# Carbon Coatings Deposited on Prosthodontic Ni-Cr Alloy
## 2 Materials and Methods  
The possibilities of using protective coatings on metal surfaces are determined both by their biological and mechanical properties. On the one hand, protective coatings should reduce the metal alloy corrosion as well as the release of harmful corrosion products and metal ions from the alloys, being harmless themselves as well, and on the other hand, they should exhibit sufficiently good mechanical strength and adhesion, in order to remain on the surface of prosthetic restorations for at least a few years. That is why the present study mainly focuses on investigating those properties of carbon coatings. Cylindrical samples made of Ni-Cr alloy, 8 mm in diameter and 7 mm in height, were prepared. The chemical composition of the alloy, determined using a fluorescent X-ray analysis method on a SIEMENS SRS 300 X-ray spectrometer, is shown in Table 1 surfaces of the specimens were subjected to grinding followed by mechanical polishing (Ra = 0.31 \(\upmu\)m). The samples were coated, by means of the magnetron sputtering method, with an interlayer of titanium, about 200 nm thick, and next, with the use of the RF PACVD method, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Element Percentage by Weight (wt.\%)**} \\ \hline Cr & Mo & Si & Fe & Co & Mn & Ni \\
24.79 & 8.89 & 1.57 & 1.33 & 0.17 & 0.13 & rest \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of Ni-Cr alloy. with a carbon layer, about 1000 nm (1 \(\upmu\)m) thick. The reaction chamber was pumped down to a pressure of 2 \(\times\) 10\({}^{-3}\) Pa. Deposition process provided 10 min etching in Ar plasma with self-bias--720 V. Process parameters are the following: power [W] for RF PACVD amounted to 50-150, power [W] for DC MS amounted to 2400-2700, methane flow [sccm] for RF PACVD and DC MS amounted to 0-30, argon flow [sccm] for RF PACVD amounted to 0-12, argon flow [sccm] for DC MS amounted to 9. The deposition time of Ti interlayer was 12 min, whereas the final DLC coating was synthesized for 30 min. More details regarding the deposition technology of gradient Ti-C coatings can be found elsewhere [30]. The samples prepared in this way were then subjected to the following tests:  
* Observation of the surface and the cross-section of the sample by means of an electron scanning microscope HITACHI S-3000N. * Determination of the chemical composition of the coating with surface element distribution by the EDS method, by means of the NORAN INSTRUMENTS system cooperating with an electron scanning microscope HITACHI S-3000N. * Chemical structure of the synthesized coatings was determined using inVia confocal micro-Raman spectrometer (Renishaw), working with 532 nm wavelength and power of 2.5 mW. * Mechanical properties. Hardness were determined using Berkovich indenter trihedral pyramid shape with an angle equaling 65.3\({}^{\circ}\) and the investigation was performed according to ISO Standards [31] on a MTS NANO INSTRUMENTS G-200 nanoindeter. The results were registered and analyzed using the TestWorksPro 4 Software. The measurement was performed with the CSM (continuous stiffness measurement) [32; 33] method. * Adhesion to substrate. Examinations were carried out by the scratch test method using a sapphire cone-shaped penetrator [34]. Rounding radius of the tip was 1 \(\upmu\)m. A 2 mm scratch was made by gradually increasing penetrator load of 0 to 200 mN. * Corrosion tests--conducted in a chemical environment of deoxidized artificial saliva in the form of a Fusayama-Meyer solution with the composition shown in Table 2 at 37 \({}^{\circ}\)C in the ST1 thermostatic chamber (POL-EKO), the measurements were carried out on the PGSTAT 30 galvanostat (Autolab) with GPES and FRA control software v. 4.9. * Corrosion tests in artificial saliva with microscopic examinations of the corrosion areas. The tests were carried out on five samples from each group. * Bacterial adhesion on the surface of the samples.