A STUDY OF THE WEAR MECHANISM OF DIAMOND-LIKE CARBON-FILMS

In the present work, the tribological mechanism of diamond-like carbon (DLC) films was investigated. DLC films were coated on metallic subst rates (M50 steel, Ti-6Al-4V alloy and AISI 440C steel) by methane ion- beam deposition. Pin-on-disc experiments showed that the DLC films pos sess excellent wear resistance and exhibit low values of friction coef ficient (f<0.1). The frictional response was characterized by an initi al break-in period that was followed by an intermediate constant f sta ge. The presence of a carbon transfer layer was observed on the wear s cars of ball surfaces. Transmission electron microscopy and electron d iffraction from the transfer layer showed that it contained a fine dis tribution of graphite nanoparticles (less than 5 nm) in a distorted di amond-like structure. Laser Raman spectra taken from the wear track re gion also revealed evidence of graphitization. In light of the present observations, the intermediate friction stage was mainly attributed t o transfer layer formation. During long-duration wear experiments, a f urther reduction in f was observed that finally reached steady state l ow friction (f=0.05-0.07) and an ultralow wear rate (about 1.6 x 10(-9 ) mm(3) m(-1) N-1). Steady state was related to the complete transform ation of diamond-like to graphite-like carbon under thermal and strain effects from the repeated friction through a precursor stage involvin g hydrogen evolution and sp(3) structure destabilization. The ultra-lo w wear rates were correlated to easy shear of low-strength atomic carb on interlayers in the graphitic structure.