Chaoliang Guan

Ultrasmooth surface polishing based on the hydrodynamic effect

This study will examine the feasibility of applying the hydrodynamic effect to ultrasmooth surface polishing. Differing from conventional pad polishing, hydrodynamic effect polishing is noncontact, as the polishing wheel is floated on the workpiece under the hydrodynamic effect. The material removal mechanism and the removal contour are analyzed in detail. Dynamic pressure and shear stress distribution on the workpiece are numerically simulated in three dimensions under different clearances between the polishing wheel and the workpiece, showing that the dynamic pressure distribution and the magnitude of shear stress on the workpiece are greatly influenced by the clearance. It is clearly demonstrated from fixed-point polishing experiments that material removal rates and contours are determined by the combined action of dynamic pressure and shear stress. A material removal analytic model is presented with the hydrodynamic effect polishing method. Finally, a polishing experiment is conducted on a quartz glass and the plastic scratches, cracks, and bumpy structures on the initial surface are clearly removed. Moreover, the processed surface roughness is improved to 0.145 nm rms, 0.116 nm Ra.

Material removal mode affected by the particle size in fluid jet polishing.

As a newly developed ultrasmooth polishing technique, fluid jet polishing (FJP) has been widely used for optical glass polishing. The size of the particle in the polishing slurry has a great influence on the material removal rate and quality of the processed surface. The material removal mode affected by the particle size is investigated in detail. Particle trajectories with different size are calculated by numerical simulations in the FJP process. Simulation results demonstrate that the particle with large size will seriously deviate from the fluid streamline and almost impact on the workpiece along a straight line in the initial incident direction. The larger is the particle size, the more deviation will occur. Impact models are established based on different particle trajectories. A polishing experiment was conducted to verify the feasibility of the mode. Experiment results show that the particle size has a great influence on the material removal mode in FJP with the same process parameters. Material is removed in the plastic mode with higher removal rate and worse surface roughness for a larger-sized particle, while the material removal occurring in the elastic mode has a much lower removal rate and smoother surface for the smaller-sized particle. Material is removed by chemical impact reaction between the particle and the surface within the elastic mode, and a smooth surface with no damage is obtained after the FJP process.

Improvement of magnetorheological finishing surface quality by nanoparticle jet polishing

Abstract. Nanoparticle jet polishing (NJP) is presented as a posttreatment to remove magnetorheological finishing (MRF) marks. In the NJP process the material is removed by chemical impact reaction, and the material removal rate of convex part is larger than that of the concave part. Smoothing thus can progress automatically in the NJP process. In the experiment, a silica glass sample polished by MRF was polished by NJP. Experiment results showed the MRF marks were removed clearly. The uniform polishing process shows that the NJP process can remove the MRF marks without destroying the original surface figure. The surface root-mean-square roughness is improved from 0.72 to 0.41 nm. power spectral density analysis indicates the surface quality is improved, and the experimental result validates effective removal of MRF marks by NJP.

Comparative analysis of oxidation methods of reaction-sintered silicon carbide for optimization of oxidation-assisted polishing.

Combination of the oxidation of reaction-sintered silicon carbide (RS-SiC) and the polishing of the oxide is an effective way of machining RS-SiC. In this study, anodic oxidation, thermal oxidation, and plasma oxidation were respectively conducted to obtain oxides on RS-SiC surfaces. By performing scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX) analysis and scanning white light interferometry (SWLI) measurement, the oxidation behavior of these oxidation methods was compared. Through ceria slurry polishing, the polishing properties of the oxides were evaluated. Analysis of the oxygen element on polished surfaces by SEM-EDX was conducted to evaluate the remaining oxide. By analyzing the three oxidation methods with corresponding polishing process on the basis of schematic diagrams, suitable application conditions for these methods were clarified. Anodic oxidation with simultaneous polishing is suitable for the rapid figuring of RS-SiC with a high material removal rate; polishing of a thermally oxidized surface is suitable for machining RS-SiC mirrors with complex shapes; combination of plasma oxidation and polishing is suitable for the fine finishing of RS-SiC with excellent surface roughness. These oxidation methods are expected to improve the machining of RS-SiC substrates and promote the application of RS-SiC products in the fields of optics, molds, and ceramics.

Research on subsurface defects of potassium dihydrogen phosphate crystals fabricated by single point diamond turning technique

Abstract. Potassium dihydrogen phosphate (KDP) crystals, which are widely used in high-power laser systems, are required to be free of defects on fabricated subsurfaces. The depth of subsurface defects (SSD) of KDP crystals is significantly influenced by the parameters used in the single point diamond turning technique. In this paper, based on the deliquescent magnetorheological finishing technique, the SSD of KDP crystals is observed and the depths under various cutting parameters are detected and discussed. The results indicate that no SSD is generated under small parameters and with the increase of cutting parameters, SSD appears and the depth rises almost linearly. Although the ascending trends of SSD depths caused by cutting depth and feed rate are much alike, the two parameters make different contributions. Taking the same material removal efficiency as a criterion, a large cutting depth generates shallower SSD depth than a large feed rate. Based on the experiment results, an optimized cutting procedure is obtained to generate defect-free surfaces.

Ultrasmooth reaction-sintered silicon carbide surface resulting from combination of thermal oxidation and ceria slurry polishing.

An ultrasmooth reaction-sintered silicon carbide surface with an rms roughness of 0.424 nm is obtained after thermal oxidation for 30 min followed by ceria slurry polishing for 30 min. By SEM-EDX analysis, we investigated the thermal oxidation behavior of RS-SiC, in which the main components are Si and SiC. As the oxidation rate is higher in the area with defects, there are no scratches or cracks on the surface after oxidation. However, a bumpy structure is formed after oxidation because the oxidation rates of Si and SiC differ. Through a theoretical analysis of thermal oxidation using the Deal-Grove model and the removal of the oxide layer by ceria slurry polishing in accordance with the Preston equation, a model for obtaining an ultrasmooth surface is proposed and the optimal processing conditions are presented.

Efficient fabrication of ultrasmooth and defect-free quartz glass surface by hydrodynamic effect polishing combined with ion beam figuring

Material removal rate has greatly relied on the distribution of shear stress and dynamic pressure on the workpiece surface in hydrodynamic effect polishing (HEP). Fluid dynamic simulation results demonstrate that the higher rotation speed and smaller clearance will cause the larger material removal rate. Molecular dynamic (MD) calculations show the bonding energy of Si-O in the silicon-oxide nanoparticle is stronger than that in the quartz glass, and therefore the atoms can be dragged away from the quartz glass surface by the adsorbed silicon-oxide nanoparticle. The deep subsurface damage cannot be efficiently removed by HEP due to its extremely low removal rate. However, the subsurface damaged layer can be quickly removed by ion beam figuring (IBF), and a thinner layer containing the passivated scratches and pits will be left on the surface. The passivated layer is so thin that can be easily removed by HEP process with a low material rate under the large wheel-workpiece clearance. Combined with the IBF process, the subsurface damage and surface scratches have been efficiently removed after the HEP process. Meanwhile there are not obvious duplicated marks on the processed surface and the surface roughness has been improved to 0.130nm rms, 0.103nm Ra.

Surface evaluation and evolution during hydrodynamic effect polishing for quartz glass

Hydrodynamic effect polishing (HEP), a noncontact machining process, can realize the processed surface roughness as small as atomic level. To investigate the subsurface structure, the HEP processed quartz glass surface was etched by the hydrofluoric acid solution. It has been proved that HEP is a polishing method with the ability to process the surface with atomic-level flatness and damage-free surface/subsurface. It has been found that the microplastic scratches on the lap prepolished glass were obviously exposed when the thin redeposition layer was removed. Then the scratches were gradually removed and surface roughness decreased quickly as the removal depth increased. The surface becomes very smooth and the surface roughness maintains at an atomic level when the subsurface damage is removed clearly. The experimental results demonstrated that the defects such as the scratches parallel to the rotational axis of the wheel were firstly removed during the polishing process, and then the defects vertical to the wheel rotational axis were removed.

Tool decentration effect in slow tool servo diamond turning off-axis conic aspheric surface

Off-axis conic aspheric mirrors are crucial components in some optical systems, such as three-mirror-anastigmatic telescopes (TMA). However, because of the swing limitation of lathe, off-axis aspheric mirrors are not easy to fabricate using a general-purpose diamond turning machine. This research demonstrates slow tool servo diamond turning process which allows fabricating off-axis conic aspheric mirrors on-axis. The figure error caused by tool centering error was studied on. An off-axis parabolic mirror was fabricated and actual machining data are discussed. The result proved that proposed approach is capable of fabricating copper off-axis parabolic mirror of 46mm diameter to a form accuracy of 0.736μm in PV error value.

Investigation on feedback control of linear motors in ultraprecision-machine feed-drive systems

Linear motors offer many advantages over ac or dc servo motors. More and more ultraprecision machines use linear motors in their feed-drive systems. The distance measured by the linear encoder is used to calculate the linear motor’s velocity. A linear-motor controller needs a high-resolution position sensor for smooth velocity control. However, the high-resolution position sensor may sense the vibration of the mechanical structure and therefore cause instability. This article presents a simulation model to analyze the impact of linear encoder feedback on a linear motor’s motion control and stability. Experiments are designed to validate the simulation model. Both simulation and experiment demonstrate that proper selection of linear encoder’s resolution can satisfy a smooth velocity control and stability requirements.