Hard diamond-like layers produced during DIII-D tokamak operations

Measurement of the hardness of carbon films, obtained by the exposure of a graphite sample to the tokamak divertor plasma and the deposition of the eroded material on Si substrates, has shown the unexpected result that their hardness was very high. The hardness was about 10-20 GPa, which is close to the hardness of sapphire. We have studied the structural and optical properties of these films by Raman and infrared spectroscopy. The Raman spectra are consistent with a high hardness. The graphite-like Raman band at 1515 cm(-1) is down-shifted, which indicates the presence of both sp(2) and sp(3) bonds in the cross-linked structure of the films which is typical for hard carbon films. From the transmission spectra it is found that the band-gap absorption edge is not lower than that of Si. Infrared spectra show a triplet around 2900 cm(-1) which is assigned to the CH stretching modes. (C) 1998 Published by Elsevier Science B.V.