Deposition and properties of carbon-based amorphous protective coatings

Using a combination of temperature-humidity testing in D2O vapour with secondary ion mass spectrometry (SIMS) depth profiling of deuterium, it has been demonstrated that moisture-tight coatings can be grown from acetylene, tetramethylsilane (TMS) and hexamethyldisiloxane (HMDSO) in an r.f. plasma deposition process, if a sufficient flux of energetic ions to the substrate surface is provided by the self-bias voltage at the substrate electrode. Using electrochemical impedance spectroscopic (EIS) investigations on films deposited on steel substrates, significant differences in protection of the substrates against corrosion in aerated electrolyte solution were found for films grown from TMS and HMDSO, respectively. Films deposited from TMS were usually stable for a period of time up to 300 h before localised corrosion began. One sample was completely stable over the whole measuring time of 460 h. Films deposited from HMDSO, however, immediately formed ionic current paths, as revealed by changes in EIS after only 24 h, and the characteristic appearance of the 0 h spectra. These results show that very good corrosion protection can be achieved by properly deposited amorphous hydrogenated silicon carbon (a-SiC:H) films, if film defects are carefully avoided.