Effect of long carbon bombardment step on the adhesion of thick amorphous carbon coating deposited by cathodic arc evaporation

The main restriction in the application of thick hydrogen-free amorphous carbon (a-C) coatings is the adhesion to the substrate, since high compressive stresses tend to delaminate the coating when a certain thickness is reached. In the present work, amorphous carbon films were deposited by unfiltered cathodic arc over a gas nitrided stainless steel substrate with metallic chromium as bond layer. An additional step was included, where carbon ions were accelerated by high bias potential before the deposition of functional a-C, to produce a carbon transition layer to act as a buffer layer. Different thicknesses of this transition layer were produced, including a sample without this step, for reference purposes. The coating adhesion was evaluated by scratch test. In addition, the microstructure of the interface was analyzed by scanning transmission electron microscopy (STEM) and the sp-type bonds were quantified by electron energy loss spectroscopy (EELS). The analysis of the microstructure close to the interface with the chromium bond layer revealed a mixing layer of Cr and C. Furthermore, it was observed the formation of the transition layer characterized by a homogeneous carbon layer with high sp2 content, when compared to the functional amorphous carbon layer. Results indicate a significant enhancement of adhesion for the samples prepared with the carbon bombardment step, which can be correlated to the presence of the mixing layer and the carbon transition layer. Moreover, a decrease in adhesion was observed for an increase in the thickness of the transition layer, which can be attributed to the lower shear strength of this sp2 rich layer.

Automated summary

In this study, amorphous carbon films were deposited on gas nitrided stainless steel substrates with the introduction of a carbon bombardment step to create a carbon transition layer, which significantly improved the adhesion of the coating. Analysis of the interface microstructure revealed a mixing layer of Cr and C, as well as the formation of a homogeneous carbon layer with high sp2 content in the transition layer.