# Carbon Coatings Deposited on Prosthodontic Ni-Cr Alloy
## 4 Discussion
These values can be treated as satisfactory, and they are comparable with those obtained by other authors on Co-Cr alloys and titanium alloys [38; 39], while being higher than those obtained by [PERSON] on titanium alloy Ti6 Al4V [23]. During the operation of elements coated with carbon layers, the latter should not become separated from the substrate. It seems that the very good adhesion is a result of the use of a titanium interlayer. [PERSON] et al. and [PERSON] et al. have also achieved improvement in the adhesion of carbon layers by applying a titanium interlayer [37; 38]. The corrosion resistance tests carried out by means of the potentiodynamic method showed that carbon coatings shift the potential E\({}_{\text{k}\text{o}}\) towards the anodic side by about 0.3 V. For the coated samples, the breakthrough potential of the passive layers Eb is about 0.2 V more anodic compared to the Eb for samples without a coating. This corrosion has a pitting character, which results from both the anodic potentiodynamic characteristics curves and the morphology of the sample surfaces examined with the use of a scanning electron microscope. No samples exhibited the occurrence of crevice corrosion. Based on the microscopic images, it can be assumed that the surface solubilization processes begin and the most intensively occur in the vicinity of the intermetallic phase inclusions present in the alloy. Summing up, it can be stated that carbon layers have a beneficial effect by increasing the corrosion resistance of the alloy. This is in agreement with the reports by other authors, in which different alloys coated with carbon layers are studied and which demonstrate an improvement of the corrosion resistance in a body fluid environment [25; 37; 38; 39; 40]. The studies of many authors have proved that there is a possibility to modify the surface of materials in order to limit the bacteria's ability to colonize them [41]. The measurements of the number of bacteria adhering to the alloy surface showed that there were much fewer of them on surfaces covered with a carbon coating. These differences are statistically significant. It can be stated that the carbon layer greatly reduced the number of bacteria adhering to the surface of the examined alloy. The obtained results are in agreement with those obtained by other authors [42; 43; 44; 45] who have also demonstrated that carbon coatings on different alloys reduce the number of adhering bacteria. It seems that these results are very advantageous from the point of view of the prophylactics in inflammatory complications connected with the introduction of prosthetic restorations into the oral cavity. The colonization of the surface of prosthetic restorations also leads to disadvantageous changes in the oral cavity. The surface of prostheses, directly after their introduction into the oral cavity, is colonized by bacteria, which form a biofilm--dental plaque. Undoubtedly, one of the pathogenic factors leading to the development of mucous membrane inflammation of the oral cavity is the dental plaque and the bacteria which form it. The harmful operation of bacteria and the products of their metabolism can lead to the development of mucous membrane inflammation of the oral cavity being in contact with the elements of the prosthesis. For example, an inflammatory reaction of the gums, which are in contact with the edges of prosthetic crowns, are observed. One of the important factors affecting the corrosion resistance and the number of adhering bacteria is surface roughness. In the present studies, smooth polished surfaces were used, which is in agreement with the dental practice, where the metal prosthesis surfaces undergo polishing, due to e.g., hygienic reasons. Full characteristics of the obtained layers should also consider the release of the metal ions from the substrate into the body fluids.