Growth of diamond coating on carbon fiber: Relationship between fiber microstructure and stability in hydrogen plasma

Stability of carbon fiber in hydrogen plasma during chemical vapor deposition growth of diamond film was investigated for a range of carbon fiber samples with different physical properties. Morphological studies using scanning electron microscopy and focused ion beam showed that pre-growth seeding with nanodiamonds was necessary both to protect the carbon fiber from atomic hydrogen attack and to promote diamond growth. Microstructural studies using Raman spectroscopy indicated that carbon fibers with larger crystallite size, which correspond to high and ultra-high modulus fibers, were less susceptible to etching compared to carbon fibers with smaller crystallite size, corresponding to intermediate modulus fibers. A model was developed to predict the diamond film coverage, following pre-seeding of the carbon fibers with nanodiamonds. Compared to larger seeds, a dense seeding with smaller sized nanodiamonds resulted in faster coalescence, which provides significant benefits as the diamond layer protects the carbon fiber from hydrogen plasma attack. The results of this study will facilitate the integration of diamond and carbon fiber into a versatile hybrid material and, in particular, pave the way towards development of novel biocompatible diamond-coated carbon fiber micro-electrodes with long-term efficacy.