Significance Ultra-precision optical processing technology is a necessary means to manufacture high-performance optical components, affecting all aspects of human production and life. With the development of modern science and technology, it has been widely and profoundly applied in various fields, from the glass screen for mobile phones to the ultra-short and ultra-powerful laser experimental devices of PW order, which are supported by ultra-precision optical processing. In the fields of integrated circuit manufacturing, chip manufacturing and other ultra-precision optical processing, ultra-precision optical processing is even the “soul of technology”, its technical level directly determines the processing quality and service life of optical elements, furthermore determines the performance limit of optical system.

As an important research direction of ultra-precision optical manufacturing, ultra-polishing has always got the researchers' attention. Many of the core techniques used in the polishing process have been studied in the past, but the acquisition of ultra-smooth surfaces severely depends on the experience of the processors. Considering the significance of ultra-polishing in exploring technical limits in many fields and the current situation that actual polishing processing relies heavily on operational experience, it is necessary to deeply discuss the physical and chemical mechanisms of material removal in the ultra-polishing process represented by classical polishing and chemical-mechanical polishing.

Progress The material removal mechanism in ultra-polishing can be simply divided into two aspects: physical process removal and chemical process removal. The physical process removal mechanism mainly includes mechanical removal mechanism and fluid mechanics removal mechanism.

In terms of the research on mechanism of mechanical removal, Preston and other researchers initially summarized the “Preston equation” and its modified equation by the relationship between macroscopic physical quantities such as workpiece surface pressure, relative velocity of workpiece and material, and removal rate in glass polishing. Zhao's research group used the Greenwood-Williamson theory to reveal the material removal process under the action of abrasive wear from a microscopic perspective, and predicted the material removal rate by calculating the actual contact area of the workpiece and polishing pad, the number of abrasive particles involved in polishing, and the embedding depth of abrasive particles, respectively. For the research on the mechanism of material removal under the action of fluid, Runnels proposed a tribology-based 3D fluid dynamics model and calculated the expression of normal material removal rate caused by fluid erosion. Sundararajan and Thakurta et al established the fluid velocity field of polishing fluid through fluid lubrication model, and then obtained the average polishing rate through mass transfer model.

The above physical removal models all ignored or simplified the chemical process of the workpiece in the polishing solution environment, and didn't reveal the important process of chemical action in the realization of the material microscopic removal process. This problem was well explained in Cook's paper, where Cook proposed a chemical tooth model for material removal (Fig.9). In order to test and verify the correctness of the model, many researchers have followed in Cook's model, carrying out numerous experiments. Yu used atomic force microscope to verify the promoting effect of aqueous solution environment on material removal (Fig.10). Zhou research group conducted experiments on this process in combination with X-ray photoelectron spectroscopy and atomic force microscope. The experiment results showed that water molecules reacted with sapphire to generate hydroxy-rich AlO(OH) and Al(OH)₃ products, thus achieving hydroxylation of sapphire surface (Fig.12); Katsuki, Shi and other researchers combined atomic force microscope and infrared spectroscopy to carry out experimental research on the important process of chemical bonding, the experimental results show that there were a lot of chemical bonds between the workpiece and the probe surface after polishing (Fig.17).

Conclusion and Prospect Each mechanism of material removal mentioned in this paper has its limitations. In the related theory of physical removal mechanisms,the “Preston” equation of phenomenology only explained the influence of material removal rate with macroscopic physical parameters such as pressure and flow rate from a macroscopic perspective. The abrasion theory based on the Greenwood-Williamson model ignored the powerful chemical action in polishing and failed to explain the mechanism of material removal at the atomic and molecular level in the final polishing. The fluid action of the polishing slurry is more concerned with the contact model between the workpiece and the polishing pad, not with fluid erosion. The chemical removal mechanism makes up for the deficiency of physical removal mechanism, but the relevant model can only give a rough description of the material removal process at the atomic and molecular level in the polishing process, which still needs to be further studied. At the same time, in the description of the mechanism of material removal in the whole process, almost all researchers hoped to use a theoretical model to explain the material removal phenomenon in the whole polishing process. However, it seems that the whole process involves multiple material removal mechanisms at the same time, and different material removal mechanisms play a key role in different processing stages. To sum up, it is still necessary to carry out continuous and in-depth research on the material removal mechanism in super polishing, so as to promote the development of the ultra-precision optical processing technology.