## 2 Materials and Methods
### Evaluation of Surface Integrity of the Steel Substrate  
Surface roughness was measured using a contact profilometer SURFTEST SJ-201 (Mitutoyo, Kawasaki, Japan). The analysis focused on two key parameters:  
* Ra (arithmetical mean deviation of the assessed roughness profile)
* Rz (maximum roughness height, defined as the sum of the highest peak and the lowest valley depth)  
Since Ra represents the average deviation from the mean profile line, it does not account for extreme protrusions or depressions. Therefore, Rz was also evaluated, as it provides a more comprehensive measure of surface irregularities. By definition, Rz is always greater than Ra, with the difference typically ranging from 4 to 8 times, depending on the surface type. Roughness profiles were filtered according to ISO 21920-2 [21], with the following measurement parameters:  
* Sampling length (\(\lambda\)c): 2.5 mm
* Number of sampling lengths: 5
* Total evaluation length (ln): 12.5 mm
* Filter type: Gaussian  
For untreated (reference) and blasted surfaces, 25 measurements were taken along both the longitudinal and transverse axes of the samples. In contrast, for ground surfaces,  
Figure 2: Principle of mechanical pre-treatment by grinding.  
50 measurements were performed perpendicular to the tool marks. Each dataset was derived from five randomly selected samples to ensure statistical robustness.  
The microhardness of the subsurface layers was assessed using a Hanemann microhardness tester, integrated into a Neophot 21 metallographic microscope (Carl Zeiss Jena, Jena, Germany). Measurements followed the Vickers hardness test (HV0.1, ISO 6507-1 [22]) using a diamond pyramid indenter with a 136\({}^{\circ}\) apex angle. For each dataset, microhardness measurements were conducted on five metallographic specimens, with five measurements per specimen taken at different depths from the surface. Due to limitations in the equipment, which prevent precise alignment of the indenter prior to testing, microhardness was measured first, followed by the determination of the indentation depth relative to the surface (with an accuracy of \(\pm\)2 \(\upmu\)m). Consequently, each dataset comprised 25 cumulative values per condition. Before measurement, each specimen was aligned parallel to the x-axis, as illustrated in Figure 3a. The measurement principle is depicted in Figure 3b. Given the influence of surface roughness, the reference (zero) position was established as the midpoint between the highest peak and the lowest valley. Accordingly, the indentation depth \(h\) (\(\upmu\)m) was defined using the following relationship:  
\[h=\frac{h_{2}-h_{1}}{2}+h_{1}\ \ \ (\mu m). \tag{1}\]  
where  
\(h_{1}\) is the distance from the indentation apex to the lowest valley.  
\(h_{2}\) is the distance from the indentation apex to the highest peak.  
The evaluated samples were sectioned using an Mikron 150 metallographic saw (MHT Hrazdil, Brno, Czech Republic), ensuring abundant cooling with a process fluid (emulsion oil mixed with water at a 1:20 ratio) to preserve the original microstructure. The specimens were then embedded in Dentacryl resin for further preparation. Metallographic grinding was performed using a Struers LaboPol-60 grinder (Struers, Ballerup, Denmark) in a sequential process with water as the process fluid. Abrasive papers of varying grit sizes were used in the following order:  
* P200
* P400
* P1000
* P2500  
The final polishing stage was conducted using diamond polishing paste (1-2 \(\upmu\)m grit size) to achieve a smooth, reflective surface. To enhance microstructural visibility, the specimens were etched with Nital (3% nitric acid in ethanol). This procedure was applied  
Figure 3: Principle of microhardness measurement: (**a**) method of specimen alignment; (**b**) determination of the indentation apex position.  
consistently across all specimens, including those with zinc-coated steel substrates. The suitability of this methodology has been previously verified [23].