Microstructure and mechanical properties of carbon-carbon composites modified by reactive diffusion treatment in niobium powder

Refractory carbide coatings are primary candidate materials for improving the high-temperature performance of carbon fiber-reinforced carbon matrix composite (C/C composites). In this paper, a niobium carbide coating was grown on the C/C composite surface employing a Powder Immersion Reaction Assisted Coating (PIRAC) method. 2D C/C composite plates were immersed into Nb powder and annealed at 800 to 1200°C for up to 16h. 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 micrometer thick uniform conformant coatings were obtained consisting of a thin Nb-rich Nb₂C layer on the top surface and a thicker sub-stoichiometric NbC_1-x layer underneath. The coating growth was dominated by short-circuit diffusion with the activation energy of approximately 155kJ/mol. At the lower PIRAC temperatures of 800-900°C, 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 (30%) was measured for C/C composites PIRAC treated at 900°C, 16h. No infiltration occurred at the higher temperatures, due to the rapid sealing of surface pores by the Nb carbide layer.