Microstructures /mechanical properties of C/C in niobium powder

Refractory carbide coating are primary candidate materials for improving the high-temperature performance of carbon fiber-reinforced carbon matrix composite (C/C composites). In this article, a niobium carbide coating was grown on the C/C composite surface exploying a powder immersion reaction assisted coating method. 2D C/C composite plates were immersed into Nb powder and annealed at 800 to 1200C for up to 16 hours. Metallic iodine was admixed to the metal powder to allow for Nb atoms transfer onto the C/C surface and into the pores by gas transport reaction. Following PIRAC treatment, several micrometers thick uniform conformant coatings were obtained consisting of a thin Bb-rich Nb2C layer on the top surface and a thicker sub-stoichiometric NbC1-x layer underneath. The coating growth was dominated by short-circuit diffusion with the activation energy of approximately 155 kJ/mole. At the lower temperatures of 800-900C, surface coating growth was accompanied by the infiltration of the volatile Nb iodide and filling of inter-bundle pores with niobium carbide. This resulted in the reduction of residual porosity and an increase in flexural strength and elastic modulus. The greatest increase in specific strength was measured for C/C composites treated at 900C, 16h. No infiltration occurred at the higher temperatures, due to the rapid sealing of surface pores by the Nb carbide layer.

Carbon fiber-reinforced carbon matrix composites (C/C composites) are promising materials for many structural applications due to their unique properties such as low density, high specific strength, high heat capacity, high thermal conductivity, low thermal expansion and good thermal shock resistance. C/C composites are mostly used in aerospace industry, for aircraft brake disks, re-entry heat shields for space vehicles and missiles, rocket nozzles, etc.

Carbon-carbon composites consist of ordered graphite fibers embedded in a carbon matrix. The carbon matrix is built-up on a fiber preform by chemical vapor infiltration or by impregnation and pyrolysis of a thermosetting resin such as epoxy or phenolic resin. CVI fabrication of C/C composites often requires several processing steps and is very slow and expensive. Resin-derived C/C composites, on the other hand, contain shrinkage voids and microcracks at matrix-fiber interfaces and between fiber bundles, and suffer from low interlaminar shear strength, especially 2D composites with graphite fabric reinforcement. The residual porosity hinders the application of C/C composites in airtight structures since the fuel and/or combustion gas may leak through the open defects. Another limitation to the high-temperatures uses of C/Cs is their insufficient resistance to ablation and erosion in hot gases. The high temperature performance of C/C composites can be significantly improved by coating with refractory carbides, such as HfC and TaC. A decrease in ablation rate was reported for chemical vapor deposition TaC-coated C/C composites. Similarly to TaC, niobium carbide, NbC, has a high melting temperature and high hardness that is retained at elevated temperatures. NbC was reported to exhibit low wear rates under dry sliding associated with high load carrying capacity. At the same time, is is much lighter than TaC and has higher specific heat, which makes it an attractive candidate material as a high-temperature protective coating on C/C composites. NbC coating were reported to protect carbon-carbon substrates from hot hydrogen.

The method of applying metal carbide coating on C/C composite substrate can have a large effect on its performance. For the coating to be adherent, it is desirable that the carbide layer is formed by reaction of the metal with the substrate. PIRAC seems an attractive method for growing such carbide layers on C/C composites. In PIRAX, the annealing of a ceramic part immersed into a metal powder leads to the formation of the metal compound coating via reactive interdiffusion at the ceramic/metal interface. PIRAC was successfully used for the coating of graphite and diamond with Cr, Ti and Nb carbides and for the coating of C/C composite with Cr carbides. It has also been shown that the transport of metal atoms can be accelerated by adding a small amount of halogen forming volatile metal halides at the PIRAX treatment temperature. PIRAC processing via this gas transport reaction provides the possibility of reactive infiltration of metal into the inherently porous C/C composites which will result in the filling of pores and make the material stronger and less permeable to hot gases in service.



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