Amorphous carbons are used in a wide range of tribological applications. Their properties vary enormously as a function of the deposition process and composition. Raman spectroscopy is a popular, non-destructive tool for the structural characterisation of carbons. It is a valuable tool to study the nature of the transfer layers and wear tracks in friction experiments. It is also commonly used to assess the quality of the carbon coatings for hard disk drives. Here we present a model for the interpretation of the Raman spectra, measured at any excitation energy for any carbon film¹. Raman scattering from carbons is always a resonant process, in which those configurations whose band gaps match the excitation energy are preferentially excited. The Raman spectra of carbons consist of three basic features, the G and D peaks around 1600 and 1350 cm^-1 and an extra T peak, for UV excitation, at ~980-1060 cm^-1. The Raman spectra at any wavelength depend on 1) clustering of the sp² phase, 2) bond length and bond angle disorder, 3) presence of sp² rings or chains, and 4) the sp²/sp³ ratio. The G peak position increases as the excitation wavelength decreases. the rate of change of the G peak position is called dispersion. The G peak does not disperse in graphite. The G peak only disperses in disordered carbons and the dispersion is proportional to the degree of disorder. Combined Raman measurements at two different wavelengths, such as 514.5 and 244 nm, allow us to derive a wealth of information not accessible by visible Raman measurements alone. The G peak dispersion can be related with the main film properties, such as sp³ content, composition, density and elastic constants. Examples of application of resonant Raman spectroscopy to characterise carbon coatings will be discussed.

¹ A.C. Ferrari, J. Robertson, Phys. Rev. B 61, 14095 (2000); 64, 075414 (2001); B, 63, R121405 (2001).