# Effect to the Surface Composition in Ultrasonic Vibration-Assisted Grinding of BK7 Optical Glass
## 3 Results and Discussions
### Analysis of the Effects in Surface Morphologies and Surface Roughness
#### 3.1.1 Effect of Grinding Parameters on Surface Morphologies and Surface Roughness
For a deeper observation, 300 times zoomed images were selected to investigate the machined surfaces morphologies of optical glass material, as shown in Figure 4. It can be seen that the material removal sizes on machined surfaces tend to be well-distributed at the feed rate of 10 mm/min, and no obvious removal areas in large sizes are found. But after the feed rate changing to 110 mm/min, several brittle-fractured pits are found on the machined surface and their sizes are obviously larger than that of other features. It is believed that the brittle-fractured pits mentioned above mainly consist of incompletely developed willow leaf shaped pits and incompletely developed double sector shaped pits. Notably, through the observation on 1000 times zoomed images of the machined surface, it can be seen that the powdering material removal can be easily found on the machined surface at the feed rate of 10 mm/min. Quite a few fracture lumps in micron level and scraps in sub-micron level produced by  
Figure 3: Influence of feed rate on surface roughness. powering material removal covered on the machined surface, and their proportion seems quite large. However, after the feed rate changing to 110 mm/min, powdering material removal is found to be much less, instead, brittle-fractured pits were induced onto the machined surface. Thus, the change of feed rate has a significant influence on the material removal modes, also has a great effect on the proportion of each removal mode. According to the measured results of surface roughness, the surface roughness (\(Sa\)) of machined surface showed in Figures 2c and 4a are 276.15 nm and 300.20 nm, respectively. Although there only exists a difference of 24.15 nm, their surface morphologies and material removal mechanism are much different. It can be seen from Figure 4c that the machined surface consists of small ductile and brittle removal pits. But no obvious powdering removal is found. Since only spindle rotation speed and feed rate changed in Figures 2c and 4a, respectively, the change of surface morphologies and material removal mechanism thus might be related to the trajectories of abrasive grains, which can also prove that the changes of spindle rotation speed and feed rate would have a significant influence not only on the morphologies and sizes but also on material removal modes. Thus, a conclusion can be drawn, at the combination of high spindle rotation speed and high feed rate, optical glass material is removed mainly by incompletely developed willow leaf shaped pits, incompletely developed double sector shaped pits and their combination in large sizes, supplemented by lumps in micron level. At the combination of high spindle rotation speed and low feed rate, optical glass material is removed mainly by lumps at the micron level, supplemented by completely and incompletely developed willow leaf shaped pits in small sizes. The influence law of grinding depth on surface roughness is shown in Figure 5. It can be seen that as grinding depth increases, the surface roughness also tends to increase. A possible reason is that the value of grinding depth is strongly related to the magnitude of grinding force and penetration depth. Another reason is that the critical cutting depth of glass material is strongly affected by the grinding depth [26]. As grinding depth increases, it would be much easier for the transferring of material removal mode from ductile removal to brittle removal. Both two reasons mentioned above will lead to the influencing results shown in Figure 5.  
Figure 4: Machined surface morphologies of optical glass material at different feed rate. Figure 6 shows the SEM images of machined surface morphologies of optical glass material zoomed by different times in different grinding depth. From Figure 5(a,c), it can be seen that, as the grinding depth changed from 10 \(\upmu\)m to 80 \(\upmu\)m, the amount of brittle-fractured pits in large sizes increases, as shown in in Figure 5(c), but not very obvious.