## 4 Results  
### SEM Imaging  
Scanning electron microscopy (SEM) showed the morphological appearance of zirconia samples surfaces (Figures 2 and 3).  
Veneered and polished zirconia samples (F1) had a slightly irregular surface, with microcracks and visible particles on the surface. Barely visible horizontal lines might indicate the direction of polishing. However, veneered and polished zirconia after ultrasonic scaling (F1a) showed an uneven surface with pores and grooves of various sizes, cavities and chipping-type defects, which average value was 55.59 \(\upmu\)m (highlighted by yellow arrow). On the verneered zirconia samples treated with brushing (F1b), small surface defects and groovelike formations were evident (yellow arrow). On the contrary, veneered and glazed zirconia samples (F2) showed a relatively regular surface texture with whitish particles without voids. The veneered and glazed samples treated with ultrasonic scaling (F2a) showed the most surface irregularities in the form of large defects, with an average value of 88.31 \(\upmu\)m. The layered breaking of the material was evident. On the sides of the defects (yellow arrow) remained a porous surface. On the surfaces of the veneered and glazed samples after brushing (F2b), there were visible protrusions and traces of scratches  
Figure 2: Representative SEM images of the veneered zirconia samples: F1—zirconia veneered and polished samples; F1a—zirconia veneered and polished samples treated with ultrasonic scaling; F1b—zirconia veneered, polished and brushed samples; F2—zirconia veneered and glazed samples; F2a—zirconia veneered and glazed samples treated with ultrasonic scaling and F2b—zirconia veneered, glazed and brushed samples.  
caused by brushing (yellow arrow). The entire surface was undulated with round-topped particules (Figure 2).  
Figure 3 shows a SEM examination of the CAD/CAM milled zirconia samples. The CAD/CAM milled and polished zirconia samples (C1) revealed an eroded surface that was created by samples made using the manufacturer's techniques and highlighted by a yellow arrow. These changes were in the forms of two types of traces, one coarser and the other finer. CAD/CAM milled and polished zirconia samples treated with ultrasonic scaling (C1a) showed a rough surface in the form of parallel scratches. Visible irregularities were those created by the effect of ultrasonic scaling (rougher, blue arrow) and others created by the effect of milling (mild, yellow arrow). As for the CAD/CAM milled and polished zirconia samples after brushing (C1b), there were slight traces, which were probably the result of the action of the brush (yellow arrow). CAD/CAM milled and glazed zirconia samples (C2) showed a slightly smoother surface layer with pale dimples and poured glaze drops (yellow arrow). On the surface of CAD/CAM milled and glazed zirconia after ultrasonic scaling (C2a), extreme irregularities in the forms of scratches and defects were visible, with a mean value of 45.38 \(\upmu\)m. CAD/CAM milled and glazed zirconia samples after brushing (C2b) showed a porous surface with black and white holes. The black holes (blue arrow) had uneven bottoms and were not covered with glaze, as the glaze remained only on the edges. It could be assumed that the tip of the bristles of the brush and the applied abrasive polishing paste broke off parts of the glaze.  
The values of the surface defects of the various groups of samples are shown in Table 2. The highest defect mean value was observed for the zirconia glazed samples treated with ultrasonic scaling, groups F2a (88.31 \(\upmu\)m) and C2a (45.38) samples.  
Figure 3: Representative SEM images of the CAD/CAM milled zirconia samples: C1—zirconia milled and polished samples; C1a—zirconia milled and polished samples treated with ultrasonic scaling; C1b—zirconia milled, polished and brushed samples; C2—zirconia milled and glazed samples; C2a—zirconia milled and glazed samples treated with ultrasonic scaling and C2b—zirconia milled, glazed and brushed samples.