## 3 Results  
The investigations revealed specific aspects for each of the groups considered. The acquired OCT data was evaluated in the _en-face_ imaging plane as well as in the B-scan imaging plane, which was important in order to control the depth of the investigations, as explained before.  
The following aspects were determined for the groups of samples studied:  
**Group N** (ceramic layers of the samples sintered at the temperatures indicated by the manufacturers, i.e., 930 \({}^{\circ}\)C): a normal distribution of the reflectivity could be noticed (Figure 4(c1)). This can also be seen in the MATLAB (Natick, MA, USA) image processed after obtaining a mean value of the OCT investigation (Figure 4(c2)), obtained from the reflectivity values on the \(y\)-axis for each \(x\)-position, as explained in the final part of the previous section. B-scan images of the samples from this group have not been relevant with regard to the influence of the temperature variations (Figure 4(c3)); neither are A-scans, i.e., reflectivity profiles on the \(z\)-axis. In fact, our study has demonstrated that _en-face_ OCT images, and only them, are capable of performing the proposed assessment.  
**Group L30** (samples sintered at 30 \({}^{\circ}\)C below the recommended temperature, i.e., at 900 \({}^{\circ}\)C): a decrease of the reflectivity could be seen; an alternation between reflective and less reflective areas (of band-like shapes) is noticed (Figure 4(b1)). These findings could be easily processed and highlighted on images that were processed using MATLAB (Figure 4(b2)).  
**Group L100** (samples sintered at 100 \({}^{\circ}\)C below the recommended temperature, i.e., at 830 \({}^{\circ}\)C): the _en-face_ OCT image of the investigation revealed a significant decrease of the reflectivity (Figure 4(a1)). The alternation between the reflective and less reflective bands is still visible in the image. However, the non-reflective area spreads much more in comparison with the more reflective area (Figure 4(a2)). A further degradation, in comparison with Group L30, can thus be concluded.  
**Group H30** (samples sintered 30 \({}^{\circ}\)C above the recommended temperature, i.e., at 960 \({}^{\circ}\)C): an overall increase of the reflectivity was noticed by comparing the mean value in the normal sample, Group N (Figure 5(a2)) with the mean value corresponding to the Group H30 (Figure 5(b2)). The alternation between the reflective and non-reflective areas disappears and a normal granular dispersion is identified (Figure 5(b2)).  
**Group H50** (samples sintered 50 \({}^{\circ}\)C above the recommended temperature, i.e., at 980 \({}^{\circ}\)C): fractures in the ceramic material can be identified in both the _en-face_ (Figure 5(c1)) and B-scan (Figure 5(c3)) images. The general aspect of the _en-face_ images also shows a significant gradient with an increasing slope in reflectivity (Figure 5(c2)). An important aspect should be pointed out by inspecting the images for Group H30: an approximately 40% increase in the reflectivity (concluded from the images processed in MATLAB, by comparing Group N and Group H30, i.e., Figure 5(a2,b2) have pointed out to a temperature increase of 30 \({}^{\circ}\)C inside the oven. Continuing to increase the temperature even more could result in thermal stress that leads to air accumulation inside the ceramic layer or even to fractures at those levels (see the defect D, Figure 5(c1)).  
Figure 4: OCT images for the samples obtained at lower than normal temperature in comparison with samples at normal temperature: column **(a)**: Group L100; column **(b)**: Group L30; column **(c)**: Group N. Top row: _en-face_ OCT images of samples of Group L100 **(a1)**, Group L30 **(b1)**, and Group N **(c1)**. Middle row, **(a2-c2)**: reflectivity average of the 500 horizontal lines making the images shown in the top row. Bottom row, **(a3-c3)**: B-scans OCT images corresponding to the three groups. The yellow lines indicate the depth position from where the _en-face_ images were rendered (measured from the zero OPD).