Basic Research On The Fabrication Of The Polyborosilazane-derived SiBNC Fibers

SiBNC ceramic fibers combine several desired properties relevant for an application in fiber-reinforced ceramic composites:mechanical strength,low density,high-temperature creep resistivity,thermal stability,and stability against oxidation or molten silicon,which could meet the demand of high-temperature engine applications at elevated temperatures.The high-temperature durability of SiBNC system is significantly influenced by Si/B ratios and the synthetic procedures.In this paper,a simple and efficient method for the synthesis of SiBNC precursor polyborosilazanes?PBSZs?with different Si/B ratios has been established via a one-pot reaction involving boron trichloride,dichloromethylsilane and hexamethyldisilazane in different molar ratios.The Si/B ratios of the derived SiBNC ceramics were consistent with that of the precursor PBSZs.When pyrolysed at 1000?,PBSZs with 0.52,0.94 and 2.12 Si/B ratios transformed into SiB_2.6N_5.0C_2.2,SiB_0.9N_2.7C_1.3 and Si_2.0BN_3.0C_1.4 ceramics respectively.The derived ceramic SiB_0.9N_2.7C_1.3.3 performed the best thermal stability and remained amorphous up to 1700?,whereas the SiB_2.6N_5.0C_2.2 and Si_2.0BN_3.0C_1.4 could resist crystallization below 1600? only.Using the precursor with 0.94 Si/B ratio for melt-spinning,green fibers were obtained.In this paper,we attempted to use the method of electron beam irradiation to crosslink green fibers.The variation of molecular structures from green fibers to cured fibers and the properties of sintered SiBNC fibers were investigated.Via electron beam irradiation,the free radicals are formed at the C atoms and Si atoms on the-N-SiH?CH₃?-main chain units and terminal-Si?CH₃? groups.The radicals react with each other to produce cross-linking,coupling and grafting among PBSZ chains,which all contribute to improvement of the cross-linking density of green fibers.Irradiation dose played an important role on the cross-linking density.Via 34 MGy irradiation dose,the cured fibers performed a gel content of 84.8% and ceramic yield of 80.4wt%.After pyrolysis at 1300?,SiBNC ceramic fibers were acquired,which exhibited a good flexibility with 12?m in diameter and 1.22GPa in tensile strength.The composition,microstructure and high-termperture behaviour of the as-received fibres were studied.Si,B,N and C atoms mainly exist in the form of SiN₃C site,SiN₄site,B-N hexatomic rings and free carbon cluster with small amount of C-Si and B-C bonds.There were ?4 at% O content,existing in the form of Si-O-Si.In N₂ atmosphere,the fibers could remain amorphous at 1600? for 2h and the tensile strength remained1.1GPa,whereas phase separation occurred from 1700 to 1900? and the tensile strength decreased dramatically.After oxidation from 1300 to 1500?,the fibers showed two-layer scale with different thickness where the outer layer was amorphous SiO₂ and the inner layer was h-BN crystallization in the amorphous oxidized SiBCNO matrix.Beneath the two-layer scale was unoxidized amorphous SiBCN fiber core.