## 3 Results
### Effect of Laser Scanning Speed on Material Removal Rate  
At repetition frequencies of 1 KHz and scanning speeds of 0.1 mm/s, 0.3 mm/s, 0.5 mm/s, 1.0 mm/s, 1.3 mm/s, 1.5 mm/s, 1.7 mm/s, and 2.0 mm/s, the nano-crystalline diamond coatings were etched at energy\(-\)flux densities of 14.2 J/cm\({}^{2}\) and 56.6 J/cm\({}^{2}\). Figure 13 displays the white-light interferometer scans of the diamond covering at various scanning speeds. Figure 14 displays the appropriate etches' widths and depths. As shown in Figure 13, as the scanning speed is increased, the area of the ablation crater gradually shrinks. The bottom of the ablation crater exhibits a clear wave structure when the scanning speed is modest (0.1\(-\)0.5 mm/s) and the energy\(-\)flux density is 14.2 J/cm\({}^{2}\). This is because the rate of excessive spot overlap causes the coating surface to gather too much laser energy, the area of the effective spot is larger, causing the phenomenon of over-melting, and the speed of laser scanning cannot keep up with the rate of material ablation. Additionally, the morphological characteristics of ablation are more obvious, with uneven edges. From Figure 14, it can be inferred that when the energy\(-\)flux density is 14.2 J/cm\({}^{2}\) and 56.6 J/cm\({}^{2}\), the etch width does not significantly change with the scanning speed. Figure 14: Ablation depths and widths at various scanning rates: (**a**) widths and (**b**) depths. Figure 13: Cross-sectional coating ablation morphologies under different scanning speeds. This is because the laser's energy has a Gaussian distribution, and because changing the scanning speed has no effect on the laser's energy density, the width of the ablation pits in the coating cross-section remains constant. The laser spot overlap rate is high when the scanning speed is slow. The number of pulses the laser emits to the coating surface when scanning the same path is high, and the accumulated energy in the same region conducts deep within the coating, producing a deep etching depth. By using a femtosecond laser to etch the diamond coating, increasing the scanning speed will increase the temperature of the diamond coating's surface. This will also hasten the vaporization of graphite on the diamond coating's surface, which will absorb some of the energy and ionize to create a dense plasma with a shielding effect on the film's surface [44; 45]. The shielding of the plasma effect causes the absorption of the pulse by the material to decrease as the plasma concentration increases; hence, as the scanning velocity rises, the etching depth of the material tends to decrease. From Figure 14, it can be inferred that when the energy\(-\)flux density is 56.6 J/cm\({}^{2}\) and the scanning speed is 0.1 mm/s and 0.3 mm/s, the laser has penetrated the coating and etched into the substrate and the diamond coating is being etched to a depth greater than 15 \(\upmu\)m. As a result, the etching results for these two instances are invalid for calculating the material removal rate. The material removal rates at various scanning speeds were calculated using the valid data in Figure 14 and the material removal rate mathematical model, and the resulting curves are displayed in Figure 15.  
The area of the incised shape and the scanning speed must be used to calculate the material removal rate. The ablation area is not the highest when the speed is 1 mm/s, but the product of the area and the speed is larger, leading to a higher rate of material removal. The material removal rates were 3.26 \(\times\) 10\({}^{5}\) \(\upmu\)m\({}^{3}\)/s and 1.7 \(\times\) 10\({}^{5}\) \(\upmu\)m\({}^{3}\)/s at 1 mm/s when the energy\(-\)flux density was 14.2 J/cm\({}^{2}\) and 56.6 J/cm\({}^{2}\), respectively, which were faster than the results at other scanning speeds. The best removal effect was attained at a speed of 1 mm/s, as illustrated in Figure 15. The material removal rate tends to increase and then drop as the scanning speed increases.