Potential of thick a-C:H:Si films as substitute for chromium plating

Abstract

Chromium plating is a well-established technique for corrosion and wear protection. One essential problem inherent with this process is the use of toxic and carcinogenic hexavalent chromium. A potential substitution of hard chromium with thick a-C:H:Si coatings (amorphous Si-doped hydrogenated carbon) has been studied. a-C:H:Si films represent chemical and electrochemical inert hard coatings with very favorable tribological and corrosion properties. Up to now it was only possible to deposit thin DLC-films because of the high intrinsic stress in the films.A series of laboratory tests including scratch, pin-on-disk and electrochemical corrosion measurements were performed in order to compare the performance of hard chromium and a-C:H:Si. By depositing a-C:H:Si films without using hazard chemicals in a dc-pulsed hot wall plasma system coatings with thicknesses up to 58μm were produced. These coatings exhibit elastic properties during scratching at the beginning and get a self-sustaining behavior with increased coating thickness resulting in a high load carrying capability. For hard chromium coating cracking was already observed at the very first stage of testing.Sliding tests under unlubricated conditions of a-C:H:Si films showed superior friction performance (friction coefficient μ=0.02 versus μ=1.1) and excellent wear resistance of the coating and the counterpart (wear rate 7∗10−8mm3/N−1m−1 versus 6∗10−5mm3/N−1m−1) compared to hard chromium. Further potentiometric observations demonstrated a much better corrosion resistance for thick a-C:H:Si coatings than hard chromium (Ecorr −0.28V vs. Ecorr −0.52V).