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 fro m acetylene, tetramethylsilane (TMS) and hexamethyldisiloxane (HMDSO) in an r.f. plasma deposition process, if a sufficient flux of energeti c ions to the substrate surface is provided by the self-bias voltage a t the substrate electrode. Using electrochemical impedance spectroscop ic (EIS) investigations on films deposited on steel substrates, signif icant 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 sam ple was completely stable over the whole measuring time of 460 h. Film s deposited from HMDSO, however, immediately formed ionic current path s, as revealed by changes in EIS after only 24 h, and the characterist ic appearance of the 0 h spectra. These results show that very good co rrosion protection can be achieved by properly deposited amorphous hyd rogenated silicon carbon (a-SiC:H) films, if film defects are carefull y avoided.