## 3 Results and Discussion
### Surface Evolution Mechanism during Polishing  
The surface quality of VO marble tile was tracked at 4, 8, 12, and 16 min during the polishing. Figure 7 shows the surface roughness (Sa) and surface peak-to-valley value (Sz) of VO marble tile during the polishing process using an SG polishing pad with a grain size of 20 \(\upmu\)m. Before polishing, both the surface Sa and Sz were very large, as shown in Figure 7, while the surface glossiness was very low, and it was almost matte. After being polished for 4 min, the Sa and Sz of VO marble tile decreased rapidly, and the surface glossiness increased rapidly, exceeding 60, while the SDD of the glossiness decreased. Although the surface scratches in Figure 8a were not completely removed after 4 min of polishing, the surface was free of deep scratches and relatively flat. Viewing from the line profile (corresponding to the red line in Figure 8a) shown in Figure 9a, the peak-valley (Rz) value is still greater than 9 \(\upmu\)m, and the contour fluctuations are relatively uniform, indicating that the surface material is removed uniformly at this time. After 8 min of polishing, the surface quality of VO marble tile reached the best state, reaching a high-gloss surface with the glossiness exceeding 80, and the SDD of the glossiness was further reduced, indicating that the surface quality consistency was further improved, while the Sa and Sz also reached their minimum values. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Abrasive Size**} & \multicolumn{6}{c}{**Optimum Polishing Time (min)**} \\ \cline{2-7}  & **VO** & **SK** & **CG** & **YG** & **FC** & **CM** \\ \hline
20 \(\upmu\)m & 8 & 9 & 10 & 10 & 12 & 14 \\
10 \(\upmu\)m & 7 & 8 & 9 & 9 & 11 & 12 \\
5 \(\upmu\)m & 5 & 6 & 8 & 8 & 10 & 11 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The optimal polishing times of different types of marbles polished by SG pads with different abrasive sizes. Figure 6: (**a**) The surface glossiness–time curves of marble tiles during the polishing process using the SG pad with a grain size of 20 \(\upmu\)m, (**b**) is the enlarged view of the red elliptical circle in (**a**). As shown in Figure 8b, the surface of the VO marble tile is very flat and smooth, and the surface has almost no scratches. The surface material has a relatively complete ductile flow and spreads across the entire surface. The line profile (corresponding to the red line in Figure 8b) given in Figure 9b indicates that the surface contour fluctuates very little at this time; the Rz value is already less than 3 \(\upmu\)m. After being polished for 8 min, the surface quality of the VO marble tile began to deteriorate: the Sa and Sz increased, and the glossiness decreased to approximately 70, while the glossiness SDD also increased, which indicated the consistency of the surface quality deteriorated. As shown in Figure 8c, some areas on the surface of the VO marble tile became rough and had deep scratches due to inconsistent material removal. The Rz value of the line profile (corresponding to the red line in Figure 8c) shown in Figure 9c had exceeded 11 \(\upmu\)m. As the polishing time further increased to 16 min, this phenomenon became more serious. The glossiness decreased rapidly, and its SDD value further increased. Figure 8: Three-dimensional surface topography of the VO marble tile after polishing for different times with 20 \(\upmu\)m diamond abrasive: (**a**) 4 min polishing, (**b**) 8 min polishing, (**c**) 12 min polishing, and (**d**) 16 min polishing. Figure 7: Surface roughness, surface glossiness, and surface peak–valley value of VO marble tile after polishing for different times with SG polishing pad with a grain size of 20 \(\upmu\)m.  
As seen in Figure 8d, most of the material on the original smooth surface is unevenly removed, making the entire surface undulating.