## Materials and Methods
### Joining Experiments  
For joining, the heating of the samples was carried out in air by means of induction. The influence of different materials and process variations on the microstructure of the joints was investigated. Different nanopastes were used, in which the nanoparticles are suspended in solvents. Moreover, dispersing agents were added because they can prevent agglomeration of the particles. A first series of experiments was performed with a Ni nanopaste. The composition of this paste, which was custom-made, is shown in Table 2. The preparation of the paste is described in Reference [26]. For joining with this nanopaste, a joining temperature of 650 \({}^{\circ}\)C, a joining pressure of 20 MPa, and a holding time of 10 min were used. \begin{table}
\begin{tabular}{c c c} \hline Particle size [nm] & Ni & 10–100 \\ Metal content [wt\%] & Ni & 49.0 \\ \hline \multirow{4}{*}{Content of organic substances [wt\%]} & \(\alpha\)-Terpineol & 24.5 \\  & p-Xylene & 24.5 \\ \cline{1-1}  & 1-Tetradecanethiol (CH\({}_{3}\)(CH\({}_{2}\))\({}_{13}\)SH) & 1.0 \\ \cline{1-1}  & 1-Octadecanethiol (CH\({}_{3}\)(CH\({}_{2}\))\({}_{17}\)SH) & 1.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of the Ni nanopaste. Figure 4: Sample geometry for joining experiments. Overlap: 7.5 mm, sheet thickness: 3 mm. Figure 3: T-t-regime of the heat treatment used for preparation of all Q&P steel samples. In further experiments, a commercially available Ag nanopaste (Harima Chemicals, Inc., Nagoya, Japan) was used for joining. The composition of the Ag nanopaste is shown in Table 3. Joining with this paste has already been investigated in References [24; 25]. For joining with this nanopaste, temperatures of 250 \({}^{\circ}\)C and 300 \({}^{\circ}\)C were tested with a holding time of 10 min and a joining pressure of 40 MPa. Furthermore, in addition to the Ag nanopaste, low-melting materials were used at the edges of the substrates as an activating material to achieve an improved bonding. The enhanced bonding in this case was obtained by the melting process of the activating material, in contrast to the use of nanomaterials that usually only bind by sintering processes, i.e., in a solid phase process [24; 25]. The use of activating materials results in a kind of sandwich structure of the joining seam (as shown schematically in Figure 5). The activating material only needs to fulfill the function of bonding between filler material and substrate and should therefore be as thin as possible. In contrast, the Ag nanopaste should be thicker than the activating material since the significantly improved properties of Ag, in particular strength and thermal stability, shall dominate the properties of the joining seam. It was also envisaged that the molten material could fill the pores that were formed by the sintering process of the nanopaste [24; 25]. Thus, a denser joining seam might be achieved, which would have a positive effect on the strength of the joints. In a first step, various SnAg solders were examined as activating material. The solders (available in the initial state as wire) were rolled into foils with thicknesses of approximately 20 \(\upmu\)m, so that the foils could be applied directly on the surface of the substrate. For the experiments, three different solders with and without flux and with different holding times were tested (Table 4). The joining temperature was 230 \({}^{\circ}\)C, and thus below the last step of the Q&P heat treatment, so that an influence on the steel's microstructure is unlikely. In the second step, pure Sn wires were rolled into foils with a thickness of approximately 20 \(\upmu\)m and were used as activating material with an increased joining temperature of 237 \({}^{\circ}\)C. In addition, different holding times and various thicknesses of the nanopastes were tested, see also Table 4.  
For preparation of the specimens, the Ag nanopaste was applied on one side of the foils (Sn and SnAg) with the thickness given in Table 4 using a screen-printing process.