Preparation And Properties Of C_f/(HfC+MC) Composite By Reactive Melt Infiltration

Carbon fiber reinforced ultra-high temperature ceramic (C_f/UHTCs) composites, with excellent high temperature performance and anti-ablation performance, are one of the most promising materials. Reactive melt infiltration (RMI), with many advantages such as short preparation, simple process, no pollution, has been demonstrated as an effective method to prepare C_f/UHTCs composites. This dissertation has investigated the preparation of C_f/(HfC+MC) composite by reactive melt infiltration, disscussed the infiltration mechanism of different Hf-based alloys into the porous C/C performs, studied the effects of the RMI process on the microstructure of the composites, and examined their anti-oxidation and ablation performance.According to the characteristics of the RMI process and the oxidation behavior of Hf, Zr, Si and Ta components at elevated temperatures, 50Hf10Zr37Si3Ta and 22Hf78Si alloys were designed and prepared. C_f/(HfC+ZrC+SiC) composites and C_f/(HfC+SiC) composites with the average densities of 1.88 g/cm~3 and 2.32g/cm~3, the apparent porosity less than 10% and 4%, respectively, were prepared by RMI.The microstructure and composition of the composites were studied by XRD and SEM and the infiltration mechanisms of both alloys were analyzed by phase diagram calculating software Pandat? combined with thermodynamics fundamentals. The infiltration of the 50Hf10Zr37Si3Ta alloy, whose melting point is 2440℃, was realized at 1900℃. The primary reason of the realization of the RMI process at a relative low temperature is that the Hf component with the most strong carbide-forming ability prefers to react with the carbon in the surface layer of the C-C preform, leading to a phase composition change of the alloy at the interface, generating liquid and forming a specific multi-layer RMI structure. Compared with the 50Hf10Zr37Si3Ta alloy, the infiltration mechanism of the 22Hf78Si alloy is that the alloy melts at temperature higher than its melting point and the melt infiltrates into the porous preform by capillary action, and reacts with carbon, which results in a dense matrix.The C_f/(HfC+ZrC+SiC) composite prepared from 50Hf10Zr37Si3Ta alloy exhibits excellent anti-oxidation ablative properties, because a layer of dense Hf(Zr, Ta)O2, which can protect the specimen from damaging, is formed on the specimen surface when tested in oxyacetylene flame and by laser. The linear ablation rate by laser (1000 W/cm2, 60 s) is only 0.008 mm/s. The infiltration depth of 50Hf10Zr37Si3Ta alloy is low, which is attributed to the RMI mechanism.For the C_f/(HfC+SiC) composite material prepared from 22Hf78Si alloy, because of the good penetrability of the alloy, the bending strength of the composite reaches 237 MPa, and the fracture behavior shows a pronounced pseudoplastic fracture characteristics. The linear ablation rate by laser (1000 W/cm2, 60 s) is only 0.012 mm/s. The static anti-oxidation ability of both composites at 1300℃in the air was adverse, because the thermal mismatch between carbide from RMI, oxide from oxidation and the composite matrix is large.