Super-smooth Surface Research Articles

Surface Roughness Measurement Using Light Sectioning Method and Computer Vision Techniques.(Dept.M)

The development of new industries has led to a requirement for super-smooth surfaces and for the ability to measure surfaces of industrial parts accurately; therefore, the measurement of engineering surface roughness is becoming increasingly important. In this work, a new approach is introduced to measure surface roughness by combining a light sectioning microscope and a computer vision system. This method has the advantage of being non-contact. The light sectioning microscope is used to view roughness profiles of the specimens to be measured and the vision system is used to capture images for the viewed profiles. A special program (named SRLSVision) has been totally developed in-house using MatlabTM software to analyze the captured images through two modules. The first module is used to extract the roughness profiles from the captured images by applying various image processing and computer vision algorithms. The second module is used to calculate the ISO roughness parameters (22 parameters) from the extracted profiles. The system has been calibrated for metric units and verified using a standard specimen. In addition, the system was used to measure various samples: machined by different operations and the obtained results were compared with the results obtained by measuring the same samples using a stylus instrument. The accuracy of the system proved to be within ±5.5% compared with the stylus instrument readings.

Machining method for controlling the behaviours of Bingham fluids in cluster magnetorheological polishing pads

Cluster magnetorheological finishing is a novel machining method for attaining super-smooth and even surfaces. The aim of the research is to strengthen the role of cluster magnetorheological polishing pads and improve the material removal rate (MRR) while attaining smooth and undamaged surfaces. To this end, a novel polishing disk with three-dimensional (3D) microstructures was formed by machining an array of holes on the surface of a polishing disk. The influences of the type of abrasives, geometric parameters of the holes, and polishing time on the roughness and MRR of the machined surface were explored. The test result showed that a polishing disk with multiple holes contributes to a higher MRR and a lower surface roughness relative to a smooth polishing disk, significantly improving the polishing effect; the diameter and distribution of multiple holes on the surface of the polishing disk exhibit a significant effect on the polishing process. The polishing effect with silica sols is most remarkably improved. The solid-state chemical reaction between sapphire and silica sols promotes material removal; relative to a smooth polishing disk, the MRR is increased by 107% and the surface roughness is decreased by 11%. The surface roughness is reduced from Ra 5.14 nm to Ra 0.24 nm by utilising the optimised technological parameters. Under the synergistic action of mechanical removal enhancement of polishing disks with multiple holes and chemical removal of silica sol sapphire can obtain super smooth undamaged surfaces efficiently.

Ground fused silica processed by combined chemical etching and CO2 laser polishing with super-smooth surface and high damage resistance.

Laser damage in fused silica, particularly ultraviolet laser damage, is still a key problem limiting the development of high-power laser systems. In this Letter, a combined process of chemical etching and CO2 laser polishing was applied to ground fused silica. A super-smooth surface with a root-mean-square roughness of 0.25 nm was achieved through this combined process. Furthermore, the combined process can reduce the introduction of photoactive metal impurity elements, destructive defects, and chemical-structure defects, resulting in a 0% probability damage threshold nearly 33% higher than a conventional chemical mechanical polished sample for a 7.6 ns pulse at a wavelength of 355 nm.

Anhydrous based shear-thickening polishing of KDP crystal

Based on the special physical–chemical characteristics of optical crystal in the field of aeronautics, a new anhydrous based shear-thickening polishing (ASTP) method has been proposed to restrain deliquescence and to improve physical properties for KDP machining. The ultra-precision polishing of KDP crystal is completed by ASTP. A kind of anhydrous based thickening polishing slurry (ATPS) was proposed in our work, and high-performance rheological properties were determined to achieve the ASTP of KDP crystal. A material removal model of ASTP in KDP machining is established, followed by the verification experiment of the prediction model. The maximum error of the predictive model is only 9.8%, which proves the validity of the material removal model for KDP polishing. The polishing experiments were carried out on the polishing platform developed by ourselves. The results show that the new polishing method can polish 20 mm × 20 mm × 5 mm KDP crystal and obtain a super-smooth surface with a surface roughness of 1.37 nm and high shape accuracy. The surface accuracy of polished KDP crystal reaches up to 0.68λ (RMS). The experimental results show that the ASTP is a potential ultra-precision machining method for KDP crystal.

High precision laser ablation of fused silica using pulsed CO<sub>2</sub> laser

Laser polishing is a novel processing method which can achieve a super-smooth surface of an optical component. However, there are residual ripples and surface distortions after laser polishing. High precision laser ablation can be used to correct the residual ripple and shape of the component, making it a promising figuring method. In this paper, the experiment of high precision laser ablation of fused silica was carried out with pulsed CO2 laser. It is verified that high-precision uniform ablation in a local region can be achieved by controlling the laser overlap rate, and different nanometer ablation depths can be obtained by controlling the laser power density. However, it has been found that there is thermal accumulation and over-ablation during the laser ablation process. A three-dimensional numerical model was established to simulate the process of pulsed CO2 laser scanning on a fused silica surface. The surface temperature distribution and evolution of fused silica with different overlap rates and pulse repetition frequencies were analyzed. The numerical results show that the increase of the overlap rate could lead to over-ablation. With a suitable overlap rate, the high pulse repetition frequency could result in significant heat accumulation.

Investigation of ultrathin carbon film growth and etching by in-situ X-ray reflectivity

The results of studies of carbon films in the process of their growth and etching in plasma of radio-frequency discharge in real time by in-situ X-ray reflectometry are presented. The film thickness, growth rate, density and surface roughness were calculated from the time dependence of the reflection coefficient of x-rays at a wavelength of 1.54 A. A method for the production of carbon films with a super-smooth surface was proposed.

超光滑表面缺陷的分类检测研究

超光滑表面缺陷的分类检测研究

Super-smooth surface demonstration and the physical mechanism of CO2 laser polishing of fused silica.

This Letter reports on the results of experiments aimed at obtaining high-quality super-smooth surfaces by investigating the laser polishing of fused silica. A maximum reduction in root-mean-square roughness to 0.156nm was achieved, and laser-polished surfaces exhibited virtually no micro-defects or damage. A subsequent analysis using a multi-physics numerical model revealed the underlying physical mechanism of laser polishing of fused silica. The model simulated the surface smoothing process of laser polishing and demonstrated the effects of surface tension, the Marangoni effect, light pressure, and gravity in the process. It was found that the surface tension dominates the surface smoothing process, and it is a critical factor for achieving sub-nanometer micro-roughness of laser polishing of fused silica. Additionally, the model was successfully applied to predict the residual surface roughness of laser polishing.

(Invited) Electrochemical Micromachining Based on Confined Etching

Micromachining is becoming the cutting-edge of high-tech manufacturing because of the increasing industrial demand for supersmooth surfaces and functional three-dimensional micro/nano-structures in microelectromechanical systems, ultra-large scale integrated circuits, miniaturized total analysis systems, precision optics and so on. Taking advantage of no tool wear, no surface stress, environmental friendliness, simple operation, and low cost, electrochemical micromachining (ECM) has an irreplaceable role in micromachining. We present here our perseverant research in the last two decades on how to “confine” the ECM processes to occur at micrometer or even nanometer scale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) to fabricate three-dimensional micro- and nanostructures (3D-MNS) on different metals and semiconductor materials since 1992. In general, there are three procedures in CELT: (1) generating the etchant on the surface of the tool electrode by electrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- or nanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usually adopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photo or electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometer scale, which ensures the nanoscale accuracy of electrochemical machining. However, because the thickness of the electrolyte solution between the tool electrode and the workpiece are usually at micro/nano-meter scale, it is a challenge to solve the problems of potential distribution and mass transfer in the ultra-thin layer electrolyte cell. In this presentation, we will introduce our recent work on the external physical field induced electrochemical reactions in which there is no need of 2-electrode or 3-electrode system in the conventional electrochemical system.

On the machining of selective laser melting CoCrFeMnNi high-entropy alloy

High-entropy alloys (HEAs) have attracted significant attention due to its superior low temperature mechanical properties. Recently, with the success of 3D additive manufacturing (AM) of CoCrFeMnNi HEA by selective laser melting (SLM), the fabrication of complex components in one step became possible. However, due to the low surface quality, post-machining is necessary. Up to date, there is no study that comprehensively reported the machinability of SLM CoCrFeMnNi HEA. Hence, this research presents a pioneer work on the machinability study of SLM CoCrFeMnNi HEA by commonly used mechanical, thermal and electro chemical machining processes. The surface and subsurface quality generated by different machining processes were quantitatively evaluated from the aspects of surface morphology and roughness, microhardness, residual stress and subsurface quality. The results show that milling and grinding smoothed the surface, enhanced surface microhardness but induced tool marks and compressed residual stresses. Wire EDM flattened the surface but caused a heat melt layer resulting in the increase of the tensile residual stress and surface microhardness. EP released the residual stress and with the combination of mechanical and electrical processes, smoother surfaces were obtained and subsurface damages were removed. A super smooth surface without subsurface damages was achieved by mechanical polishing.

Magnetorheological finishing of silicon for nanometric surface generation: An experimental and simulation study

Silicon mirrors are essential for guiding the X-ray beam and focusing it to a specific location. These mirrors using total internal reflection require super smooth surface finish due to small wavelength of X-ray. Magnetorheological finishing is a computer-controlled technique used in the production of high-quality optical lenses. This process utilizes polishing slurries based on magnetorheological fluids, whose viscosity changes with the change in magnetic field. In this work, polishing potential of silicon mirrors by magnetorheological finishing process is examined to achieve nanometric surface finish for X-ray applications. The individual effect of parameters such as magnetizing current, working gap, rotational speed on surface roughness is investigated, and optimized parameters are identified. To investigate the physical essence underlying magnetorheological finishing process, the molecular dynamics simulations are used. Molecular dynamics simulation is used to study the atomic-scale removal mechanism of single-crystalline silicon in magnetorheological finishing process and attention is paid to study the effect of gap between the tool and the workpiece on surface quality. The outcome is promising and the final surface roughness achieved is as low as 6.4 nm. The surface quality is analyzed in terms of arithmetic roughness, power spectral density, and image analysis of scanning electron microscopy for uniform evaluation.

Development and testing of glass substrate Wolter-1 X-ray focusing mirror

The wolter-1 X-ray focusing mirror can reflect grazing incidence X-ray to the focal plane, which plays an important role in the astronomical detection and other fields due to its good image detecting capability. A geometric model of the optical system is established for theoretically deriving the optical path equations which is useful in this glass based focusing mirror designing, all the design parameters of the focusing mirror can be obtained by solving these equations. In the manufacturing process, the D263T glass is chosen to be the structural material of the focusing mirror due to its light weight and super smooth surface, after a slumping process, the flat glass mirror will have the shape of Wolter-1 X-ray focusing mirror. This slumping process has been used successfully in the manufacturing process of an American mission named The Nuclear Spectroscopic Telescope Array, which was launched in 2012. According to X-ray reflecting theory, the reflectivity of the Wolter-1 mirror can be improved significantly by coating metal film on the surface of the mirror. In this work, an iridium film is coated on the surface of the glass mirror through a vacuum evaporating process. In order to learn the influence of the focal spot caused by the mirror shape tolerance, the morphology of the mirror is tested by using a 3-D laser scan instrument. The results show that 50% of the total test points are located in the tolerance range of-10-10 m, in which the tolerance represents the difference between the actual lens profile and the ideal lens profile. Then the focal spot test is carried out with the help of a visible light test system:a laser collimator is installed in front of focusing mirror as an incidence light source, and a charge coupled device (CCD) is placed in the focal plane to gather the image of the focal spot, by calculating the gray level distribution of the focal spot image taken by the CCD, the energy distribution characteristic of focal spot can be obtained. The experimental results show that the focal length of the focusing mirror is 1.6 m, and the half-power surrounding diameter of the focal spot is 0.33 mm, corresponding to the angular resolution of 0.7 arc min.

Water dissolution ultra-precision polishing of KDP crystal and its precision cleaning

KDP crystal is a kind of excellent electro-optic material used in many laser facilities, which is also widely acknowledged to be extremely hard to machine. In this work, we developed a process to finish KDP crystal to a precision and clean engineering specimen. A micro-nano water dissolution principle and its planarisation mechanism in the machining process are illustrated, which turns the disadvantaged deliquescence property of KDP crystal into the driving force for ultra-precision polishing of the crystal. Micro emulsion fluid with nano water nuclei can precisely control the material removal and realize the selective polishing, thus forming a super smooth and precise surface. Then a subsequent cleaning process developed specially for this ultra-precision polishing method helps to get rid of the polishing fluid, and finally reduces the residue off the crystal to a clean and tidy surface with 1.964 nm rms roughness for engineering application.

Water dissolution ultra-precision polishing of KDP crystal and its precision cleaning

KDP crystal is a kind of excellent electro-optic material used in many laser facilities, which is also widely acknowledged to be extremely hard to machine. In this work, we developed a process to finish KDP crystal to a precision and clean engineering specimen. A micro-nano water dissolution principle and its planarisation mechanism in the machining process are illustrated, which turns the disadvantaged deliquescence property of KDP crystal into the driving force for ultra-precision polishing of the crystal. Micro emulsion fluid with nano water nuclei can precisely control the material removal and realize the selective polishing, thus forming a super smooth and precise surface. Then a subsequent cleaning process developed specially for this ultra-precision polishing method helps to get rid of the polishing fluid, and finally reduces the residue off the crystal to a clean and tidy surface with 1.964 nm rms roughness for engineering application.

Experimental and simulation study of nanometric surface roughness generated during Magnetorheological finishing of Silicon

Silicon Mirrors are essential components for guiding the X-Ray beam and focusing it to a particular location. These mirrors using total internal reflection require super smooth surface finish because X-Ray wavelength is very small, otherwise it suffers from strong scattering. The polishing possibility of silicon mirrors by Magnetorheological finishing process is examined. Magnetorheological finishing (MRF) is a computer-controlled technique that is used in the production of high quality optical lenses. This novel finishing process utilizes polishing slurries based on magneto rheological fluids, whose viscosity changes with the change in magnetic field. In this study, the MRF is used to polish the silicon mirror in order to achieve nanometric surface finish to be used for X-rays applications. The individual effect of parameters like magnetizing current, working gap, rotational speed on surface roughness is investigated. Based on the experimentation, the optimized process parameters are identified. The final surface roughness achieved is as low as 6.4nm. The surface quality is analyzed in terms of arithmetic roughness (Ra) and Scanning Electron Microscopy for uniform evaluation. In order to investigate the physical essence underlying MRF process, the Molecular Dynamics Simulations (MDS) is used. MDS is used to study the atomic-scale removal mechanism of single crystalline silicon in Magneto Rheological Finishing (MRF) process and particular attention is paid to study the effect of gap between the tool and work piece as well as cutting velocity on surface quality.

Oleophobic properties of the step-and-terrace sapphire surface

Sapphire is widely used in production of optical windows for various devices due to its mechanical and optical properties. However, during operation the surface can be affected by fats, oils, and other organic contaminations. Therefore, it is important to improve the oleophobic properties of sapphire windows. In this study, we investigate the interaction of a supersmooth sapphire surface with oleic acid droplets, which imitate human finger printing. It is established that chemical–mechanical polishing with additional annealing in air, which leads to the formation of an atomically smooth sapphire surface, makes it possible to significantly improve the oleophobic properties of the surface. The results are analyzed using the Ventsel–Deryagin homogeneous wetting model.

Ultra-fine polishing of glass-ceramics by disaggregated and fractionated detonation nanodiamond

Ultra-fine polishing of glass-ceramics is extraordinarily difficult because of structure inconsistency, chemical inhomogeneity and high rigidity. Detonation nanodiamond particles(NDs) were disaggregated and fractionated by serial centrifuge separation to obtain four stable suspension with average sizes of 54.9nm，103.8nm，145.6nm and 245.4nm respectively. Those suspensions were then employed to polish glass-ceramics. The evolution of NDs has been carefully characterized by FTIR, Raman and XRD analysis and the finish surfaces of glass-ceramics were investigated with AFM. Results showed that disaggregation treatment modified only nanodiamond surface other than its structure. The amount of nitro-groups on NDs surface increased and the absolute value of ξ-potential was enhanced by least 6 times at the pH 4–5, which was propitious to disperse NDs in mild acidic solution and polish glass-ceramics. Coarse particles performed both higher removal rate and average roughness (Sa) than the finer one. By using the suspension with average size of 54.9nm, a super smooth surface with Sa 0.208nm (5µm×5µm) was achieved.

Nanoscale surface characterization using laser interference microscopy

Nanoscale surface characterization is one of the most significant parts of modern materials development and application. The modern microscopes are expensive and complicated tools, and its use for industrial tasks is limited due to laborious sample preparation, measurement procedures, and low operation speed. The laser modulation interference microscopy method (MIM) for real-time quantitative and qualitative analysis of glass, metals, ceramics, and various coatings has a spatial resolution of 0.1 nm for vertical and up to 100 nm for lateral. It is proposed as an alternative to traditional scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods. It is demonstrated that in the cases of roughness metrology for super smooth ( Ra >1 nm) surfaces the application of a laser interference microscopy techniques is more optimal than conventional SEM and AFM. The comparison of semiconductor test structure for lateral dimensions measurements obtained with SEM and AFM and white light interferometer also demonstrates the advantages of MIM technique.

Electrochemical micro/nano-machining: principles and practices.

Micro/nano-machining (MNM) is becoming the cutting-edge of high-tech manufacturing because of the increasing industrial demand for supersmooth surfaces and functional three-dimensional micro/nano-structures (3D-MNS) in ultra-large scale integrated circuits, microelectromechanical systems, miniaturized total analysis systems, precision optics, and so on. Taking advantage of no tool wear, no surface stress, environmental friendliness, simple operation, and low cost, electrochemical micro/nano-machining (EC-MNM) has an irreplaceable role in MNM. This comprehensive review presents the state-of-art of EC-MNM techniques for direct writing, surface planarization and polishing, and 3D-MNS fabrications. The key point of EC-MNM is to confine electrochemical reactions at the micro/nano-meter scale. This review will bring together various solutions to "confined reaction" ranging from electrochemical principles through technical characteristics to relevant applications.

Sputtering of carbon using hydrogen ion beams with energies of 60–800 eV

This article presents the result of a study on the sputtering of carbon films by low-energy hydrogen ions. In particular, the etching rate and surface roughness were measured. The range of energies where the sputtering switches from pure chemical to a combination of chemical and physical mechanisms was determined. It is shown that Sigmund’s theory for ion etching does not work well for fields of energy less than 150eV and that it accurately describes the dependence of a sputtering coefficient on ion energy for energies greater than 300eV. A strong smoothing effect for the surface of carbon film was also found. This result is interesting in itself and for its significance for the manufacture of super-smooth surfaces for X-ray applications.