Preparation And Ablation Resistance Of C_f/C-SiC-MoSi₂ Composites

With the rapid development of hypersonic transportation,reentry vehicles and propulsion applications,great challenges for ultra high temperature materials used in thermal protection systems?TPS?appeared.The materials not only possess excellent heat-insulating property and ablation resistance,but also light mass,high strength.Carbon/carbon?C/C?composites are able to be used in TPS because of its convincing performance.However,the C/C composites are vulnerable to oxidation at elevated temperature in aerobic environment,which seriously limits their application in advanced space systems.At present,besides the coating method,introducing ultra-high temperature ceramics into C/C composites?C/C-UHTCs?has been considered as an effective way to improve the oxidation and ablation resistance.In this paper,we fabricated C_f/C-SiC-MoSi₂ composites by using a vacuum filtration infiltration,hydrothermal penetration processes with a later hydrothermal densification.The composition and microstructure of the C_f/C-SiC-MoSi₂ composites were investigated by XRD,SEM,EDS and XRF.The ablation behaviors of the composites were particularly studied,and the ablation mechanism was researched.The main conclusions are listed as follows:C_f/C-SiC-MoSi₂ composites with gradient composition and microstructure were prepared by vacuum filtration infiltration process with a later hydrothermal densification.The distributions of SiC and MoSi₂ were homogeneous and gradient along the cross-section of the composites,respectively.From the inner part to t he outer part of the composites,the increase in de nsity and thermal conductivity was achieved.The change of porosity was opposite.After being exposed to the oxyacetylene flame at 2 273 K for 30 s,the linear and mass rates of the as-prepared composites?1.72 g×cm-3?were only 0.0051 mm×s-1 and 0.76 mg×s-1.In addition,the SiC/MoSi₂ mass ratio had a strong impact on the prpoperties of C_f/C-SiC-MoSi₂ composites.With the increase of SiC content,the bending strength and ablation resistance of the composites were improved;the porosity of the composites was decreased.It was because that the introduction of SiC enhance d the interface bonding between carbon fibers and matrix.SiC can act as a transitional phase between C/C and MoSi₂ during the carbonized process,and the unexpected cracks caused by the mismatch in the CTE between carbon fibers and ceramic matrix would not appear.During the ablation test,the formation of SiO₂ molten phase and the generation of Mo_4.8Si₃C_0.6 phase could protect the carbon from further oxidation.The escape of gaseous products was also benefical to ablation performance.The C_f/C-SiC-MoSi₂ composites prepared by hydrothermal methods had compact structures.Results showed that the activation energy of penetration process was only 51.3 k J×mol-1.The large pressure from closed system acting on the ceramic particles overcame resistance in a way.And the residual pressure pushed the particles to the inner part of the porous C/C composites.With the hydrothermal temperature rising,the distribution depth of ceramics along the cross-section of the composites increased.Meanwhile,the increase in densities and the decrease in porosities of the composites were achieved.When the hydrothermal penetration temperature was 413 K,the flexure str ength of the composites was 58 MPa.But when the temperature reached 473 K,the bending strength was 111 MPa.After ablation under a plasma flame at 2573 K for 60 s,the mass ablation rate of the composites was 2.28 mg×cm-2×s-1,decreased by 65 %,54 % and 29 % compared to samples prepared at 413 K,433 K and 473 K,respectively.During the ablation,the composites suffered both chemical erosion and mechanical denudation.In different ablation region,the ablation results were different.It was because that the temperature and scouring force of plasma flame decreased from abltion center to brim.The carbon fibers were ablated into noodle shape in ablation center;SiO₂ protective coating was found in central boundary area;there were plate-like MoO₂ in transition area;the damage in brim area was the slightest.