Graded layer design for stress-reduced and strongly adherent superhard amorphous carbon films

Diamond-like carbon thin films for tribological applications were deposited by d.c.-magnetron sputtering of a graphite target in a pure argon atmosphere or in a reactive hydrogen or methane atmosphere at pressures between 0.1 and 1 Pa in a graded constitution to improve adhesion and reduce residual stress. The temperature of the metallic, carbon- and ceramic-like substrates was below 100 degrees C. The mechanical, thermal, electronic and optical properties of the carbon thin films show a significant dependence on the ion energy. Below 220 eV, strongly adherent black conductive films with hardness values up to 2000 HV0.05 were obtained. Hard and superhard diamond-like carbon thin films were deposited in an energy range between 220 and 370 eV with hardness values up to 4000 HV0.05. They are insulating, optically transparent and show a high degree of hardness combined with high compressive stress in the order of 4 GPa as well as a low adhesion, which means that the critical loads of failure are below 10 N. Above 370 eV, weak black conductive films with a poor adhesion were deposited. A new concept has been realized, which allows the conservation of the positive properties of superhard films, such as high hardness, sp(3) content as well as a low friction coefficient (0.08-0.17 against 100Cr6 and Al2O3) with a simultaneously decreasing stress and increasing adhesion. First. a thin TIC interface layer was deposited, and then, the ion energy was gradually increased during the deposition of the carbon layer. Critical loads of failure in a scratch test of up to 50 N were reached when applying this concept. These amorphous carbon films have shown excellent tribological properties. especially low friction coefficients and low wear under dry sliding wear conditions against 100Cr6 and Al2O3. X-ray diffraction and TEM examinations confirmed a fully amorphous structure of hard carbon films. Substrate temperatures above 200 degrees C result in the deposition of nanocrystalline graphite-like carbon films shown by Raman spectroscopy. (C) 1999 Elsevier Science S.A. All rights reserved.