Effect of amorphous silicon interlayer on the adherence of amorphous hydrogenated carbon coatings deposited on several metallic surfaces

The effect of an amorphous hydrogenated silicon (a-Si:H) interlayer on the adherence of amorphous hydrogenated carbon (a-C:H) coatings deposited on four metallic surfaces: AISI M2 steel, AISI 304 stainless steel, Nitinol alloy, and Ti6Al4V alloy was studied. The interlayers and the coatings were deposited employing an asymmetrical bipolar pulsed-DC PECVD system with an active screen. Multilayer a-C:H coatings were also deposited, with the aim of obtaining thicker films. The interlayers were synthetized by varying the applied negative pulse amplitude from -0.8 kV to -10 kV, keeping their thickness constant at 250 nm. The coatings' adhesion was evaluated using classical scratch and VDI 3198 indentation tests. Raman spectroscopy was used to analyze the films' atomic arrangements. The total compressive stress was determined through the measurement of the substrate curvature before and after the film deposition, while nanoindentation experiments allowed determining the films' hardness and elastic modulus. In order to determine the chemical bonding between a-Si:H and the metallic surfaces, X-ray photoelectron spectroscopy (XPS) was used. The obtained results showed high values of critical loads, allowing a high degree of adherence of the a-C:H coatings to all the metallic materials. The highest Lc1 critical load values (>= 25 N) were determined when the a-Si:H interlayers were deposited using the highest negative applied voltage (from -6 kV to -10 kV) on the Nitinol alloy surfaces. The XPS results suggested that the high degree of adhesion of the a-C:H coatings to Nitinol could be attributed to chemical bonds of Ti-Si and Ni-Si formed in the interface, while for the Ti6Al4V alloy the Ti-Si bonds predominated. On the other hand, on steel surfaces the adhesion was due to Fe-Si bonds. A combination of a modified pulsed-DC PECVD system with an active screen and an a-Si:H interlayer allowed depositing hard, adherent, and low-stress a-C:H coatings.