Study On The Properties Of Carbon/Carbon Composites In Simulated Space Extreme-Temperature Environment

With the rapid development of the space industry,more and more satellites,space stations and other spacecraft enter the space environment.When serving in the low-earth orbit(LEO)between 100 km and 1000 km above the earth’s surface,spacecraft need to pass in and out of the earth’s shadow,which leads structural materials to suffer the long-term periodic extreme-temperature changes(-120°C～120°C).Up to now,there are few systematic reports on the space extreme-temperature environmental properties of carbon/carbon(C/C)composites.Studying the properties change and damage mechanism of C/C composites in space extreme-temperature environment is of great significance to promote the application of C/C composites in the space environment,enrich the database of extreme-temperature properties of C/C composites,and optimize the design criteria of C/C composites.In this work,at first,C/C composites are prepared by isothermal chemical vapor infiltration and 2.5D needle-punched carbon fiber preform.Then,C/C composites are modified by matrix and coating: SiC,Zr B2,and ZrC ceramic modified C/C composites(C/C-SiC-ZrC-Zr B2)are prepared by precursor infiltration and pyrolysis,and SiC coated C/C composites(SiC-SiCnw-C/C)are prepared by pack cementation.Finally,by using ground equipment to simulate the space extreme-temperature environment.The evolution of the microstructures,thermophysical properties and mechanical properties of C/C composites and their modified materials under simulated space extreme-temperature environment are systematically studied.The effects of space extreme-temperature environment damage on the oxidation resistance of modified C/C composites are investigated by oxidation test between room temperature(RT)and 1500 ℃ in air.The main research contents and results are as follows:Microstructural evolutions of C/C composites in space extreme-temperature environment were studied.The results show that by the calculation of the finite element model of “CF/PyC interface” in C/C composites,it shows that the shrinkage effect of carbon fiber and pyrolytic carbon matrix causes local debonding of CF/PyC interface at low-temperature(RT ～-120℃),while when the temperature rises from-120℃ to 120℃,the debonding interface gradually "closes" and the interface "reconstructs".In space extreme-temperature environment,the microstructural damage of C/C composites caused by extreme-temperature thermal cycling(ETC)is mainly manifested in the debonding of carbon fiber/pyrolytic carbon interface,the generation of microcracks and the weakening of the polymerization between the fibers in the carbon fiber bundles.Space extreme-temperature thermal cycling causes microcracks perpendicular to and parallel to the CF/PyC interface in C/C-SiC-ZrC-Zr B2 composites.The alternately propagation of this two kinds of microcracks results in the microstructure damage of the composites with the increase of cycle number.The interfacial bonding strength of C/C matrix and SiC coating in SiC-SiCnw-C/C composites decreases gradually with the increase of extreme-temperature thermal cycles.The effects of extreme-temperature thermal cycling on thermal expansion and thermal conductivity of C/C composites were discussed.The results show that the defect effect of thermal mismatch and the crystal structure transition effect of pyrolytic carbon exist simultaneously during extreme-temperature thermal cycling.The change of thermophysical properties of C/C composites mainly depends on the competition of these two effects.The long-term extreme-temperature thermal cycling makes the defect effect far greater than the crystal structure transformation effect.On the one hand,the gradual increase of defects in the material can absorb and offset part of the expansion,resulting in the decrease of the coefficient of thermal expansion.On the other hand,these defects could aggravate the phonon scattering and even cut off the channel of heat conduction,resulting in the decrease of the thermal conductivity of C/C composites.The thermal expansion and thermal conductivity of C/C-SiC-ZrC-Zr B2 composites and SiC-SiCnw-C/C composites decreased overall in space extreme-temperature environment.This is mainly due to the existence of the multi-component interface in these two kinds of composites,which is conducive to the generation of defects under cyclic thermal stress,providing space for thermal expansion.At the same time,the increase of defects strengthens phonon scattering,the average free path declines,and the ability of heat transfer of the composites receded.Based on the variation of flexural strength,shear strength and compression strength of C/C composites after different extreme-temperature thermal cycles,the evolution mechanism of mechanical properties of C/C composites in space extreme-temperature environment were expounded.The results show that the "high-stress contact point" produced by the negative expansion effect of carbon fibers in the pyrolytic carbon matrix is the main reason for the enhancement of the flexural strength of C/C composites after 50 cycles,then the flexural strength decreases with the increase of cycle number,it maintains 85.84% of the initial strength after 200 cycles.The flexural strength of C/C-SiC-ZrC-Zr B2 composites increased by 25.88%after 50 cycles,and it remains the initial strength of 83.03% after 200 cycles.The flexural strength of SiC-SiCnw-C/C composites decreases with the increase of cycle number.After 200 cycles,the flexural strength of SiC-SiCnw-C/C composites is52.74% of the initial strength.The flexural fracture failure of C/C composites and their modified materials shows a tendency from brittle mode to the similar pseudo-plastic mode with the increase of extreme-temperatures thermal cycles.The oxidation resistance of C/C-SiC-ZrC-Zr B2 composites and SiC-SiCnw-C/C composites and its failure mechanism after experiencing space extreme-temperature environment were revealed.The results show that the mass loss of C/C-SiC-ZrC-Zr B2 composites increases with the increase of extreme-temperature thermal cycles during the oxidation test,and the mass loss after 200 cycles reached 14.96%.In the process of oxidation test,there are two types of oxidative damage leading to the failure of the C/C-SiC-ZrC-Zr B2 composites subjected to space extreme-temperature environment:(i)a large number of micropores and microcracks produced on the surface of the composites;(ii)the matrix cracking and the exposed carbon fibers eroded in the internal structure of the composites.The mass loss of SiC-SiCnw-C/C composites decreases firstly and then increases with the increase of extreme-temperature thermal cycles.The lowest value was 3.68% after 100 cycles and it increased to 14.31% after200 cycles.The improvement of oxidation resistance of SiC-SiCnw-C/C composites after 100 extreme-temperature thermal cycles can be attributed to the appropriate extreme-temperature thermal cycling combined with the toughening mechanism of SiC nanowires,which can make the molted Si O2 glass phase effectively heal some defects in the coating.