# Carbon Coatings Deposited on Prosthodontic Ni-Cr Alloy
## 3 Results  
Figure 1 presents the surface of a sample with a carbon coating and an EDS spectrogram made on that surface, whereas Figure 2 shows a microscopic image of the cross-section of the sample together with the surface distributions of carbon, titanium, nickel and chromium.  
Microscopic observations showed that carbon layers on prosthodontic Ni-Cr alloy are solid and tight, without surface defects. X-ray microanalysis showed that only carbon exists in the examined layers, there are no other impurities. Therefore, this layer may be considered as a pure carbon layer.  
Figure 3 presents typical Raman spectra of manufactured carbon coatings with their deconvolution employing four-peak fitting method. Namely, D1, D2, G1 and G2 Gaussians, according to the literature [36]. The obtained de-convolution results resemble those, typical for diamond-like carbon coatings (DLC), widely presented in the literature [37].  
Figure 4 presents hardness the carbon coatings in the function of the depth of the forced-in penetrator. The hardness of layers amounted to 20-22 GPa. The hardness of the alloy amounted approximately 3 GPa.  
Adhesion in the examined layers amounted to 150 mN. These values can be considered as entirely satisfactory. During exploitation of elements covered with carbon layers, layer should not be separated from a substrate.  
Figure 1: Surface of a sample with a carbon coating (**a**) and an EDS spectrogram from the sample surface (**b**).  
Figure 2: _Cont._Figure 3: Raman spectra of examined samples.  
Figure 2: Microscopic image of a sample’s cross-section with the surface element distributions (**a**) microscopic image, (**b**) surface distribution of carbon, (**c**) surface distribution of titanium, (**d**) surface distribution of nickel, (**e**) surface distribution of chromium.  
Figure 5 shows exemplary current-potential characteristics of samples with a carbon layer (1) and a sample without a coating (2). It is clear from the presented characteristics that the potential at which the process of intensive corrosive pulping of the passive layers begins is visibly shifted towards the anodic side for samples with carbon layers, compared with those without coatings. The linear density characteristics of the current intensity j from the polarization potential E were used to determine the breakthrough potentials of the passive layers Eb. The mean breakthrough potential values have been given in Table 3.  
Figure 6 shows microscopic images of the corrosion areas after the tests.  
Figure 4: Hardness in the function of the depth of the forced-in penetrator.  
Figure 5: Exemplary current-potential characteristics; 1–Ni-Cr alloy sample with a carbon layer, 2–Ni-Cr alloy sample without a coating.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Mean Value** & **Standard Deviation** \\ \hline Ni-Cr with a coating & 0.619 V & 0.010 \\ Ni-Cr without a coating & 0.823 V & 0.003 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Mean values of breakthrough potentials of the examined samples Eb.  
After the corrosion examinations, corrosion pits appeared on the surface of the samples, 50-100 m in size. An effect of the intensive corrosive processes is also removal of the carbon layer from the area near the formed pit, which is visible in Figure 6b.  
Figure 7 shows exemplary micro-images of the surface of the examined samples with adhering bacteria. Table 4 presents the number of bacteria adhering to the surfaces of the particular groups of samples.  
The results were subjected to a statistical analysis, which used the one-way analysis of variance test (ANOVA) without repetitions (\(p<0.05\)). For the evaluation of the normality of the distribution of the examined variables, the W Shapiro-Wilk test was applied, while, for the evaluation of the variance homogeneity--the Levene test. The performed statistical analysis demonstrated that the differences in the number of adhering bacteria on the surfaces of the samples with and without a carbon layer were statistically significant.