Friction induced phase transformation of pulsed laser deposited diamond-like carbon

Structural transformations in the sliding friction of hydrogen-free diamond-like carbon (DLC) films prepared by pulsed laser deposition are investigated. Stainless steel disks were coated with 0.5 μm thick DLC films, and ball-on-disk sliding experiments were performed with steel and sapphire balls in humid air, a nitrogen atmosphere, and under vacuum. Friction coefficients of about 0.1 are reported. The low friction is related to a friction induced transformation of the surface into a graphite-like phase and the formation of an adherent transfer film of this material on the counterface. Surface enhanced micro-Raman studies of the wear tracks, wear debris and the transfer film demonstrated that an sp 3 to sp 2 phase transition has occurred in the wear tracks on the DLC film surface. The formation of a graphite phase after several thousands of cycles caused a humidity sensitive behavior of the DLC films and an increase in the friction coefficient in high vacuum conditions. A lubricating sp 2-rich layer on the surface of the hydrogen-free DLC films is proposed as the reason for their extremely low wear rates in ambient environments.