## 3 Results and Discussion  
### Joining without Activating Materials  
For joining without activating materials, a Ni nanopaste and an Ag nanopaste were used. Relatively high temperatures of at least 650 \({}^{\circ}\)C are required for joining with Ni nanopaste [26]. Even if an influence on the Q&P microstructure was to be expected when using such high joining temperatures, the basic feasibility of joining with a Ni nanopaste should still be examined. Figure 6 shows that joining of the Q&P steel with a Ni nanopaste is basically possible (joining temperature: 650 \({}^{\circ}\)C, holding time: 10 min, joining pressure: 20 MPa). A bonding can be successfully achieved. However, the optical micrographs already show that the microstructure of the Q&P steel is influenced considerably by the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Activating Material**} & \multirow{2}{*}{
\begin{tabular}{c} **Flux (According to** \\ **DIN EN ISO 9454-1)** \\ \end{tabular} } & \multicolumn{3}{c}{**Parameters for Joining**} \\ \cline{3-5}  & & **Temperature** & **Holding Time** & **Thickness of Paste** \\  & & & & **Application** \\ \hline Solder SnAg3.5 & Without flux & 230 \({}^{\circ}\)C & 30 min & 20 \(\upmu\)m \\ Solder SnAg4 & 2.2.3 & 230 \({}^{\circ}\)C & 10 min & 20 \(\upmu\)m \\ Solder SnAg5 & 2.2.2 & 230 \({}^{\circ}\)C & 10 min & 20 \(\upmu\)m \\ Sn (purity: 99.5 wt\%) & Without flux & 237 \({}^{\circ}\)C & 3 min & 40 \(\upmu\)m \\  & & & 5 min & 80 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table}
Table 4: Specifications of the activating materials and parameters used for the joining process.  
Figure 5: Structure of the joining seam (schematic) with the use of activating materials before joining.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Thickness of PVD Sn Coating**} & \multirow{2}{*}{**Joining Temperature**} & \multirow{2}{*}{**Holding Time**} & \multicolumn{2}{c}{**Thickness of Paste**} \\  & & & **Application** \\ \hline
10 mm & 237 \({}^{\circ}\)C & 3 min & 20 \(\upmu\)m \\
20 \(\upmu\)m & & & 5 min & 20 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table}
Table 5: Parameters for the joining process using physical vapor deposition (PVD) Sn coatings.  
joining process (Figure 6) compared to the microstructure in the initial state (Figure 7). Therefore, joining of Q&P steels with the Ni nanopaste is not expedient.  
Therefore, in further experiments, joining with an Ag nanopaste was investigated, which allows for significantly lower joining temperatures [24; 25]. Joining tests with the Ag nanopaste were carried out at temperatures of 250 \({}^{\circ}\)C and 300 \({}^{\circ}\)C with a holding time of 10 min and a joining pressure of 40 MPa. These parameters have been successfully used to join copper as substrate [24; 25]. However, when using Q&P steel as substrate, no bonding could be achieved. This is most likely a consequence of the immiscibility of Ag and Fe. Moreover, when joining with nanoparticles, sintering processes have a bigger influence than melting processes [24; 25], so that diffusion is limited. Therefore, the addition of different activating materials was investigated in further experiments, which were supposed to lead to an improved bonding between filler material and substrate.