## 4 Discussion
As already described in other work [8] and shown in Figures 5-7 an increase in shear strength caused by the annealing process can be stated, which is a quite well known effect. While after plasma treatment and curing at room temperature the shear strength increased by a factor of two, after performing the annealing process it increased again. For uncoated optics the shear strength increased with increasing annealing temperature as can be seen in Figure 5. The NPBS-coated optics reached a higher shear strength with an annealing temperature of 100 \({}^{\circ}\)C than PBS-coated optics does with an annealing temperature of 200 \({}^{\circ}\)C. One explanation for this is the greater overall layer thickness of the PBS coating and its material selection. This combination can lead to the formation of layer stresses, which worsen the adhesion of the layer to other surfaces during bonding or optical contacting. Moreover, the particular temperature-dependent increases do not confirm the results observed for untreated fused silica. This behaviour could be attributed to differences in composition of the outermost and thus contacting surfaces: in the case of coated optics, a high purity of the deposited silicon dioxide layers is achieved. Classically polished fused silica surfaces are used for the preliminary investigations; however, they feature a significantly different chemical composition of the near-surface material. In the course of the manufacturing process, residues from the used tools and operating materials such as polishing agents are implanted into the surface [33; 34; 35; 36] where the lateral distribution of such contaminants can even be strongly inhomogeneous [37]. As a result, the impact of plasma treatment is most likely different for coated surfaces on the one hand and uncoated, polished surfaces on the other hand. Moreover, differences in cleanliness of the initial surfaces may further contribute to different efficiencies of plasma treatment. The process developed and reported here offers the possibility of bonding even very sensitive surfaces such as functional coatings gently and permanently without joining materials. In order to further expand the application possibilities, the transferability of the process to other coatings and layers should be investigated in further studies. Furthermore, with regard to material compatibility, it should be investigated whether the process can be transferred from planar optics to spherical optics such as achromatic doublets. When transferring the method, the different expansion coefficients of the involved glasses must be taken into account, which can lead to loosening of the bond during the curing process. If the process can be transferred to other glass materials and even to different glass pairs, it creates quite a number of new possibilities in optics manufacturing. Finally, it should be noted that the bond surfaces were additionally inspected for defects using microscopy and spectroscopy. No significant defects that would impair the use of the plasma-treated and bond optics were found across all samples. Since the surfaces are in specification with respect to the requirements for cemented optics, the presented approach fulfils its intended purpose of offering an alternative solution to the classical cementing process.