PORES IN HYDROGENATED AMORPHOUS-CARBON FILMS ON STAINLESS-STEEL

The aim of this work is to investigate the influence of surface precleaning procedures prior to deposition of hydrogenated amorphous carbon films onto stainless steel. In industrial coating processes the substrates are first degreased and then Ar-ion sputter-cleaned in the deposition chamber just prior to film deposition. This sputter cleaning, which removes the native surface oxide layer as well as unknown contaminations, is carried out rather thoroughly to assure a good adhesion of the hard carbon him. In these films holes or pores are found when inspected with the scanning electron microscope. We demonstrate that these are not pores in the coating but holes in the stainless steel substrate at locations of manganese-sulphur inclusions. They are produced during the sputter-cleaning procedure owing to a higher sputter yield of the manganese-sulphur inclusions with respect to the stainless steel substrate. By carefully selecting the surface preparation procedures, the depth of these holes may be reduced below the surface roughness and can then be neglected. We also show that the Ar-ion sputter etching increases the surface roughness by less than 2% of the thickness of the removed layer and that the amorphous carbon film growth is extremely uniform, the increase in surface roughness being less than 1% of the film thickness for 100 nm thick coatings.