Preparation And Properties Of C/ZrC Composites Via Reactive Melt Infiltration

The increasing server performence enviroment of the hypersonic aircraft vehicles proposed higher requirment for the thermal structure components, but conventional ultrahigh temperature structure materials cannot withstand the ruthless application condition. Thus it is pressing to research and prepare continuous fiber reinforced unltrahigh temperature ceramics(UHTCs) matrix composites with execellent properties, such as mechanical properties, toughness, thermal shock resistance and ablation resistance. Continuous carbon fiber reinforced zirconium carbide(C/Zr C) composites have been spotlighted and widely researched for its high melting point, excellent thermal shock resistance and ablation resistance. Reactive melt infiltration(RMI) was highly focused among the various fabrication process of C/Zr C composites for the low cost, effective time-consuming and near net shaping. Up to the present, more work were needed to applied on the systemic research of preparation process and mechanism research of RMI process. In this study, effects of processing parameters, properties of C/C preform and post-treatment on the properties of C/Zr C composites were reasearched systematicly. The wetting, infiltration and reaction process in the RMI process were analysised and discussed.The process parameters of the preparation of C/Zr C composites by RMI process were studied and optimized, and the optimal parameter were 2000°C,30 min and vacumn. Firstly, effects of preparation temperature in the range from 1900°C to 2100°C on the microstructure and preperties of C/Zr C composites fabricated by RMI process were investigated. Thus, 2000°C was selected as the optimal preparation temperature taking account of mechanical and oblation properties. The composites were considered to obtain the optimal properties with flexural strength and modulus of 148 MPa and 15.8 GPa, fracture toughness of 6.8 MPa?m1/2, mass loss and linear recession rate of 0.0037 g/s and 0.0033 mm/s under an oxyacetylene torch. Secondly, effects of holding time in the range from 5min to 120 min on the microstructure and preperties of C/Zr C composites fabricated by RMI process were investigated. 30 min was selected as the optimal holding time taking account of mechanical and oblation properties. Once the holding time excedeed 30 min, fiber degradation and Zr element diffusion induced fiber erosion would lead to decrease of mechanical properties. The composites prepared by 30 min holding time obtained flexural strength and modulus of 175 MPa and 16.7 GPa, fracture toughness of 8.2 MPa?m1/2, mass loss and linear recession rate of 0.0039 g/s and 0.0027 mm/s. Thirdly, effects of furnance pressure in the range from 0.01 k Pa to 0.15 MPa on the microstructure and preperties of C/Zr C composites were investigated and the results indicated that the effects were very weak. But vacumn condition was selected for its process feasibility and the higher density of the corresponding composites.The effects of C/C preform properties, such as carbon matrix type, matrix amount(or density) and graphitization treatment on the microstructure and preperties of C/Zr C composites were studied, and the C/C preform with 1.40 g/cm3 density prepared by CVI process was selected as the optimal C/C preform. Firstly, effects of carbon matrix type on the microstructure and preperties of C/Zr C composites were investigated. Three types of C/C preforms prepared from phenolic resin pyrolysis, pitch pyrolysis and chemical vapor infiltration were utilized to progress the research. The C/Zr C composites prepared from CVI C/C preform obatianed the optimal mechanical and ablation properties. Secondly, the effects of density(or carbon matrix amount) of C/C preform in the range from 1.12 to 1.60g/cm3 on the microstructure and preperties of C/Zr C composites were investigated. The mechanical properties of C/Zr C composites increased and ablation resistance decreased with density increase of C/C preform. The composites prepared from C/C preform with 1.40g/cm3 density obtained flexural strength and modulus of 203 MPa and 15.5 GPa, fracture toughness of 8.5 MPa?m1/2, mass loss and linear recession rate of 0.0047 g/s and 0.0052 mm/s. Thirdly, graphitization treatment was employed for the C/C preform, but no distinct effects of graphitization on the microstructure and preperties of C/Zr C composites was found.The effects of introducing Si C matrix on the microstructure and preperties of C/Zr C composites were investigated. Si C matrix was introduced by PIP or VSI process to densefy the residual open pores in C/Zr C composites. Then the dendity of C/Zr C composites both increased. The flexural strength and modulus, fracture toughness of C/Zr C composites(CZ) increased from 155 MPa, 12.8GPa and 5.0 MPa?m1/2 to 235 MPa, 17.3GPa and 7.0 MPa?m1/2 of CZS-VSI after VSI process, and increased to 263 MPa, 33.1GPa and 11.0 MPa?m1/2 of CZS-PIP after PIP process. Si C improved the oxidation resistance of C/Zr C at 1600°C. Furthermore the retention ratio of flexural strength for CZS-VSI was over 95%, revealing the execellent oxidation resistance. The linear recession rate of C/Zr C composites was almost constant after introducing VSI-Si C, and the mass loss rate increased from 0.0031g/s to 0.0071g/s, but the ablatin morphology was improved.The effects of heat-treatment in the temperature range from 1200°C to 2000°C on the microstructure and preperties of C/Zr C composites were studied. After heat-treatmen, the residual zirconium amount decreased and the mechanical and ablation properties of C/Zr C composites were both enhanced. 1600°C was selected as the optimal heat-treatment temperature and the C/Zr C composites treated at 1600°C obtained the flexural strength and modulus of 192 MPa and 17.7 GPa, mass loss and linear recession rate of 0.0040 g/s and 0.0017 mm/s.The effects of PIP-C, CVD-C and PIP-Si C interphase on the microstructure and preperties of C/Zr C composites were studied. PIP-C interphase didnot increase the mechanical properties of C/Zr C composites and affected the oblation resistance barely.PIP-Si C interphase decreased the mechanical and ablation properties of C/Zr C composites. CVD-C interphase had scarely any effects on the ablation resistance of C/Zr C composites, but increased the mechanical properties. The optimal interphase was prepared by CVD process for 15 h, and the flexural strength and modulus 121 MPa and 9.8GPa, respectively. C/C preforms in this chapter were prepared in other department and it was hard for us to control the final properties of the preforms. Though the properties of C/Zr C composites here deviated from the antecedent composites, the effects of interphase on the microstructure and preperties of C/Zr C composites were the same.The wetting, infiltration and reaction behaviors and mechanism during RMI process were investigated and discussed. Firstly, the wettability between zirconium and graphite or C/C preforms were studied. Zr-C reaction would drive the melt spreading to the reaction front, thus the wettability between zirconium and reacion product was fine. Secondly, the infiltration process was investigated by melt capillary rise and the infiltration mechanism was discussed. The infiltration behavior of zirconium melt into C/C prefrom can be appropriately describled by the modified Washburn equation. The maximun infiltratin height was determined by 1r, effective radius based on capillary pressure, and initial infiltration rate was determined by 2r, effective radius based on viscous force. 1r and 2r varied as reaction progressed. Lastly, thermodynamics and dynamics anylysis for Zr-C reaction were carried out and the reaction mechanism in RMI process was investigated. It can be concluded from thermodynamics computation that Zr-C reaction can happens spontaneous at room temperature and it was exothermic heavily. DTA and Zr-C mixture powders reaction results indicated that solid-solid reaction between Zr-C began at ~1000°C, and solid-liquid reaction began after melting of zirconium. During RMI process, continuous Zr C layer formed fast by heterogeneous nucleation and growth of Zr C grains caused by solid-liquid reaction. The growth of continuous Zr C layer was governed by diffusion-reaction and dissolution-precipitation mechanism, and the major factor was the former. The growth of Zr C layer was parabolic curves, proving that the growth was dominated by element diffusion. The dynamic model of Zr C layer growth was established, and the diffusion activation energy ED and effective diffusivity constant 0eD were calculated to be 251 k J/mol and 0.45×10-6cm2/s.