The effect of deposition parameters on structure, mechanical and adhesion properties of a-C:H on Ti6Al4V with gradient Ti-a-C:H:Ti interlayer

Diamond like carbon (DLC or a-C:H) is a coating material with broad application in many technical fields such as tribological, optical and biomedical applications. However, its outstanding mechanical and wear properties are counteracted by its poor adhesion to steel or titanium alloys. Therefore, to widen their usability in vast technological fields, the improvement and understanding of adhesion/cohesion of the DLC-metal substrate system is mandatory. In this study, a-C:H was deposited using a radio frequency magnetron plasma enhanced chemical vapor deposition (rf-magnetron-PECVD) process onto medical Ti Grade 23 (Ti6Al4V ELI) substrates, provided with a chemical gradient Ti-a-C:H:Ti interlayer. The deposition parameters, like Ar/C2H2 ratio and rf-power during gradient Ti-a-C:H:Ti interlayer deposition as well as working pressure and substrate bias voltage during the deposition of top a-C:H were varied. The layer systems mechanical and adhesion properties were characterized by nanoindentation and scratch testing. The samples microstructure after coating was examined by etching and SEM imaging. SEM cross sections revealed a columnar structure for gradient interlayer and amorphous structure for top a-C:H coating. The Ti-a-C:H:Ti-a-C:H coating system increased the substrate's surface hardness from 5.8 to 13.5 GPa. Simple Ti interlayer could improve a-C:H adhesion to substrate but led to huge area hertzian spallation under scratch load of approximately 98 mN. Adding the Ti-a-C:H:Ti gradient interlayer could prevent huge area delamination for critical loads up to 262 mN without significantly affecting the top a-C:H mechanical properties. Additionally, the COF between substrate and Berkovich stylus during scratch testing decreased from 1.26 for uncoated sample to 0.03 for a-C:H coated sample. No change in grain structure near substrate surface was examined for deposition temperatures up to 135 °C.