## 7 Conclusions  
A study of a GESA modified coating-substrate material was performed using transient grating excitation and laser beam deflection detection. Two types of experiments were carried out. By scanning the position of the excitation grating with respect to the detection position, the spatiotemporal thermoelastic response of the tangential component of the gradient of the normal displacement at the sample surface was obtained. Transformation to wavenumber-time domain allowed extraction of the elastic and thermal properties of the coating layer. The effective coating thickness found for the GESA modified coating was found to be consistent with the thickness range found by SEM analysis. The value of Young's modulus of the coating layer as calculated from the coating's Rayleigh velocity was in accordance with a nano-indentation result. However, as a result of fitting covariance, the relatively large uncertainty on the extracted coating density was transferred into a large most squares uncertainty on the value of Young's modulus. Narrowing this uncertainty would thus require _a priori_ knowledge of the coating's density. From the same experiment, it was possible to extract a thermal dispersion curve, provided fixing the thermal diffusivity of the substrate material at a known literature value. The fitted value of the thermal diffusivity of the GESA coating was found to be about 2 times smaller than the one of the T91 steel substrate.  
The second experiment aimed at the visualization of topographic thickness and elastic strength variations. This was achieved by scanning the coincident pump and probe beams along a two-dimensional grid spanning a surface area of 2 mm \(\times\) 2 mm for a set of four well-chosen grating spacings. To cope with signal quality deterioration due to optical scattering, an adaptive algorithm was used to iteratively modify the relative positioning between the scanning grid position and the probing location till a satisfactory signal to noise ratio was obtained. The resulting mappings showed that on one hand, for the current measurement technique and sample under investigation, quantitative information about the topographic variations of the three coating parameters could not be extracted simultaneously. Using a larger number of excited wavelengths would reduce the margin of uncertainty, and topographic variations should become visible. A complementary approach, i.e., limiting the unknowns to the coating thickness only, led to satisfactory results, with coating thickness maps that were found to be consistent when comparing scanning results obtained according to different patterns.  
B.V.: Software, Formal Analysis, Investigation, Writing--Original draft preparation; J.S.: writing--Review and Editing; T.V.d.D.: investigation, Resources, Validation; PS.: supervision, project administration, funding acquisition; C.G.: conceptualization, writing--Review and Editing, Supervision, Project administration  
Figure 18: Simulated thermal dispersion curve (solid gray line) for the GESA modified material using the obtained fitting results. The dispersion curve is bounded by the thermal diffusivities of the coating and substrate. Also, effective thermal diffusivities extracted from each decay trace in Figure 15 were added (open red circles). The scatter on these points reveals the experimental uncertainty.  
**Acknowledgments:** The authors would like to thank KU Leuven (C1-project OPTIPROBE--C14/16/063), FWO-V/SCK-CEN (aspirant scholarship BV) for financial support. [PERSON] is acknowledged for his kind help with typesetting.  
**Conflicts of Interest:** The authors declare no conflict of interest.