## 4 Discussion
### Impact of Plasma Post-Processing
Plasma-induced surface cleaning by the removal of carbon can also be expressed in the form of surface coverage as shown in Figure 8. Here, the surface coverage as given by the ratio of carbon and the glass component silicon (C/Si) is shown. Silicon was chosen for calculating this ratio since silicon dioxide is the main compound of the investigated glass, representing 60-70 wt % of the entire glass weight [37]. Figure 8: Comparison of the surface coverage by carbon (i.e., the ratio of carbon and silicon) of zinc crown glass samples before and after plasma post-processing vs. polishing suspension concentration including trend (dotted line). It turns out that a high surface coverage is found for untreated samples where the amount of carbon is even higher than the amount of silicon since the surface coverage value exceeds one in any case. It can further be seen that the degree of surface coverage decreases with increasing polishing suspension concentration (compare also Figure 4). After plasma post-processing, the surface coverage approaches a saturation value of approximately 0.3, corresponding to a total content of carbon of about 7.82% on average. This value does not depend on the initial carbon content before plasma treatment and is thus independent on the polishing suspension concentration. It can consequently be stated that surface-adherent carbon was efficiently removed in all cases in the course of plasma post-processing. However, there is a residual constant carbon content which was not removed by the plasma. It can be assumed that this remaining carbon is given by contaminants from the used tools and working materials which have accumulated within digs and micro cracks underneath and/or within the hydrated silica gel layer. The plasma is thus capable of removing surface-adherent contamination by carbon such as wear debris from the polyurethane polishing pad or residues from cleaning agents but does not affect deeper regions of the polished glass surface. Even though the final amount of remaining carbon after plasma post-processing does not depend on the initial carbon content, the LIDT shows a certain dependency on this value and the polishing suspension concentration, respectively, where the LIDT increases with increasing polishing suspension concentration. This indicates that the observed increase in LIDT does not exclusively follow from plasma-induced removal of surface-adherent carbon, but further subsurface effects and mechanisms are involved additionally. Such possible mechanisms are constructive interference of incident laser light, an increased absorption and a reduced mechanical strength of the glass due to the presence of micro cracks [3] where absorbing and scattering residues from working materials can moreover accumulate [4]. It is known that the crack density increases with increasing polishing suspension concentration [55]. It can thus be assumed that the crack depth features the same behaviour since, in the case of increasing polishing suspension concentration, the high number of polishing grains between the polishing pad and the glass surface leads to a reduction in local pressure and load. Consequently, surface stress as expressed by the stress intensity factor \(K\) is reduced as shown by _[PERSON]_ and co-workers [56]. Since this factor is inversely proportional to the crack depth [57], a higher crack depth and crack volume arise in this case, so the available volume for the accumulation of laser light-absorbing contaminants is increased. Based on this theoretical approach, the LIDT should finally feature a systematic dependency on the polishing suspension concentration. A trend towards this behaviour was observed in the present work, see Figure 4. One has to consider that this dependency was not only observed for polished and plasma post-processed samples, but also for polished ones, see Figure 6. This fact supports the assumption that the polishing suspension concentration and the resulting crack density may play an additional and essential role for the observed behaviour of the LIDT.