Do DLC-like features in Raman spectra of tribofilms really mean they are DLC formed by friction?

Many previous studies of tribofilms have interpreted D- and G-bands in Raman spectra as evidence that diamond-like carbon (DLC) was formed during sliding. DLC and other amorphous-carbon films are produced by high-energy processes or high-temperature pyrolysis. Since neither of these conditions commonly occurs in a sliding interface, it seems unlikely that such materials could be produced during simple frictional sliding. To understand this apparent contradiction, we systematically analyzed tribofilms produced from vapor and liquid lubrication experiments using Raman spectroscopy with varied laser power and wavelength. The results provide evidence that DLC-like features in Raman spectra of tribofilms formed from organic molecules originate, not by tribochemical synthesis in situ during the tribo-testing as suggested previously, but rather from post-synthesis photochemical degradation of carbonaceous organic matter during the Raman analysis.