Third Bodies and Tribochemistry of DLC Coatings

Much of the literature on diamond-like carbon (DLC) tribological coatings emphasizes the processing—structure—tribological property interrelationships but often ignores or speculates on the mechanisms which control friction and wear. In reality, third-body processes, such as transfer films that form in the moving contact, are responsible for the long life of DLC coatings. In this chapter, the friction and wear behavior of amorphous diamond-like nanocomposite (DLN) coatings, Ti- and W-doped DLC, and hydrogenated DLC coatings in low speed, dry sliding contact have been investigated using a home-built in situ Raman tribometer. In situ optical microscopy identified how third-body processes controlled friction and wear behavior of these DLC coatings in reciprocating sliding against sapphire hemispheres in dry (~4% RH) and humid (~20–60% RH) air between contact stresses of 0.7 and 1.1 GPa. In situ visual observations monitored the health (e.g., formation, thickening, thinning, and loss) of transfer films at the sliding (buried) interface. For most of the coatings, interfacial sliding between the transfer film and underlying wear track was the dominant velocity accommodation mode (VAM) responsible for steady-state low friction coefficients between 0.03 and 0.2, with lower values obtained at high contact stress and lower RH percentage. Transfer film behavior with the W-doped DLC coating was also studied when lubrication changed from dry air to a perfluoropolyether (PFPE) lubricant.