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

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 58 mu m 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 mu = 0.02 versus mu = 1.1) and excellent wear resistance of the coating and the counterpart (wear rate 7 * 10(-8) mm(3)/N-1 m(-1) versus 6 * 10(-5) mm(3)/N-1 m(-1)) compared to hard chromium. Further potentiometric observations demonstrated a much better corrosion resistance for thick a-C:H:Si coatings than hard chromium (E-corr -0.28 V vs. E-corr -0.52 V). (C) 2013 Elsevier B.V. All rights reserved.