Friction-induced structural transformations of diamondlike carbon coatings under various atmospheres

Structural transformations that occur in diamondlike carbon coatings with increasing hydrogen content have been investigated by Raman spectroscopy, transmission electron microscopy, electron diffraction, and electron-energy-loss spectroscopy. Friction tests were performed with uncoated steel balls against coated substrates at contact stresses of 1 GPa in ambient air (RH=30–40%), dry air (RH<1%), and dry nitrogen (<1%). The lowest friction coefficient (f<0.02) was obtained for the most hydrogenated sample in dry nitrogen, where the formation of a third-body layer was observed on the steel surface. Raman spectra obtained from the counterfaces after sliding in humid and dry air revealed an increase and narrowing of the ‘D’ and ‘G’ peaks with decreasing humidity. Analysis of peak positions and I(D)/I(G) ratios suggest an increasing order and an enlargement of the sp2 clusters under friction. The shape and position of the carbon K-edge spectra for the transfer layer are affected the same way, although evidence of extended graphite layer formation was not observed. Development of these differing trends was correlated with the hydrogen-to-carbon ratio of the gas precursor used during the synthesis and testing environment.