Preparation,Oxidation Resistance And Ablation Mechanisms Of ZrB₂/SiBCN Composites

Because of their excellent stability,high temperature creep resistance and oxidation resistance performance,SiBCN ceramics gradually become a hotspot in the field of aerospace materials.However,the high temperature performance of ceramics still need to be improved.Ultra high temperature ceramic ZrB₂,with good chemical stability,thermal conductivity,resistance to erosion corrosion,make it suitable for hypersonic flight,atmospheric reentry,and rocket propulsion under extreme chemical environment and thermal environment.However,due to high covalent bond and low self diffusion coefficient,the densification of ZrB₂ ceramics are quite difficult.According to the above problems,this paper used the sol-gel method to introduce ZrB₂ to SiBCN matrix,and then through the spark plasma sintering process to prepare high performance ZrB₂/SiBCN ceramics.X-ray diffraction?XRD?,SEM,TEM,thermogravimetric analysis,mechanical property testing,oxidation performance test,thermal shock and erosion resistance test were used to study the reaction mechanism,phase composition,microstructure and properties of ZrB₂/SiBCN ceramics.The influenc of ZrB₂ on the mechanical properties,oxidation resistance,thermal shock and ablation properties were carefully studied.Results show that after high temperature sintering,the main phases were Si C,BN?C?and ZrB₂ for two different ways of sol-gel method introducing ZrB₂ to SiBCN matrix.For SZ system,after high-energy ball mill,amorphous BN?C?was more active.A carbon/boron reduction reaction happened with ZrO₂,forming a ultra high temperature phase ZrB₂.The second phase ZrB₂ evenly dispersed in the matrix with a particle size around 800-1000 nm.For SZB system,ZrB₂ was fully introduced by sol-gel precursor,with a small particle size around 300-600 nm.The introduction of ZrB₂ particles could effectively promote the densification of duplex ceramic,the density SZB15 could be up to 98 %.The mechanical properties of ceramics were increased with ZrB₂ content and this value arrived to highest when ZrB₂ content is 20 %.The fracture mode for both ceramics are mainly transcrystalline fracture for the introduction of the ZrB₂ phases which propagated the bending cracks.The pulling out of layered structure BN?C?could improve the fracture toughness to some extent for both SZ and SZB system.Oxidation tests show that the oxidation phenomenon of ZrB₂/SiBCN ceramics are more obvious with the extension of oxidation time.SZB15 ceramics presents a better oxidation resistance compared with SZ15 ceramics.The antioxidant performance decreased with the increase of ZrB₂ content and when the ZrB₂ contentwas more than 15 %,a significant oxidation phenomenon could be observed.ZrB₂ began to oxidation firstly at a relatively low temperature and at high temperature,the generated B2O3 would evaporate adding the diffusion rate of oxygen atoms.With the increase of oxidation temperature,amorphous Si O2 can fill pores and protect the ceramic particles from further oxidation.B2O3 and SiO2 would form silicon borate glass on the surface of ceramics with a relatively good protection effect.Strong covalent bond B-N-C can effectively prevent the spreading of the oxygen atoms.The dense SiO2 layer can effectively prevent further internal diffusion of oxygen atoms.The oxidation mechanisms were reaction and diffusion control.The thermal conductivity decreased and thermal expansion coefficient increased with the increasing of temperature during thermal shock test.Both SZB15 and SZ15 ceramics showed a downward trend of thermal shock strength as temperature risees.Oxidation on the surface of the ceramics and internal thermal expansion coefficient mismatch were the main cause of the fracture.The second phase ZrB₂ introducing to SiBCN matrix obviously improved the ablation resistance.Under the oxyacetylene flame ablation for 30 s,the mass and liner ablation rate of SZ15 ceramics were 4.75x10-4 g/s and 0.004 mm/s.And the mass and liner ablation rate of SZB15 ceramics were 7.04x10-6 g/s and 0.0005 mm/s.Phase changes after ablation for both SZ15 and SZB15 showed the same trend.Analysis of the material after thermal ablation testing showed that ablation products mainly consisted of the Zr Si O4,SiO2 and ZrO₂ phases.A reaction occurred between the SiO2 and ZrO₂ phases in the central region of the ceramic forming Zr Si O4 that protected the material from further thermal damage.A loose and porous oxidation layer was found from the matrix based on analysis of a cross-section image.Main ablation mechanisms of ceramics were high temperature air erosion and scour,etc.During sintering process,the matrix will react with C_f causing C_f degradation.The thermal conductivity of the C_f/SZB15 ceramic was increased and thermal expansion coefficient was decreased,that would obviously improve the thermal shock resistance.Under the oxyacetylene flame ablation for 60 s,the mass and liner ablation rate of C_f/SZB15 ceramics were 3.27x10-5 g/s and 0.0013 mm/s.No catastrophic damage was found after ablation.