Parameters study of carbon bombardment step for enhanced adhesion of thick DLC coating deposited by cathodic arc evaporation

Abstract

The main limitation for the deposition of thick (> 5 μm) DLC coatings is to achieve adhesion to the substrate. In the present work, the adhesion of thick (~20 μm) amorphous carbon coatings was analyzed considering the combination of metallic chromium as bond layer and a carbon bombardment step before the deposition of the functional coating. The carbon bombardment step was included to generate an interfacial structure composed by a mixing layer of Cr and C and a carbon transition layer rich in sp2 bonds. The complete deposition process was made by cathodic arc and the substrate was nitrided steel. Different sets of parameters were used during the carbon bombardment to change the interfacial structure and to analyze the effect on adhesion. The parameters evaluated were the bias voltage, duration, cathode current and temperature. The adhesion of a-C layer was assessed by scratch tests and the interface structure was evaluated by scanning transmission electron microscopy (STEM). Results indicate that a lower bias voltage possibly increased the sp3 content of the carbon transition layer, mitigating the negative impact of a thicker carbon transition layer. Moreover, an increase in the carbon cathode current may increase the adhesion, possibly by an increase in the thickness of the mixing layer. Finally, improved adhesion was achieved when the temperature of the bombardment was reduced. A diagram of expected behavior for the adhesion (in terms of critical load during scratch tests) as a function of carbon transition layer thickness and sp-bonds was proposed based on the available results.