| SiBCN ceramics is a kind of thermal protection materials due to its high thermal resistance and oxidation resistance.With the brittleness as other common ceramic materials,the SiBCN ceramics need to be toughened by adding carbon fibers.Since the surface inertia of carbon fibers is large,the bonding strength of the composite is not high,the fiber's pulling out are easy to occur when the carbon fibers reinforced ceramic matrix composite is damaged.Therefore,we determined the idea of preparing Cf-SiCnws/SiBCN composite material by surface modification of carbon fibers with SiC nanowires.In this paper,SiC nanowires were grown on the surface of carbon fibers by means of chemical vapor deposition.It was noticed that the nanowires were distributed evenly on the carbon fibers and basically grew along the radial direction of the carbon fibers,with some crosses,bends and bulges.Nanowires are single crystal structure,composed of?-SiC.By changing the reaction temperature,the length of nanowires can be controlled.The length of nanowires grown at1000°C is about 24 microns,and the length of nanowires grown at 1100°C is about 20 microns.Then,the modified carbon fiber made at 1100°C was selected as the reinforcement,and the SiBCN ceramic matrix was introduced into the unmodified and modified carbon fibers by means of polymer impregnation pyrolysis at 1400°C to prepare Cf/SiBCN and Cf-SiCnws/SiBCN composites.The porosity and density of the composite are adjusted by the number of PIP times from 1 to 10.The compressive properties,tensile properties,thermal conductivity and thermal expansion coefficient of the material were measured.It was noticed that the SiBCN ceramics were amorphous.It was noticed that the composite material made by modified fiber was well combined with the fiber reinforcement,and the ceramic grains were small and evenly distributed,without obvious defects.The mechanical properties and thermophysical properties of the two composites were tested.It was found that the tensile strength and compression modulus of the composites made with modified fibers were significantly better than those made with unmodified fibers,and the mechanical properties of the composites increased with the increase of the number of PIP times.The thermal conductivity of the two materials increases with the increase of temperature,and the thermal conductivity of the composites made with modified fibers is generally higher than that made with unmodified fibers at the same temperature.With the increase of the number of PIP times,the thermal conductivity of the material also increases gradually.The thermal expansion coefficient firstly decreases and then increases.At the same temperature,the thermal expansion coefficient of the modified fibers is higher than that of the unmodified fibers.With the increase of the number of PIP times,the thermal expansion coefficient of the material increases gradually.Finally,a Ta-Mo based coating was prepared by spraying with TaSi2,MoSi2,SiC whisker,SiB6 and borosilicate glass.Ablative tests were carried out at 1500°C for 1000s and 1800°C for 500s.The measured linear ablative rate of the material at1500°C was only 1.4×10-4mm/s,the mass ablative rate was only 1.7×10-5g/s,the mass ablative rate at 1800°C was 6.2×10-5mm/s,and the mass ablative rate was1.1×10-3g/s.It was shown that during the ablative process at 1500°C,the reaction among Ta elements,Si elements and Mo elements took place,generating Ta2O5,SiO2,Mo4.8Si3C0.6.6 and other substances.In this process,the coating can still insulate oxygen.When ablative test was conducted at 1800°C,Ta2O5 is the main component remaining in the coating.At this time,the coating structure becomes loose and porous,unable to insulate oxygen. |
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