# 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
#### 3.1.2 Effect of Ultrasonic Amplitude on Surface Morphologies and Surface Roughness  
Ultrasonic vibration can provide separation effect between grinding tool and glass workpiece, and also can affect grinding force, critical cutting depth, and many other factors. Thus, it is necessary to investigate the influence of ultrasonic vibration on surface roughness and material removal mechanism. Figure 5: Influence of grinding depth on surface roughness. Figure 6: Machined surface morphologies of optical glass material at different grinding depth. The influence of ultrasonic vibration amplitude on surface roughness is shown in Figure 7. It is found that the surface roughness decreases as the ultrasonic vibration amplitude increases. A possible reason is that, the material removal process is much similar to that in conventional grinding while the ultrasonic vibration amplitude is in small value, the advantages of separation effect mentioned above cannot be reflected. Besides, the removal volume of a single abrasive grain would increase with a decrease in the separation effect. Another reason is that, the comprehensive and repeated impact and ironing pressing effects of abrasive grains on glass workpiece are weakened when ultrasonic amplitude is small, the motion strength of abrasive grains along axis direction is also weakened. The third reason it that, ultrasonic vibration can provide cracks shield effect, that is, the second indentation will not cause a median crack or this median crack will not propagate entirely if the distance between the first indentation and the second indentation is too small. If ultrasonic vibration amplitude is in small value, this effect will be restrained, the interference of median crack thus becomes weak, the lateral crack and brittle-fractured sizes will correspondingly become large, the surface roughness thus becomes large. Figure 8 showed the machined surfaces morphologies of BK7 optical glass material zoomed by different times and in different ultrasonic vibration status. Through the macro observation on Figure 8,c, it is found the amount of ductile and brittle-fractured pits increases to a certain level when ultrasonic vibration was introduced into grinding process. 1000 times zoomed images are used in order to further observe the differences of morphologies and material removal modes. It can be seen from Figure 8 that, while the material was machined without the help of ultrasonic vibration, machined surface would consist of comprehensively distributed ductile, brittle-fractured pits, and their combinations. The powdering removal also can be obviously found to be in a large proportion, including fracture lumps in micron level and scraps in sub-micron level which covered the machined surface. Figure 8: _Cont_. Figure 7: Influence of ultrasonic vibration amplitude on surface roughness. After ultrasonic vibration was introduced into the grinding process, the material removal mechanism obviously changed. From Figure 8d, when ultrasonic vibration amplitude is 7 \(\upmu\)m, the amounts of brittle-fractured pits in large sizes become much less, instead, ductile removal pits are induced. Meanwhile, the powdering removal in sub-micron level nearly disappear. The fracture lumps in micron level still have a certain proportion, but their amounts become much less. Instead of covering the machined surface in conventional grinding process, these powdering removal defects still connected to the machined surface. Hence, a conclusion can be drawn that the introduction of ultrasonic vibration changes the proportion of three material removal modes (i.e., ductile removal, brittle removal and powdering removal). Besides, the proportion of ductile removal mode increases while the proportion of brittle removal mode decreases. The powdering removal in sub-micron level are strongly restrained by ultrasonic vibration, the proportion of fracture lumps in micron level decreases.