## 3 Results and Discussion
### Joining with Activating Materials  
#### 3.2.1 Joining with SnAg Solders as Activating Material  
Low melting materials were used as activating material in combination with the Ag nanopaste to achieve an enhanced bonding through melting of the activating material and miscibility between substrate and activating material. In a first step, SnAg solders were investigated as activating materials. Three different solders with and without flux were tested (Table 4). Joining was again carried out at a temperature of 230 \({}^{\circ}\)C, i.e., below the last step of the Q&P heat treatment. For the joining experiments,  
Figure 6: Light microscopy image of the Q&P steel joined with a Ni nanopaste (joining temperature: 650 \({}^{\circ}\)C, holding time: 10 min, joining pressure: 20 MPa). Figure 7: Light microscopy image of the Q&P steel in the initial state. different holding times were tested (Table 4). The joining process with the activating materials was performed without pressure. Figures 8-10 show optical micrographs of the joints with the different solders. The micrographs demonstrate that bonding can be achieved with all three solders. When using the solder SnAg3.5 without flux (Figure 8), a comparatively long holding time of 30 min was chosen to achieve a sufficiently good bonding to the Q&P steel even without flux. However, this long holding time leads to a distinctive formation of intermetallic compounds (IMC) in the joining seam. On the one hand, EDX analyses show that these are the typical Ag\({}_{3}\)Sn phases (Figure 11). On the other hand, FeSn\({}_{2}\) phases also formed due to the long holding time, and thus the possibility of intensive diffusion between joining seam and substrate. Compared to Ag\({}_{3}\)Sn, much more FeSn\({}_{2}\) phases formed. In comparison to the other two SnAg solders SnAg4 and SnAg5, where a short holding time of 10 min was used, with SnAg3.5 more IMC formed due to the long holding time of 30 min (Figures 8-10). Therefore, it must be assumed that the mechanical properties were negatively influenced when using a long holding time. In future investigations, an optimum holding time needs to be identified, with the goal of obtaining a minimized formation of IMC but also a sufficient bonding to the substrate. Figure 12, which depicts the interface between substrate and joining seam (solder: SnAg3.5), clearly shows that good bonding can be achieved even without flux. Pronounced diffusion processes take place between activating material and substrate--even in areas far from the substrate, the joining seam contains approximately 2 at% Fe. In the substrate, an interesting diffusion behavior along the  
Figure 8: Light microscopy image of a joint with SnAg3.5 (without flux) (joining temperature: 230 \({}^{\circ}\)C, holding time: 30 min). Figure 9: Light microscopy image of a joint with SnAg4 (with flux 2.2.3) (joining temperature: 230 \({}^{\circ}\)C, holding time: 10 min). typical Q&P structure can be observed, Figure 12. Sn from the solder can be detected in the substrate up to a depth of approximately 4 \(\upmu\)m. This pronounced diffusion indicates that the holding time can be reduced to minimize the formation of brittle phases. Therefore, in further tests the holding time was varied (see Section 3.2.2) to achieve an optimum between minimized formation of IMC and sufficient bonding. Figure 11: SEM image of a joint with SnAg3.5 as activating material (without flux; joining temperature: 230 \({}^{\circ}\)C, holding time of 30 min). Figure 10: Light microscopy image of a joint with SnAg5 (with flux 2.2.2; joining temperature: 230 \({}^{\circ}\)C, holding time: 10 min). The marks in Figure 13 show residues of the Ag sintered seam (activating material: SnAg3.5). Contrary to the assumption that the molten solder might fill the pores of the sintered seam, the sintered Ag material is distributed in the liquid solder. In addition, Ag does not dominate the joining seam.