## 6 Conclusions  
Two different photothermal methods for the determination of the thermal parameters of PBMA coatings were applied, i.e., PTR and BDS. The first goal of the investigation was to determine the set of thermal parameters of the PBMA samples, i.e., thermal diffusivity, thermal conductivity, and thermal effusivity. The second goal of the investigation was to determine the changes in thermal parameters with the change in parameters for annealing the samples. The third goal of the investigation was to check the correlation between the values of the thermal parameters of the samples obtained with these two nondestructive methods. All these goals were achieved and are presented in this paper. Two different thermal models of the PTR signal have been discussed. The appropriate thermal model of the PTR signal of the thin, optically transparent coating on a thick, optically and thermally opaque substrate was established. It was proved that this model can also be applied for further investigation of the thermal parameters of other thin, optically transparent coatings deposited on optically and thermally opaque substrates investigated by the PTR method.  
The changes in the thermal parameters of the coating after annealing at various temperatures were examined experimentally. The values of the thermal parameters (thermal conductivity, thermal diffusivity, and thermal effusivity) of a series of corrosion-protective coatings on structural steel were presented. These parameters were extracted using the PTR and BDM methods in the frequency mode. It turned out to be possible to simultaneously determine the thermal diffusivity and thermal conductivity of the coatings. This was accomplished by fitting the theoretical to the experimental amplitude and phase characteristics of the PTR and BDS signals in the frequency domain.  
It can be seen that both the thermal diffusivity \(\alpha\) and the thermal conductivity \(\lambda\) increased with the increase in the value of the annealing temperature. The thermal diffusivity, thermal conductivity, and thermal effusivity increased by 9% with the increase in annealing temperature. From the obtained results, no significant differences in the obtained values of the thermal properties determined by the two techniques were observed. The results clearly show that both the PTR and BDS methods used are attractive from the point of view of the thermal characterization of thin, optically transparent layers of the PBMA-siloxane coating. The above-described methods are very attractive from the practical point of view due to their nondestructive, non-contact character and low apparatus costs. The extracted thermal parameters provide information about the quality of anti-corrosion coatings.  
Conceptualization, L.C.; methodology, L.C. and D.K.; software, L.C. and S.J.-L.; formal analysis, L.C., M.M. and D.K.; investigation, L.C. and D.K.; resources, D.K.; data curation, L.C. and D.K.; writing--original draft preparation, L.C. and D.K.; writing--review and editing, L.C., M.M. and D.K.; visualization, L.C. and S.J.-L.; supervision, L.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the Slovenian Research and Innovation Agency, grant number P2-0393.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.