Preparation,improving And Properties Of C/C-ZrC Composites

The development of hypersonic vehicles proposed higher requirement for the thermal structure materials.It is urgent to develop ultra-high temperature materials with high-temperature durability,anti-ablative and good mechanical properties.Based on shortcomings of conventional high temperature materials,the preparation and ceramic transformation of a meltable ZrC-C precursor were studied.Moreover,C/C-ZrC composites were prepared by melt infiltration with the precursor.The matrix was modified by slurry infiltration to improve the mechanical properties and ablation resistance of the composites.The ZrC-C precursor was prepared by condensation reaction of zirconium acetylacetonate at 190?C.The mechanism of polycondensation reaction,variation of high temperature viscosity and ceramic transformation of the precursor were studied.Zirconium acetylacetonate melted at 190?C for 35min,and then reacted to polyzirconoxane?ZrC-C precursor?with the number of molecular weight of 829¹212by losing some acetyl acetone ligands,which volatilized in the form of acetylacetone.The viscosity of ZrC-C precursor rose with increasing holding time.During 40¹00min,the viscosity kept in 12¹48mPa?s,which was fit for infiltration in PIP process.The precursor was transformed into?ZrC?1.0?HfC?0.0074?ZrO2?0.131C0.553 ceramics at 1650?C with a yield of²9.6%.C/C-ZrC composites were prepared by melt infiltration with zirconium acetylacetonate as the raw material and¹.0g/cm³ as the reinforcement.In order to improve efficiency of densification and performances of the composites,the optimization of process parameters were as followed:self-pressurization infiltration at190?C,inorganic treatment at 1200?C,carbo-thermal treatment at 1650?C and 12 PIP cycles.The C/C-ZrC composites had a flexural strength of 146 MPa,flexural modulus of 22.0 GPa,and fracture toughness of 8.68 MPa?m^1/2.The linear ablation rate and mass ablation rate were 15?m/s and 11.6 mg/s after oxyacetylene ablation for 60s.The effect of graphitization treatment of C/C reinforcement in the temperature range from 1650?C to 2700?C on the properties of C/C-ZrC composites was studied.With temperature increasing,graphitization degree of C/C reinforcement rose and the thermal conductivity became bigger,which could reduce the surface temperature of the composites during the ablation process and was conducive to the ablation resistance.The linear ablation rate of C/C-ZrC composites with treated C/C at 2700?C as the reinforcement reduced by 35%.To improve high porosity,low strength and oxidation resistance of C/C-ZrC,SiC and ZrB2 were introduced to the composites by slurry infiltration.In the matrix,ZrC particles were connected by SiC and the pores were filled.The density of the composites increased and the porosity decreased.The flexural strength and modulus increased by 1.58².02 times and 2.28³.24 times respectively.A ZrB2 particle layer was formed on the surface of the composites by impregnating ZrB2 slurry.During the ablation,volatilization of SiO2 and B2O3 could reduce the surface temperature of the composites and ZrO2 increased,which protected the composites.The linear ablation rate reduced from 46?m/s to 24.7?m/s.In the preparation process of the composite,matrix components could be controlled by introducing ZrC and SiC alternately.A strong interface bonding between the matrix and the fiber bundle was formed due to SiC.The composites showed a brittleness damage mode and the flexural strength decreased to 252²71 MPa.Meanwhile,the volume fraction of ZrC was reduced in the matrix and the linear ablation rate increased to 26.8⁴0.0?m/s.The tensile and flexural properties of C/C-ZrC matrix-modified composites?C/C-ZrC-SiC-ZrB2 composites?at 1500?C,1650?C and 1700?C/1800?C were studied.The interface of fiber-PyC coating was improved at high temperature,and the tensile strengths of the composite were bigger than that at room temperature,which was the highest?145MPa?at 1500?C.At 1700?C,the tensile strength was as high as 136MPa.The flexural strength at high temperature decreased slightly,which was 223MPa at1800?C.The oxidation resistance of C/C-ZrC composites before and after matrix modification at 1500?C and 1700?C was studied.C/C-ZrC composites were destroyed at 1500?C.ZrB2 and SiC were good for the oxidation resistance.Their oxidation products formed a SiO2-B2O3 film on the surface of the composites at 1500?C,which could inhibit oxygen exposure.However,the composites were seriously oxidized from the sideface and the flexural strength decreased to 117MPa.The composites were destroyed at 1700?C.The ablation resistance properties of C/C-ZrC-SiC-ZrB2 composites were investigated in flowing oxyacetylene torch and plasma environment.A ZrO2 protective layer was formed on the surface of the composites in the oxyacetylene torch environment.The linear ablation rate decreased from 14.5?m/s to 5.8?m/s and the mass ablation rate decreased from 10.0 mg/s to 2.4 mg/s with ablation time increasing from 60s to 600s.The temperature of the plasma jet is 2500?C,which was higher than the boiling point of SiO2 and lower than the melting point of ZrO2.The ZrO2 and SiO2could not protect the composites and the linear ablation rate increased from 5.5?m/s to8.8?m/s with the ablation time increasing from 60s to 120s.