## 4 Conclusions  
A detailed analysis of the thermal behavior of thin films of tungsten nitride in vacuum was conducted. The main purpose was to assess their viability as constituents of solar coatings, as well as active and robust materials for capacitive energy storage and plasma-facing uses.  
Thin films were annealed in vacuum at medium-to-high temperatures (470 \({}^{\circ}\)C, 580 \({}^{\circ}\)C) and characterized by means of X-ray diffraction (XRD), AFM, micro-Raman, FTIR, UV-VIS-NIR, sheet, surficial energy and wetting angle measurements. The overall set of analyses  
Figure 12: WCA measurements of annealed WN\({}_{\mathrm{x}}\) samples, (**a**) WN300, (**b**) WN500 and (**c**) WN700.  
evidenced some modifications and differences between the samples (which would be useful for selecting them for specific applications) and, at the same time, provided insight into the mechanism of nitridation occurring in a plasma reactor, where kinetic constraints modulate the reduction of nitrogen to nitride compounds by means of sputtered W metallic clusters' ability to adsorb, form complexes with and react with the strong N\({}_{2}\) triple bond.  
After annealing, the 300 W film, which was of poor crystalline quality as deposited, released nitrogen entrapped in the lattice and kept its W\({}_{2}\)N structure until 580 \({}^{\circ}\)C. In spite of there being no phase transition, there was an increase in sheet resistance but overall optical reflectance stability, even though they were part of the cermet. The 500 W film proved to be the best material for solar thermal applications, having a crystalline structure that remained stable in the desired range and a metallic character that was unmodified by increasing temperature. The 700 W film, whose structure as deposited was almost amorphous, gave raise to modifications after annealing, both as a thin film and as part of a cermet. In facts it crystallized and disproportionately to metallic tungsten and W\({}_{2}\)N.  
All materials were slightly hydrophobic, with varying wettability with respect to AlN. This behavior can be useful for spheroidizing tungsten nitrides and obtaining good cermet materials. The study of thermal behavior suggested that the thermal evolution of a sputtered WN\({}_{x}\) film is strictly dependent on the dimensions of the W clusters obtained during the fabrication process, which in turn depend on process parameters and hence can be controlled. The thermodynamically favored phase, W\({}_{2}\)N, tends to form when energy is supplied to the system, but if the nitrogen content is low, a concomitant disproportion can occur with bcc W lattice formation and nitrogen molecule evolution. The redox process seems to be reversible and, in such a direction, a potential catalytic application of thin films as a coating for water-splitting electrodes can be considered.  
Conceptualization, A.C. and E.G.; methodology, A.C. and E.G.; data curation, E.G. and L.L.; writing--original draft preparation, A.C.; writing--review and editing, A.C. All authors have read and agreed to the published version of the manuscript.  
The results presented in this paper have been obtained within the framework of the project \"SOLARGID: Sistemi sOlari termodinamici e fotovoLtaici con Accumulo peR co-GeneRazlone e flessibilita Di rete\"--cod. ARSO1_00532, jointly funded by the European Union and Italian Research and University Ministry (MUR) under the Programa Operativo Nazionale \"Ricerca e Innovazione\" 2014-2020 (PON \"RKI\" 2014-2020).  
Not applicable.  
The authors declare no conflict of interest.