## 3 Results and Discussion
### Coating Thickness Measurement  
Research has shown that there is a strong linear relationship between the number of laser applications and the ablation depth of the sample [34]. It is possible to attempt to establish a linear model relating the number of laser applications to the ablation depth of fireproof coatings (applied to galvanized steel surfaces) in order to characterize their coating thickness. When creating the linear model for the thickness of the fireproof coating with respect to the number of laser applications, it is essential to control the number of laser applications to prevent the laser from penetrating through the fireproof coating. Figure 5: Spectra of galvanized steel and fireproof coating: (**a**) GAlvanized steel spectrum analysis, (**b**) Fireproof coating spectrum analysis. Figure 6: Variation of the spectral line intensity of fireproof coatings with changes in air pressure. The air pressure was set at 101 kPa, 60 kPa, and 50 kPa, the laser energy was set at 85.5 mJ, the laser frequency at 1 Hz, and the number of laser applications was set at 5, 10, 15, 20, 25, and 30. The 3D morphology of the ablation pits on the sample surface was observed using a confocal microscope, with results shown in Figure 7.  
As shown in Figure 7, the confocal microscope reveals the 3D ablation morphology of the fireproof coating sample's surface. The surface's 3D ablation morphology roughly resembles an inverted cone, but it is not uniform. This unevenness is due to the high temperature of the plasma generated during the laser ablation process, which causes the melted material to become fluid and splatter, resulting in inconsistencies in the ablation morphology of the sample. For a more detailed quantitative analysis, the built-in analysis software of the confocal instrument was used to statistically evaluate the specific information regarding the depth of the ablation pits of the samples under different air pressures and laser pulse counts, as illustrated in Figure 8.  
Figure 7: Ablation morphology under different laser pulse counts at 101 kPa air pressures: the ablation morphology under 5, 10, 15, 20, 25, and 30 laser pulses, respectively, from (**a–f**). Figure 8 shows that the depth of laser ablation pits increases with the number of laser pulses at gas pressures of 101 kPa, 60 kPa, and 50 kPa. When the number of laser pulses is 5, the depth of the ablation pits is approximately 150 \(\upmu\)m; at 30 laser pulses, the depth reaches about 400 \(\upmu\)m. Furthermore, there is a strong linear relationship between the number of laser pulses and the ablation depth across all pressure levels, with coefficients of determination of 0.963, 0.997, and 0.981, respectively. The fitting results are satisfactory, indicating that the ablation depth of the sample can be determined by the number of laser pulses. As the atmospheric pressure decreases, the linear fitting curve of the number of laser pulses versus the depth of the ablation pit gradually rises. This means that with the same number of laser pulses, the depth of the ablation pit increases as the pressure drops. The reason for this is that, as the pressure decreases, the expansion speed of the plasma increases, the plasma shielding effect weakens, and the interaction between the laser and the sample becomes more effective, leading to deeper ablation. This indicates that atmospheric pressure influences the measurement of coating thickness, therefore, it is necessary to establish a relationship curve between ablation depth and laser pulses at different pressures, in order to correct for the effects of pressure on measurements. The characteristic spectral line of the Zn element (Zn I 334.56 nm) was chosen as the basis for determining whether the fire-retardant coating had been penetrated by the laser. Laser penetration tests on the fire-retardant coating were conducted under conditions of 101 kPa and 50 kPa, with the laser energy set at 85.5 mJ and the number of laser shots set at 60.