## 4 Discussion  
Observations correlate well with the typical features characterizing the sintering process. Such processes are commonly divided into three stages [37]. In an initial first stage, a network of particles starts to form, marked by neck formation. The individual powder particles are still distinguishable. In this stage, the relative density of the material increases by roughly 3%. Most of the densification, as well as the most significant micro-structural changes, occur in what is termed as the intermediate, second stage, where voids form continuously pore channels along three-grain junctions. The pore channels are formed when the individual powder grains become well-bonded during the initial stage. After that, the pore channels tend to shrink due to the diffusion of the atomic vacancies away from them; however, the grain growth occurring at the same time leads  
Figure 5: OCT images for the samples obtained at higher than normal temperature in comparison with samples at normal temperature: column (**a**): Group N; column (**b**): Group H30; column (**c**): Group H50. Top row: _en-face_ OCT images of samples of Group N (**a1**), Group H30 (**b1**) and Group H50 (**c1**). Middle row, (**a2–c2**): reflectivity average of the 500 horizontal lines in 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 position where the _en-face_ images were rendered from (measured from the zero OPD). to a reduction in the total number of pores and tends to increase their average diameter and length as a consequence of coalescence. The process continues until the network of pores undergoes the so-called Rayleigh breakup [37; 38], first into progressively fewer interconnected elongated pores on the grain edges and finally into isolated pores at the grain corners; at this point, the final, third stage of sintering begins. When the density of a material exceeds 90-94% of the final (theoretical) density, the isolated pores are formed and the densification is slowed down. These can be seen in Figure 4(a1,b1) for Groups L100 and L30, respectively, as the grain density is less than in Figure 4(c1), where the material has been brought to its final, prescribed temperature, for Group N. In the former figures, the process was interrupted, while, in the latter, it has been completed, and a uniform distribution of grains was achieved; this is demonstrated by a more uniform reflectivity in Figure 4(c2) in comparison to the non-uniform reflectivity in Figure 4(a2,b2). In this third stage of sintering, the isolated pores are eliminated by the transport of mass from the grain boundary to the pore. The densification is pinned by pores and a considerable amount of energy is necessary for further densification. In this stage, the grain growth dominates the sintering process. Therefore, as the temperature is increased above the value prescribed by the manufacturer, as for Groups H30 and H50, the growth of the grains induces mechanic stress inside the material. The worst results are cracks, as seen in Figure 5(c1)--obtained for the highest temperature used for sintering (Group H50). Even for temperatures in the oven that are not so dramatically different from the normal temperature prescribed, a further increase of the grains in size results in a noticeable increase in the reflectivity in the _en-face_ image in Figure 5(b2) for Group H30. In terms of granular dispersion, the _en-face_ images were processed further in MATLAB. The result is Figure 6, which is obtained after removing the noise, so after the image is converted into a binary one; in contrast, the graphs in Figures 4 and 5 have been obtained using raw data. The results confirm the reflectivity increase, which is a consequence of the significant increase in the granulation from Group L100 (sintered at 830 \({}^{\circ}\)C) towards Group N (sintered at 930 \({}^{\circ}\)C, the normal temperature) and Group H50 (sintered at 960 \({}^{\circ}\)C).