Graphitization processes in wear tracks of Bi-layer carbon coating deposited by PVD-HiPIMS technique

Abstract This paper focus on a statistical Raman analysis to study the graphitization processes produced during a reciprocating tribological test on a design free hydrogen Bi-layer carbon coating deposited by PVD-HiPIMS. The design consists of a first layer L with a low sp3 content to reduce stresses and improve adhesion to substrate, the second layer H has a high sp3 content to increase the hardness and produce and smooth surface. The thickness and sp3 ratios H/L were fixed at 2 by monitoring the HiPIMS discharge current. Images of the cross section verify a different growth mechanism for each layer. The tribological tests against alumina ball showed a coefficient of friction lower than 0.2 for applied loads in the range 0.75–1.50 N. Statistical Raman analysis revealed that as the tribo-test progress, the more sensitive fitting parameter in the Raman spectra was the G position followed of the coupling coefficient. Such parameters showed absence of graphitization at low load, and partial graphitization as the loads increase. All loads produced adhesive wear on the coating.