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

• Published in: Surface & coatings technology Vol. 281; pp. 76 - 81
• Main Authors: Yin, Xiaowei, Gotman, I., Gutmanas, E.Y., Weiss, R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-11-2015
• Subjects: Carbides; Carbon-carbon composites; Coatings; Diffusion; Flexural strength; Infiltration; Niobium; Niobium carbide; Porosity; Powder Immersion Reaction Assisted Coating; Reactive infiltration; Surface chemistry; Niobium carbide; Reactive infiltration; Carbon–carbon composites; Powder Immersion Reaction Assisted Coating; Flexural strength
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.09.020

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 NbC1−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.
•Original reactive treatment of carbon–carbon composites in Nb–iodine powder mixture•Highly uniform, conformant Nb carbide (NbC) coatings on carbon–carbon composites•Coating growth controlled by short-circuit diffusion with low activation energy•Reactive infiltration of inter-bundle pores by NbC concurrent with coating growth•Reduced residual porosity and increased strength of carbon–carbon composites