Evidence for and role of boron carbide icosahedra in the hardness of amorphous hydrogenated boron carbide

We report both infrared absorption and B-11 nuclear magnetic resonance measurements that provide evidence for the presence of boron carbide icosahedra in amorphous hydrogenated boron carbide (a-B:C:H) thin films. The infrared absorption spectra are dominated by a broad line at 1280 cm(-1) with a FWHM of 320 cm(-1), and the B-11 nuclear magnetic resonance spectra are dominated by a line with a chemical shift of 4 ppm and FWHM of 35 ppm; similar features have been previously reported in polycrystalline boron carbide, where boron carbide icosahedra make up the unit cell. We also suggest that it is these icosahedra that increase the hardness of these films over those films without boron, by playing the role of nanocrystals in a nanocrystal/amorphous matrix composite system.