The importance of unbound hydrogen and increased aromatic structure on the friction and wear behaviour of amorphous hydrogenated carbon (a-C:H) coatings

Abstract

It has been observed that amorphous hydrogenated carbon (a-C:H) coatings, which have a wear life in dry nitrogen, contain a significant fraction of unbound hydrogen and are characterized by larger aromatic clusters as seen in the enlargement of the D peak in the Raman spectra. A correlation is made between the presence of unbound hydrogen, which is associated predominantly with the surfaces of aromatic clusters, and the wear life of a-C:H coatings in dry nitrogen. It is hypothesized that high-energy edge-site/edge-site interactions of graphitic clusters is responsible for accelerated wear under dry nitrogen test conditions analogous to the dusting wear of graphite. However, where unbound hydrogen is present its release in sufficient quantities leads to passivation of these high-energy sites, reducing the coefficient of friction (COF) to values as low as 0.02. Under ambient laboratory conditions passivation of these sites occurs with the aid of water vapour. The interplay of the various factors, including material and environmental, that influence the value of the COF under ambient conditions is discussed.