| The extremely severe working environment of the hypersonic vehicles proposes urgent requirement for advanced structure materials that can withstand mechanical damage, oxidation and ablation at ultrahigh temperature. Therefore, it is pressing to develop novel thermal-structure materials. Continuous carbon fiber reinforced zirconium carbide(C/ZrC) composites are considered as one of the most promising candidates for the aerospace applications, because of their anti-temperature capability, high mechanical properties, good thermal shock resistance and excellent ablation resistance. Among the fabrication processes for C/ZrC composites, the reactive melt infiltration(RMI) has attracted increasing attention due to its advantages including short fabrication period, low cost, high ZrC yield and density, and near net shape. However, due to the high melting point of pure zirconium used as infiltrator, RMI process can only be operated at temperatures above 1900 °C, resulting in some drawbacks such as high processing temperature, difficulty of reaction control, and great damage to the fibers. In this study, C/ZrC composite was fabricated by reactive melt infiltration at relatively low temperature. The parameters and mechanism of RMI process, and the properties of C/ZrC composites were researched systematically.The low-temperature reactive melt infiltration process adopted to prepare the C/ZrC composites was designed elaborately. Since the melting points of zirconium-copper(Zr-Cu) binary alloys are lower than 1200 °C, the process temperature will decrease largely when Zr-Cu alloys instead of pure Zr are used as infiltrator. What is more, based on the thermodynamics calculations, Zr-Cu alloys are certified to react with carbon at relatively low temperature, implying that the preparation of C/ZrC composites using Zr-Cu alloys as infiltrator is theoretically feasible. For these reasons, Zr-Cu alloys were chosen as the infiltrator and prepared by arc melting; also physical characters of the Zr-Cu alloys were investigated. In order to obtain the C/ZrC composites with optimal properties, the C/C preform was designed particularly from four aspects: structure of fabric, type of carbon matrix, interphase and porosity, and then fabricated by resin infiltration and carbonization(IC) process, with needle-punched carbon fiber felt and phenolic resin as reinforcement and carbon precursor, respectively. The obtained C/C preform had great parts of open pores with straight tubular structure, and its porosity was adjustable.The C/ZrC composites were fabricated by low-temperature RMI process, and both process parameters and mechanical properties of the C/ZrC composites were studied and optimized. It was found that mechanical properties of the C/ZrC composites were affected significantly by the C/C preform density, Cu content of Zr-Cu alloy, reaction temperature and time. With the increase of C/C preform density from 1.02 g·cm-3 to 1.24 g·cm-3, flexural strength and modulus of the obtained C/ZrC composites firstly increased and then decreased. The mechanical properties of C/ZrC composites derived from ZrCu alloy were better than that from Zr2 Cu or Zr7Cu10. With the increases of reaction temperature from 1200 °C to 1800 °C and reaction time from 0.5h to 3h, flexural strength and modulus of the C/ZrC composites firstly increased and then decreased. When C/C preform with density of 1.12 g·cm-3 and ZrCu were used as raw materials, and the reaction temperature and time were fixed at 1400 °C and 1.5h respectively, the obtained C/ZrC composites showed the highest flexural strength and modulus of 126.9 MPa and 58.0 Gpa.Effects of C/C preform density, Cu content of Zr-Cu alloy and RMI process parameters on ablation resistance of the C/ZrC composites were investigated. The results revealed that, linear recession rate of the composites firstly decreased and then increased with increasing C/C preform density from 1.02 g·cm-3 to 1.24 g·cm-3. The ablation resistance of C/ZrC composites derived from ZrCu alloy was better than that from Zr2 Cu or Zr7Cu10. With the increases of reaction temperature from 1200 °C to 1800 °C and reaction time from 0.5h to 3h, linear recession rate of the C/ZrC composites firstly decreased and then increased. When C/C preform with density of 1.12 g·cm-3 and ZrCu were used as raw materials, and the reaction temperature and time were fixed at 1500 °C and 1.5h respectively, the obtained C/ZrC composites exhibited the best ablation resistance, and the linear recession rate was only 0.0004mm·s-1. The ablation of C/ZrC composites was a combination of oxidizing reaction and mechanical erosion, and the formation of a dense Zr O2 protective layer and the evaporation of residual Cu were the reasons for excellent ablation resistance of the C/ZrC composites.Structure and properties evolutions of the C/ZrC composites with increase of temperature were investigated. When heat-treatment temperature reached 1600~1800 °C, the open porosity and strength retention ratio of C/ZrC composites were about 14% and 77%, respectively. Due to the injury of carbon fiber and ZrC crystals, the strength retention ratio of C/ZrC composites would further decreased to 54.1% when the heat-treatment temperature increased to 2000 °C.Thermal physical properties of the C/ZrC composites, including coefficient of thermal expansion, specific heat capacity, thermal diffusivity and thermal conductivity, were tested and their evolution with the increase of temperature from 25°C to 1200°C were investigated. The results revealed that, with the increase of temperature from 25 °C to 1200 °C, thermal expansion coefficient of the C/ZrC composites firstly increased and then decreased, with the maximum value of 2.86×10-6 K-1 at about 1200 °C; the specific heat capacity increased from 0.60 J·g-1·K-1 to 1.28 J·g-1·K-1; and both the thermal diffusivity and thermal conductivity decreased gradually, in detail the thermal diffusivity decreased from 17.45 mm2·s-1 to 7.18 mm2·s-1 and the thermal conductivity from 47.27 W·m-1·K-1 to 41.37 W·m-1·K-1.The infiltration and reaction mechanism during the fabrication of C/ZrC composites were studied on the basis of microstructure characterization. It was found that the produced ZrC matrix was either single- or polycrystalline, and some nano sized Cu-Zr-C eutectic was produced inside the ZrC matrix. In addition, the infiltration model of Zr-Cu melt were constructed and the infiltration depth into the preform as a function of time was estimated. Besides, the growth mechanism of ZrC grain was defined. As indicated, the formation of ZrC was controlled by solution-precipitation mechanism, i.e. carbon dissolved into alloy liquid and diffused toward the lower temperature area. Then homogeneous ZrC nucleus precipitated from the supersaturated solution and then grew up.The C/ZrC composites were doped by Si C to enhance their anti-oxidative and anti-ablative properties, and the doping effects of Si C into composites were investigated. Both heat treatment and acid bath methods were tried to remove the residual melt, and then Si C was introduced into the C/ZrC composites by two approaches including liquid silicon infiltration(LSI) and precursor infiltration pyrolysis(PIP). It was demonstrated that the residual Cu melt in the composites could be removed more completely by heat treatment than acid bath, and the PIP-Si C doped C/ZrC composites exhibited obvious improvement of both mechanical properties and ablation resistance. Linear recession rate of the doped C/ZrC composites was as low as 0.0007±0.0003mm·s-1, which was about 37% of the composites without doping. The formation of a dense Zr O2-Si O2 layer was in favor of the increase of ablation resistance of the doped C/ZrC composites.The performance of C/ZrC composites was further tested under the hot firing test. After tested at 1800 °C for 689 seconds, the throat of C/ZrC composites thruster showed an expansion rate of 0.1%, much lower than the one of C/Si C composites thruster, implying a broad application prospect of the C/ZrC composites. |
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