## 4 Conclusions  
In conclusion, two different but combinable types of electroless deposition techniques were tested to obtain palladium films without cracks, exfoliations, or blisters on copper substrates. Applying palladium displacement deposition, we obtained very thin metal  
Figure 6: OM images of sample (a) #6C; (b) #8C and (c) #10C are shown in the figures. For samples #6C and #8C the presence of cracks can be observed (e.g., the features highlighted by the red circles) while for sample #10C the surface results were smooth and free of defects.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Average Thickness (\(\upmu\)m)** & **Std. Dev. (\(\pm\upmu\)m)** \\ \hline \#6C & 1.59 & 0.16 \\ \#8C & 1.69 & 0.18 \\ \#10C & 1.78 & 0.06 \\ \hline \hline \end{tabular}
\end{table}
Table 5: XRF thickness determination of palladium coatings. The thicknesses of the palladium coatings derived from PDP+PCP procedure are reported in the second column.  
films with equivalent thicknesses ranging from 6.1 to 22.2 nm, measured with a SEM-EDS K-ratio method. Moreover, the possibility to plate only the metallic surface of the substrate, which is the main feature of electroless plating based on galvanic displacement, represents an interesting option in fuel cell electrodes fabrication with novel geometries in which palladium surface contours must be finely tuned.  
Palladium films obtained by displacement plating on copper were used as protective undercoat to perform palladium chemical plating from an ammonia-based bath, which otherwise could not be applied due the corrosive behaviour of ammonia for copper substrates. A high-quality palladium film with thickness of 1.78 um (sample #10C) were obtained applying the combined procedure we studied using the Pearlstein and Weightman pre-deposition (solution C). This result is proof of the integrity of the palladium coating obtained by displacement plating represents a valuable alternative for applications where thick palladium films on copper are requested. The combination of PDP + PCP studied in the present work could represent a valuable alternative to palladium electroplating in jewellery and luxury good industries, where this metal is widespread as an intermediate layer or topcoat. Palladium electroless plating is a valid alternative to electroplating thanks to the lower investment required for industrial plants and the electricity savings required for the process.  
Future developments will regard the refinement of the control of Pd thickness both in single and in combined techniques mode. Moreover, Pd film needs to be characterized more in depth from the mechanical and compositional point of view based of specific areas of application to which it will be applied.  
Conceptualization, W.G. and L.F.; methodology, W.G. and L.F.; software, W.G.; validation, F.V. and M.I.; formal analysis, W.G., L.F. and F.B.; investigation, W.G., L.F. and F.B.; resources, F.V. and M.I.; data curation, W.G., L.F. and F.B.; writing--original draft preparation, L.F.; writing--review and editing, W.G., M.B., H.M., C.F. and F.V.; supervision, M.I. and W.G.; project administration, F.V. and M.I.; funding acquisition, F.V. and M.I. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Ministero dell'Istruzione, dell'Universita e della Ricerca (PRIN \"Progetti di Ricerca di Rilevante Interesse Nazionale\", which made possible the project \"Novel Multilayered and Micro-Machined Electrode Nano-Architectures for Electrocatalytic Applications (Fuel Cells and Electrolyzers)\"), grant number 2017 YH9 MRK.  
The data presented in this study are available in the body of the text.  
PRIN \"Progetti di Ricerca di Rilevante Interesse Nazionale\" is acknowledged for the financial support.  
The authors declare no conflict of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.