## 3 Results and Discussion  
### Influence of Effective Number of Pulses on CVD Diamond Coating Ablation  
It can be found from Equation (1) that the effective number of pulses correlates to both repetition rate and scanning speeds. In this part, the effective number was adjusted separately by changing scanning speeds. SEM photographs of the diamond coating surface irradiated at different scanning speeds, with a pulse energy of 100 \(\upmu\)J, and repetition rate of 1 kHz are demonstrated in Figure 2. It can be seen from Figure 2 that the sharp pointedness of diamond grains was polished by laser ablation and the initial homogeneous structure was taken over by deep subwavelength ripples. The periodical ripples or laser-induced periodic surface structure (LIPSS) derive from the interference of the incident laser with quasi-monochromatic plasmons or among themselves. In this regime, the produced periodicity of LIPSS, approximately \(\lambda/4\), is much smaller than \(\lambda\)[27], and can be calculated as \(\Lambda=\lambda/2n=167\) nm for normal incidence, where \(\lambda=800\) nm is the wavelength of the fs laser applied in this work and \(n=2.40\) is the real part of the complex refractive index of diamond [28]. The orientation of the LIPSS is perpendicular to the laser polarization direction. When irradiated at high scanning speed, the laser-induced surface modification is less in extent for the reason of the small effective number of pulses, \(N\). Although there is LIPSS in the irradiated region, it is not obvious because the gaps between the LIPSS have been filled by re-solidified materials. As the scanning speed decreases, which means a bigger \(N\), excessive energy accumulates in unit area, leading to a strong incubation effect and inducing clear division among the periodical structures. Moreover, the width of ripples becomes narrower and longer, which is in line with the previous results found by [PERSON] et al. [29]. It has also been observed that a large number of melt slags showed up in the irradiated region. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Fixed Parameters**} & \multicolumn{3}{c}{**Controlled Parameters**} \\ \hline Wavelength/nm & Pulse duration/fs & Speed/mm.s\({}^{-1}\) & Pulse energy/\(\upmu\)J & Repetition rate/Hz \\
800 & 100 & 1.2–2.0 & 50–100 & 500–100 K \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The processing parameters applied for fs laser etching. The WL interferometer reconstructed topographies are displayed in Figure 3, with the upper profiles (blue) derived from the cross-section along the yellow horizontal line. The red profiles on the right are the cross-section obtained along the black vertical line. It can be found that there is little difference between the height of the irradiated region and that of the laser unaffected region when scanning at speed of 2 mm/s. With the decrease of the scanning speeds, the depth of the ablated grooves, which figures out the degree of ablation, increases gradually though the single pulse energy does not change. The tendency of the ablation depth indicates that the effective number of pulses (or the degree of incubation effect) plays a major role in the machining of the diamond coating. The surface roughness Ra and material removal rate (MRR) at the employed laser conditions are both based on the WL interferometer topography measurement. The obtained Ra and MRR vs. different scanning speeds are depicted in Figure 3f. As expected, the greater the scanning speed, the better the surface roughness. As the scanning speed rises from 1.2 mm/s to 2.0 mm/s, Ra decreases from 0.356 \(\upmu\)m to 0.314 \(\upmu\)m, indicating that the reduction of the effective number of pulses is beneficial for the polishing of the diamond coating surface. However, unlike the tendency of Ra variation, the MRR declines with the decrease of scanning speed, which obviously results from the fixed focal distance from the focus point to the diamond coating surface.