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[Featured Article] Investigation of the trajectory uniformity in water dissolution ultra-precision continuous polishing of large sized KDP crystal

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Release Date: 2020-09-24 Visited: 


Zhipeng Cheng, Hang Gao, Ziyuan Liu and Dongming Guo

1. Introduction

Large sized potassium dihydrogen phosphate (KDP) crystal is an irreplaceable nonlinear optical component in inertial confinement fusion (ICF) project. Restricted by the size, the previous studies mainly aimed at the removing principle and surface roughness of small size KDP crystal, and less research on the flatness. Owing to the low surface damage and high machining efficiency, water dissolution ultra-precision continuous polishing (WDUCP) has become a good technique to process the large sized KDP crystal. In this technique, the trajectory uniformity of water droplets’ can directly affect the surface quality, such as flatness and roughness. Specifically, uneven trajectory distribution of water droplets on KDP crystal surface derived from the motion mode obviously affect the surface quality. Prof. Hang Gao, Prof. Dongming Guo, Dr. Zhipeng Cheng and Dr. Ziyuan Liu from Dalian University of Technology, China, wrote a paper "Investigation of the trajectory uniformity in water dissolution ultra-precision continuous polishing of large sized KDP crystal" on IJEM. In this study, the material removal mechanism of WDUCP was firstly introduced. The simulation of water droplets trajectory on the KDP crystal under different eccentricity motion modes was then performed. Meanwhile, the coefficient of variation (CV) was utilized to evaluate the trajectory uniformity. Furthermore, to verify the reliability of simulation, some experimental tests were also carried out by employing the large continuous polisher. Under the motion mode of uncertain eccentricity, the initial surface texture of the 100 mm×100 mm×10 mm KDP crystal can achieve uniform planarization. The surface root-mean-square roughness is reduced to 2.182 nm, and the flatness is reduced to 22.013μm. Therefore, the feasibility and validity of the WDUCP for large sized KDP crystal are verified.

2. Background

Potassium dihydrogen phosphate (KDP) crystal is an excellent electro-optic nonlinear optical material, which is the only material for electro-optic switches and laser-frequency conversion applications in high-energy laser system for inertial confinement fusion (ICF). For the ICF project, the machining requirements of KDP crystal are large size, low surface roughness, and high accuracy of flatness. However, owing to its high brittleness, soft texture, easy deliquescence, and sensitivity to temperature, KDP crystal is regarded as difficult-to-machine material. In recent years, water dissolution polishing is considered as a promising technology to achieve a super-smooth and super-clean surface of the KDP crystal. In this study, the WDUCP was developed to achieve the process of large sized KDP crystal by the trajectory uniformity.

3. Recent Advances

Water dissolution polishing is a special chemical mechanical polishing technology; whose slurry is abrasive-free slurry. The water-in-oil microemulsion is one of the abrasive-free slurry which is studied for KDP crystal by our research team. The water-in-oil slurry is composed of water, oil and surfactant, and water molecules are caged into micelles to form the water droplets. The polishing pad is in direct contact with the crystal surface under a certain polishing pressure F in the polishing zone, as shown in Fig. 1(b). The mechanical movement between the polishing pad and the crystal is produced by the rotation. Under the effect of pressure and friction force, water droplets crush and deform, which lead to the release of water. Then the deformed and broken water droplets dissolve materials of the dissolve layer. The dissolved product is carried away through the groove with the flow of the new slurry and the rotary motion of polishing pad, as shown in Fig. 1(a). Thus, the planarization polishing of the KDP crystal can be achieved.


Fig. 1. Schematic of the WDUCP: (a) the removal process; (b) removal mechanism.

The motion trajectory Eq. (1) of water droplets on the KDP crystal surface under certain motion mode by the graphical transformation method can be shown as follows:


Similarly, the motion trajectory Eq. (2) of initial position water droplets P on the KDP crystal surface under uncertain motion mode can be shown as follows:



Fig. 2. The mesh of the KDP crystal.

To calculate trajectory uniformity, the crystal surface is divided into a plurality of grids which is shown in Fig. 2. The number of track point in each region, such as S1, is counted. And the number of sample points on the in the entire divided region can be obtained by the region dividing strategy and the region statistical strategy. This amount of the sample points makes up the sample Q. And the standard deviation σ and arithmetic mean μ are calculated.

It is noteworthy that the standard deviation and arithmetic mean to have their limitations, which are easily affected by different sample data. The CV can be assessment standard of dispersed extent of trajectory uniformity. The CV can directly show the absolute value of dispersion degree under different factors. The CV is defined as the ratio of sample standard deviation σ to arithmetic mean μ, which is used to describe the degree of dispersion between two sets of samples with different expectation. And it can be written as the Eq. (3):


The CV is not only affected by the discrete degree, but also by the average level. The smaller the CV, the better the trajectory uniformity, and vice versa. The trajectory uniformity is quantified by the CV.

The simulations result of certain and uncertain eccentricity are shown in Fig. 3. The results show that as the speed ratio is in the range of 0-1, the CV varies from 0.67 to 2.02 under the certain eccentricity motion mode, and varies from 0.48 to 0.65 under the uncertain one. As the speed ratio is 0.1, the difference between the certain and uncertain eccentricity is the largest. The comparison of removal times is shown in Fig. 4. On the contrary, the difference is the smallest at the speed ratio of 1 (Fig. 5). At the same speed ratio, the CV is small under the uncertain eccentricity, which illustrates the trajectory uniformity is better. What’s more, the material removal times are almost the same in different regions and the fluctuation is small. Therefore, it is beneficial to achieve the high-quality processing of large sized KDP crystal for uncertain eccentricity.


Fig. 3. The CV under certain and uncertain eccentricity motion modes.

Fig. 4. The removal times at a speed ratio of 0.1: (a) certain eccentricity; (b) uncertain eccentricity.

Fig. 5. The removal times at a speed ratio of 1: (a) certain eccentricity; (b) uncertain eccentricity.

The WDUCP tests were conducted at different speed ratio on the large continuous polisher of 1080 mm polishing plate, which can operate the certain and uncertain eccentricity motion modes. The concrete structure is shown in Fig. 6.

Fig. 6. Structure model of large continuous polisher.

The comparison of surface roughness under certain and uncertain eccentricity is shown in Fig. 7. The measurement results show that the surface RMS roughness after the certain and uncertain eccentricity’s processing can reach 4.678 nm and 2.182 nm, respectively. The surface RMS roughness is small under uncertain eccentricity motion mode, and a super-smooth surface was obtained.

Fig. 7. Surface RMS roughness after WDUCP: (a) certain eccentricity; (b) uncertain eccentricity.

4. Perspectives

Water dissolution ultra-precision continuous polishing is an effective technology to achieve ultra-precision processing of large sized KDP crystal. The WDUCP combine with the trajectory uniformity is bound to play an important role in promoting the high quality processing of large sized KDP. However, there is still a certain gap between the ultra-precision processing of large sized KDP crystal (400mm × 400mm). What’s more, the uneven polishing problem of KDP crystal has not been completely solved, which leads to the edge phenomenon collapse still exists. Therefore, it is extremely significant to deeply understand the water dissolution ultra-precision continuous polishing technology, which is conducive to realize the ultra-precision processing of large sized KDP crystal combined with practical problems.

5. About the Authors

Hang Gao is a full professor at DLUT, where he is also the subject leader of Mechanical Manufacture. He is the recipient of the State Council government allowance and the honorary Medal of the 70th anniversary of the founding of the people's Republic of China.

Prof. Gao’s research focuses on high performance composite, abrasive flow machining, soft brittle crystalline (KDP and DKDP), water jet machining and ultra-precision grinding technology. He has obtained more than 30 authorized invention patents, published more than 200 academic papers, and published 3 monographs or teaching materials as chief editor.

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