Precursor-Derived SiBCN Ceramic Coatings For Oxidation Resistance Of C/C Composites

C/C composites are considered to be ideal high-temperature structural materials due to their excellent mechanical properties and thermal stability.However,C/C composites are easily oxidized above 450°C under oxidizing atmosphere,which results in mechanical strength degradation,thus limits their applications under aerobic environment.Protective coating technology is an effective way to improve oxidation resistance performance of C/C composites.Current coating methods such as pack cementation,vapor deposition and thermal spraying,either relies on expensive equipment or needs ultra-high sintering temperature.In recent years,the preparation of high-temperature anti-oxidation ceramic coatings by precursor-derived method has attracted extensive attention,which ascribes to its low pyrolysis temperature,its versatile coating process and the easiness to adjust the ceramic composition.However,the pyrolysis process of precursor is accompanied with volume shrinkage and gas release,which easily leads to cracks and pores in the obtained coatings.Therefore,multiple impregnation-pyrolysis(coating-sintering)cycles have to be employed to increase thickness and minify defects of the coatings,which makes the process tedious and low efficiency,and thus hinders the real application of this promising method.Therefore,under the premise of taking advantages of precursor-derived method,further improving the coating’s preparation efficiency becomes the focus of researchers.During the transition process from polymer to ceramic,chemical composition of polymer precursor is the key to influence its ceramization behavior,as well as the ceramic’s structure and basic properties.To promote the preparation efficiency of ceramic coating by choosing a proper polymer precursor is feasible.Multicomponent SiBCN ceramic derived from polyborosilazane(PSNB)precursor has been demonstrated to possess superior thermal stability and high-temperature anti-oxidation performance compared with binary ceramics such as SiC and Si3N4,as well as ternary ceramics such as SiCN and SiOC.At present,a great deal of research work has been done on the pyrolysis mechanism of PSNB precursors and oxidation resistance mechanism of SiBCN ceramic at home and abroad,but the preparation of high-temperature anti-oxidation ceramic coatings using PSNB precursor received little attention.In this work,the basic properties of SiBCN ceramic coating derived from PSNB precursor were firstly investigated.And then,the SiBCN ceramic coatings without cracks and pores were prepared on the surface of C/C composites by one impregnation-pyrolysis(coating-sintering)cycle.Finally,the high-temperature anti-oxidation performance of SiBCN ceramic coatings was investigated.The main works and results are summarized as follows:(1)The basic properties of SiBCN ceramic coating derived from PSNB precursor.A crack-free SiBCN ceramic coating with thickness of 480 nm was achieved using PSNB precursor.The gas generated by PSNB precursor during heat-treatment process can escape in time to reduce the stress concentration,and the as-formed pores owing to gas release disappear after heat-treatment process.The hardness and Young’s modulus of SiBCN ceramic coating are approximately 10.16 GPa and 63.70 GPa,respectively.Therefore,the SiBCN ceramic coating possesses high hardness and good toughness,which will enhance resistance to cracking.The SiBCN ceramic shows excellent oxidation resistance performance with 3.11%weight loss after oxidation at 1500oC for 20 min.The BN(C)and SiC(N)phases in SiBCN ceramic can react with O2 to form B2O3 and SiO2 molten oxide layers at high temperature,which effectively prevents penetration of oxygen and further oxidation of internal SiBCN ceramic.(2)The preparation and characterization of low-density carbon bonded carbon fiber(CBCF)-SiBCN(CS)composites.The SiBCN ceramic coating was successfully prepared on the surface of the chopped carbon fibers in CBCF composites by one impregnation-pyrolysis cycle using PSNB precursor.The results show that the SiBCN ceramic can significantly enhance the mechanical properties and oxidation resistance performance of CBCF composites.When the concentration of PSNB precursor reaches 30%,the compressive strength of composites increases42%in xy plane,and 50%in z direction.The initial oxidation temperature of CS composites increases from 500°C to 750°C.In addition,the CS composites still have lower density(0.47g/cm~3),higher porosity(56.09%)and lower thermal conductivity(0.172 W/Mk),which exhibit excellent comprehensive performance.(3)The preparation and characterization of SiBCN-based gradient ceramic coatings for C/C composites.A gradient anti-oxidation ceramic coating for C/C composites was prepared via dip coating of the mixed slurry of PSNB precursor,ZrB2 and SiC powders,followed by pyrolysis and low-temperature densification process.The coated C/C composites show excellent oxidation resistance performance with 0.86%weight loss after oxidation at 800oC for 120 min and only0.06%weight loss after oxidation at 1500oC for 30 min.The formed dense ZrO2-B2O3 layer affords the low-temperature oxidation protection,while the ZrO2-SiO2 layer formed above 930oC endows the C/C composites with high-temperature anti-oxidation property.Besides,the gradient ceramic coating possesses superior thermal-shocking resistance property with no peeling off or detachment of coating from C/C composites after 12 cycles-air quenching from 1500oC to room temperature.