:1:6was realized at210 ℃with pressure of0.4MPa, and the continuous PBSZ-1:1:6fibers were sized byacetonitrile and stretched by the spool at a rotation rate of ca.400m min-1to produceflexible endless fibres with lengh of>1000m. The continuous PBSZ-1:1:6fibers werecured at95℃for4h under HMeSiCl2. The continuous cured PBSZ-1:1:6fibers werepyrolyzed to1000℃(10℃min-1, at600℃holding for2h) under NH3atmospheres.Continuous SiBN ceramic fibers with a near-stoichiometric composition of Si0.91BN2.1were obtained with ca.0.6GPa in tensile strength and ca.10μm in diameter.