# Carbon Coatings Deposited on Prosthodontic Ni-Cr Alloy
## 4 Discussion  
Providing the coatings applied on prosthetic elements with the proper protective properties first of all requires the appropriate density (no cracks, scratches, pores, etc. ), \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Average Value** & **Standard Deviation** \\ \hline Ni-Cr with a coating & 84 & 10 \\ Ni-Cr without a coating & 173 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The number of bacteria adhering to the surfaces (150 \(\times\) 100 m) of the particular groups of samples. Figure 6: Image of the pits after corrosion tests, (**a**) sample without a coating, (**b**) sample with a coating. Figure 7: Sample surfaces with adhering bacteria (**a**) Ni-Cr alloy sample without a coating. (**b**) Ni-Cr alloy sample with a carbon layer. which will ensure anticorrosive properties. Fulfilling this condition will provide protection from the formation of corrosion products and their release into the oral cavity, as well as from ions transferred to it from the metallic substrate. The microscopic observations showed that the examined carbon coatings on prosthetic alloy Ni-Cr are monolithic and hermetic (Figures 1 and 2), without surficial defects. Here, there are also no cracks, pores or delaminations. Therefore, it is possible to assume that they can provide the proper density, thus constituting a barrier separating the environment of the oral cavity of the patient from the alloy from which the prosthetic elements are made. The X-ray microanalysis demonstrated that, in the examined coatings, only carbon is present (Figure 1b), without any other additives. Therefore, we can state that this is a pure carbon coating. Unfortunately, based on the performed chemical composition tests, it is impossible to clearly establish the type of the coating. However, the Raman spectrometry made it possible to state that, in the examined coating, both nanocrystalline diamond and nanocrystalline graphite are present. On the cross-sections of the samples (Figure 1), one can clearly see a titanium interlayer located between the carbon layer and the substrate. Protective coatings should also characterize in other properties so that they can fulfil their role. Such properties include the wear resistance. The latter mostly depends on the coatings' hardness. The higher the hardness, the higher the wear resistance. The examined coatings characterize in hardness of about 20 GPa, with the hardness of the substrate alloy of about 3 GPa. The values obtained by other authors are similar [22; 38]. The obtained hardness values of the test samples make it possible to provide significant wear resistance. While the wear resistance was not tested, it can be inferred from the reports of other authors that carbon coatings of this type characterize in high wear resistance, which is much higher than that of the substrate alloy. This is an additional advantage behind their use for prosthetic elements. There are not many literature reports referring to carbon coatings on prosthetic elements, yet tests performed on other alloys being applied in e.g., orthopedics exhibit elevated resistance of this type [22; 38; 39; 40]. It should be noted that many alloys used in orthopedics have a composition which is very similar to that of the alloys applied for prosthetic and orthodontic elements used in the oral cavity. The operation conditions of e.g., hip joint endoprostheses are significantly harder than those of dental prosthetic restorations. In the case of protective coatings, it is very, or even the most, important to provide them with the proper adhesion to the substrate, as only this will make it possible to fully take advantage of their properties. Even the best coatings which do not possess sufficient adhesion to the substrate are incapable of properly fulfilling their role. This is so, as each spalling of the layer causes exposition of the substrate material and loss of tightness, with all its consequences. In the examined layers, the adhesion equaled about 150 mN.