The friction coefficient between a steel ball and a Si-DLC film was investigated in an open atmosphere without lubrication. Our earlier work demonstrated that the friction coefficient of Si-DLC with the Si content adjusted to achieve low friction（about 0.1 or fewer）correlates with the hydrogen content within the region of a low friction coefficient（0.06–0.12）. The friction coefficient is considered to have changed because of the change in hardness associated with the change in the Si-DLC film hydrogen content. Therefore, for this study, we conducted ball-on-disk friction tests against a steel ball made of high carbon chromium bearing steel（SUJ2, JIS）for Si-DLC films with hardnesses of HK1924, 1468, 1462, and 1290 obtained using different deposition methods. The results reaffirmed that Si-DLC hardness is associated directly with a higher friction coefficient. Moreover, post-test SEM and EDS measurements of the wear scars on the ball side detected Si derived from the Si-DLC and Cr derived from the ball, revealing that the friction coefficient increased with the increased Cr/Si ratio at higher film hardness. These results suggest that friction coefficient increases when more ball-derived metal wear debris penetrate into the friction interface Therefore, for suppressing generation of wear debris derived from the steel ball, a friction test was conducted between a Si-DLC film and a steel ball coated with an a-C:H film. The friction coefficient was 0.04 when using a steel ball coated with an a-C:H film, but it was 0.08 when using a steel ball without any coating.
