The Effects Of Fe³⁺ And H-BN Additions On The Microstructure And Properties Of C_f/BAS Composites

The hypersonic missile flying at high mach speed will cause severe friction with atmosphere,resulting in a large thermal gradient on the radome.Connecting-ring acting as a key component between radome and cabin serves under hash environment.It not only bears large load,but also has a good dimensional and high temperature stability.Due to its high density and low thermal stability,the Invar currently in use is no longer sufficient.Barium aluminosilicate?BAS?glass-ceramics have a good thermal stability and high temperature resistance.They can be combined with fibers to provide excellent mechanical properties,which are very promising alternative materials.However,studies about continuous carbon fiber reinforced BAS glass-ceramic?C_f/BAS?composites is fewer.The topic is based on this background.The preparation of BAS and C_f/BAS composites,phase composition and interface regulation were investigated in detail,and the following results were obtained:The BAS sol was prepared by mixing silica sol,boehmite sol and barium acetate aqueous solution.The BAS glass powder was obtained by calcination at 600 ? for 4 h,and then BAS glass-ceramics were obtained by heat treatment at different temperatures.The effects of Li⁺on microstructure evolution process of hexagonal-monoclinic?H-M?phase transformation in BAS glass-ceramics were studied by XRD,SEM and TEM.The results showed that Li⁺can dissolve into hexagonal phase at low temperature,resulting in the increase of system energy which can promote the precipitations of paracelsian and celsian at high temperature.The paracelsian was unstable at high temperature and will be converted into celsian.A small amount of Li⁺doping can effectively promote the H-M phase transformation to obtain low-expansion celsian phase,and also can reduce the viscosity of the BAS glass system and improve the sintering properties of BAS glassceramics.The flexural strength and fracture toughness of the BAS glass-ceramic with 2wt.% Li⁺?BAS2?sintered at 1400 ? were 82 MPa and 1.36 MPa·m1/2,respectively,which were increased by 30.2% and 27.9 % compared to that without Li⁺doping.However,the mechanical properties of BAS glass-ceramics were too low to meet the requirements of connection-ring.So it needed enhancing and toughening treatment.Carbon fiberreinforced glass-ceramic composites?C_f/BAS?were prepared by slurry impregnation and hot-press sintering.The effects of Li⁺content on the microstructure and mechanical properties of C_f/BAS glass-ceramic composites were investigated.The main phase of the undoped C_f/BAS glass-ceramic composite is hexacelsian,which has large thermal mismatching with the fiber and low mechanical properties.Adding Li⁺can effectively increase the density of composites,increase the content of celsian in matrix materials,and reduce the thermal mismatching between BAS matrix and fibers,so 2 wt.% Li⁺doped C_f/BAS glass-ceramics composites have strength and toughness up to 373.5 MPa and 6.9MPa·m1/2,which is 25.3% and 18.9% increase compared to undoped C_f/BAS composites.However,after excessively increasing the Li⁺content,the lithium feldspar content in the BAS matrix increased,which was detrimental to the high temperature performance of the C_f/BAS composite.The interface bonding of C_f/BAS glass-ceramic composites was relatively strong,which was not conducive to the improvement of the properties of ceramic matrix composites.Therefore,the influence of Fe³⁺content on the interfacial structure of C_f/BAS composites was studied.The effects of Fe³⁺on the densification,phase composition and mechanical properties of BAS glass-ceramic substrates were investigated.The results showed that the Fe³⁺content less than 3 wt.% can improve the sintering performance of BAS glass-ceramics by liquid phase sintering,but can inhibit the precipitation of celsian.While doping 4 wt.% of Fe³⁺is not conducive to the sintering of the BAS matrix,it can effectively promote the precipitation of celsian at 1100 ?.The obtained BAS sample has less residual stress,and the flexural strength and fracture toughness were 87 MPa and 1.5MPa·m1/2,respectively.The effects of Fe³⁺on the interfacial structure,mechanical properties and thermal expansion coefficient of C_f/BAS composites were investigated.The results showed that 1 wt.% Fe³⁺doping can improve the mechanical properties of C_f/BAS composites to 558 MPa and 10.8 MPa·m1/2.And it should be attributed to the formation of discontinuous pyrolytic layer in interface region,which can effectively weaken bonding strength of the composite material.So fiber pull-out was obvious,and the characteristics of ductile fracture were exhibited.However,with the increase of the content of Fe³⁺,and the strength of C_f/BAS composites decreased.The reason may be that the fibers were damaged remarkably caused by Fe³⁺etching,leading to poor ability to transfer loads.Sothe mechanical performances of composites were bad.The effect of h-BN addition on the interfacial structure and mechanical properties of C_f/BAS glass-ceramic composites was investigated.When the content of h-BN exceeded2 wt.%,the precipitation celsian was inhibited,and the densification of BAS glass-ceramic was also disadvantageous.h-BN was partially oxidized to boron oxide during sintering to promote densification of BAS and C_f/BAS,but only for addition in small amounts.When the amount of addition was increased,more h-BN remained in the matrix,which increased the viscosity of the matrix and hindered the diffusion of the elements,which was disadvantageous to the mass transfer densification process.Meanwhile,when the content of h-BN addition was less than 2 wt.%,the celsian can be obtained,which was very helpful for decreasing thermal mismatching.In addition,as received B2O3 can react with Li2 O to weaken the interface bonding strength of composites.Therefore,attributed to the lower thermal mismatching and interfacial strength,C_f/BAS composites with 2 wt.%h-BN exhibited the most flexural strength and fracture toughness,592.6 MPa and 12.7MPa·m1/2,respectively.