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 substrates (M50 steel, Ti-6Al-4V alloy and AISI 440C steel) by methane ion-beam deposition. Pin-on-disc experiments showed that the DLC films possess excellent wear resistance and exhibit low values of friction coefficient (f<0.1). The frictional response was characterized by an initial break-in period that was followed by an intermediate constant f stage. The presence of a carbon transfer layer was observed on the wear scars of ball surfaces. Transmission electron microscopy and electron diffraction from the transfer layer showed that it contained a fine distribution of graphite nanoparticles (less than 5 nm) in a distorted diamond-like structure. Laser Raman spectra taken from the wear track region also revealed evidence of graphitization. In light of the present observations, the intermediate friction stage was mainly attributed to transfer layer formation. During long-duration wear experiments, a further reduction in f was observed that finally reached steady state low 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 transformation of diamond-like to graphite-like carbon under thermal and strain effects from the repeated friction through a precursor stage involving hydrogen evolution and sp(3) structure destabilization. The ultra-low wear rates were correlated to easy shear of low-strength atomic carbon interlayers in the graphitic structure.