Aspects of tribological testing of diamond coatings

Friction and wear were investigated under unlubricated vibrating contact conditions in air at room temperature. Tests were performed in a ball-on-disk configuration with a steel bah (100 Cr6) or an alumina ball acting as counterbody, respectively. For comparison of the tribological performance of six different diamond coatings on polished SIC substrates. standard tests with identical test parameters were performed. The effect of relative humidity was investigated by testing in dry, in normal and in moist air, respectively. To study running in effects and wear propagation, the number of sliding cycles was varied between 10(4) and 1.2 x 10(6). Against steel, the friction was high. Nearly no wear was found at the diamond coating, but the wear at the steel ball was high and affected by coating roughness and by relative humidity. In contradiction, in tests with Al2O3 against diamond, a pronounced running in of friction and wear was found with high friction and high wear rates at the beginning of each test, followed by a stationary phase with extremely low friction coefficients (less than or equal to 0.05) and with wear rates below the limit of resolution. This running in depends on grain size and roughness of the coating. (C) 1998 Elsevier Science S.A.