**3. Results**

#### *3.1. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM)*

According to Figure 1, the AFM analysis showed an irregular surface topography in non-coated samples with roughness Rq 9.58. In contrast, SiC-coated samples were slightly smoother with Rq 7.98 (*p* > 0.05). Non-coated, SiC-coated, and glazed disks demonstrated significant qualitative di fferences in surface roughness (Figure 2A–F).

**Figure 1.** Roughness means values and standard deviation in non-coated (reference) and coated (ref SiC) samples (**A**). Atomic force microscopy (AFM) images of surface topography of non-coated (**B**) and coated SiC (**C**), with amplifications of 10 μm.

**Figure 2.** SEM images of roughness of non-coated and coated samples. (**A**,**D**) Non-coated fluorapatite disk, (**B**,**E**) glazed fluorapatite disk, and (**C**,**F**) SiC-coated fluorapatite disk.

#### *3.2. Water Contact Angle Measurements (WCAs)*

The outcomes indicate different WCAs between the non-coated and coated surfaces (Table 1). The coated SiC surface presented higher contact angles than the non-coated surface (*p* > 0.05).



#### *3.3. Bacterial Growth*

After 24 h, the amount of polymicrobial biofilm (*S. mutans* and *S. sanguinis*) was less on the SiC-coated disks. The biofilm coverage on these disks was 16.9%, whereas uncoated reference samples showed a significantly higher biofilm coverage of 91.8% (*p* < 0.0001) (Figure 3).

**Figure 3.** Live coverage of *Streptococcus mutans* and *Streptococcus sanguinis* after 24 h of culture on the non-coated (control) and coated surfaces.

The fluorescence images in Figure 4 demonstrate higher polymicrobial biofilm (*S. mutans* and *S. sanguinis*) formation on the control group than on the coated group. The SEM images confirmed the results from the live assays, showing a reduction in the number of adherent biofilms on the coated group for *S. mutans* and *S. sanguinis* after 24 h of culture (Figure 5).

**Figure 4.** Live fluorescence images of *S. mutans* and *S. sanguinis* cultured for 24 h on the non-coated (**A**,**B**) and coated (**C**,**D**) surfaces. The cultures were stained with SYTO 9 to dye the living bacteria green.

There was a significant reduction in the number of CFU/mL after 24 h contact with ceramic SiC coating compared to the control group (Figure 6), showing *p* = 0.0003 for *S. sanguinis* and *p* ≤ 0.0001 for *S. mutans*. The reduction of the monomicrobial biofilm of *S. sanguinis* and *S. mutans* were also similar (*p* = 0.2528).

**Figure 5.** SEM adhesion of polymicrobial biofilm of *S. mutans* and *S. sanguinis* after 24 h of cultivation on non-coated (**A**,**B**) and coated (**C**,**D**) ceramic disks.

**Figure 6.** Mean (±SD) of colony-forming units (CFU)/mL of monomicrobial biofilms of *S. mutans* (**A**) and *S. sanguinis* (**B**) presented in the non-coated (control) and coated groups.

#### *3.4. Biocompatibility Testing*

In order to determine whether the SiC coating presented biocompatibility, the cytotoxicity was evaluated by the CellTiter-Blue assay. After 24 h of HPdLF cultivation on the samples, no obvious cytotoxicity was observed, as evidenced by the absorbance of the cells cultured on SiC coating being comparable to the control group (*p* = 0.3904) (Figure 7).

**Figure 7.** Cytotoxicity of the non-coated (control) and coated groups after 24 h of human periodontal ligament fibroblasts (HPdLF) culture assessed by CellTiter-Blue absorbance.

The SEM images showed the interaction of cell extensions with the SiC-coated and non-coated surfaces. SEM results demonstrated that cells adhered and covered the full surface of the disk after 24 h in culture. Additionally, the cell morphology was similar for cells cultured on SiC-coated and non-coated samples, where HPdLFs appeared oval-shaped and flattened on the surface (Figure 8).

**Figure 8.** SEM images showing the adhesion of HPdLF on non-coated (**A**,**B**) and coated (**C**,**D**) surfaces after 24 h of incubation.
