Diamond-like carbon (DLC) coatings are known to provide beneficial mechanical and tribological properties in harsh environments. Their combination of high wear resistance and low friction has led to their extensive use in any number of industries. The tribological performance of a DLC coating is varied however, and the frictional response is known to be strongly dependent on the surrounding environment, as well as the material composition and bonding structure of the DLC coating. This paper presents an up-to-date review on the friction of DLC coatings in a water environment, with a special focus on transfer layer formation and tribochemistry.
Three commercially available Diamond-Like Carbon (DLC) coatings were investigated to help understand the dynamics of transfer layer formation and decay, when sliding against AISI 52100 steel balls in distilled water.Optimum tribological behaviour was observed during interfacial sliding between the transfer layer and DLC coating. Alternatively, shear of the carbonaceous transfer layer from the contact region resulted in growth of an iron oxide layer comprised of magnetite, maghemite and hematite, as identified by Raman spectra. Threebody abrasive wear involving iron oxide wear particles explained the high wear rate of the DLC coatings in the case of shear.Friction was controlled by the formation of a transfer layer, reducing adhesive interactions between surfaces.Subsequently, a gradual increase in friction was observed, and suggested to relate to an increase in the shear strength of the transfer layer due to adsorption of oxidative species. This was modelled using the Elovich equation for gas adsorption kinetics.
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