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 sp3 to sp2 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 sp2-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.

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